FOUS30 KWBC 130749
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
249 AM EST Wed Nov 13 2024

Day 1
Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE DEEP SOUTH...

...Southeast U.S and Ohio Valley...

A 1034mb surface ridge is currently analyzed over the Great Lakes
and adjacent Ontario Province with an expected translation eastward
over the next 24 hrs leading to an increasing return flow pattern
over the Southeastern U.S. Current v-vector anomalies pinpoint the
strongest return flow between sfc-850mb bisecting areas from LA
into MS and Western AL leading to ample moisture being pulled
poleward as far north as the Great Lakes. PWAT anomalies on the
order of +2 to +3 deviations will be co-located with the strongest
v-vector component leading to a broadening area of elevated
moisture and accompanying theta-E's indicative of greater
instability, especially the closer you are to the Gulf. Across the
Plains, a potent shortwave trough will shift eastward with
increasing mid and upper level ascent across areas downstream
within the Low to Mid-Mississippi Valley area. The combination of
the pattern evolution will aid in increased convective development
and coverage, especially as we work through the late-morning time
frame and beyond during the period.

Heavy rain potential will be on the rise with the introduction of
the thunderstorm risk with rainfall totals from the latest HREF
blended mean now sufficiently into the 1.5"+ territory with
associated CAMs signaling isolated totals >4" during the time frame
of convective impact. This has ramped up probabilistic signals with
the neighborhood probs now upwards of 40-70% across portions of
South-Central LA through much of MS with the highest probs located
over the MS Delta over to Jackson. Organized convective pattern is
most likely to occur over this area thanks to the approach of a
cold front upstream stemming from surface low development over the
Southern Plains, moving northeast through the Mississippi Valley
after nightfall. Hourly rates within the heaviest convection across
the Deep South is likely to range from 1-2"/hr on average, but some
non-zero prospects for 2.5-3"/hr persists within the CAMs and
expressed through the probability fields on the 00z HREF output.
Typically these signals would warrant a MRGL risk for excessive
rainfall, but considering the soils across parts of LA and MS still
recovering from the onslaught of heavy precip from a few days
prior, the antecedent environment is a bit more conducive for flash
flood prospects. In coordination with the local WFOs within LA and
MS, a Slight Risk was added to encompass areas where heavier
convection is most likely considering the pattern evolution, as
well as covering the areas where top layer soil moisture anomalies
lie above the 70th percentile as reported by the updated NASA SPoRT
data.

The remainder of the Southeast and points up into the Ohio Valley
will maintain the previous MRGL risk forecast with only a minor
expansion to the north and east to account for some of the trends
of increasing heavy rainfall potential through Northern KY as the
system remains fairly progressive in its forward propagation.
Limited instability will also cap the risk for significant rainfall
rates, but instances of >1"/hr are certainly plausible considering
the prime mid-level evolution and approaching cold front providing
surface based forcing. Totals will be closer to the 1-2" range for
most, but 2-3" is within reason in the heaviest rain cores.

...Pacific Northwest...

Atmospheric River from this evening will continue to unfold across
the Pacific Northwest with the IVT pulse orienting further south
through the morning and early afternoon with heavy rain impacting
areas of Southwest OR through much of Northwestern CA with the
heaviest rain focused at the immediate coast into the interior
terrain aligning from the Klamath Mountains down towards the King
Range south of Eureka. The forecast has been very consistent on the
threat the past several days with little deviation in the expected
outcome with totals ranging from 2-4" at the immediate coast from
WA down through Northern CA with 3-6", local maxima of 8" during
the 48-hr period of impact over places like the Olympic Range in WA
state and the Klamath Range down in Southwestern OR. The premise of
the MRGL risk was mainly for 3 or 6 hour FFG exceedance intervals,
along with some 24 hour FFG potential, mainly located over the
Olympics for the latter index. Hourly rates will be majority sub-
FFG exceedance, however some 1-1.5"/hr rates during the height of
the IVT surge Wednesday morning over the coastal mountain ranges
could be sufficient for isolated flooding prospects before becoming
more of an accumulative rainfall concern over time. The one area
where the heavier precip is not anticipated but still a relative
concern remains in the area located south of Mount Shasta and
points west where remnant burn scars maintain locally lower FFG
indices leading to a heightened threat for flash flooding where the
scarring is located. This is the reason for the inland extent
across Northern CA and to maintain consistent messaging with the
local WFO for the threat.

Kleebauer


Day 2
Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Fri Nov 15 2024 - 12Z Sat Nov 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

$$
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FOUS30 KWBC 140754
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
254 AM EST Thu Nov 14 2024

Day 1
Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Maturing surface cyclone off the VA/NC coast will amplify
rainfall prospects over the VA Tidewater into Eastern NC late this
afternoon and evening with a developing axis of deformation driving
elevated convective concerns in the aforementioned areas. Totals
between 1.5-3" are forecast within these zones with some minor
flooding possible over urban zones and the immediate coast,
coinciding with coastal flooding likely occurring. Rainfall rates
will fall short of the necessary criteria to meet FFG exceedance,
however a non-zero threat will exist for flash flood concerns
across Northern NC into Southern and Southeast VA in urban zones
that are more prone to flooding prospects. This is still below the
5% threshold necessary for flash flooding leading to a continued
nil ERO, but non-zero threat.

Kleebauer


Day 2
Valid 12Z Fri Nov 15 2024 - 12Z Sat Nov 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

$$
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FOUS30 KWBC 150755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Fri Nov 15 2024

Day 1
Valid 12Z Fri Nov 15 2024 - 12Z Sat Nov 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 2
Valid 12Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Campbell


Day 3
Valid 12Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

An upper trough traversing through the West this weekend is
expected to split which will send energy in the Northern
Plains/Upper Midwest along with a strong southern stream cutoff low
into the Four Corner that will exit into the Central/Southern
Plains for the start of the week. A surface low pressure system
will forecast to spin up in the lee of the Rockies and deepen as it
tracks from the Plains through the Midwest. Scattered to widespread
moderate to heavy rainfall will be supported by this setup across
portions of the Southern Rockies and the adjacent Southern/Central
Plains.

Guidance is suggesting 1 to 2 inches of rain for central and
eastern portions of New Mexico, particularly over some of the same
areas that had several inches of snow last week. With rain on top
of snow expected some locations will have an increased risk for
flooding concerns. The Marginal Risk area that was already in
effect was expanded westward to now cover central and eastern
portions of New Mexico. The highest concentration of rainfall will
likely focus across parts of northern Texas/panhandle into western
Oklahoma. Consensus suggests areal average of 3 to 4.5 inches with
locally higher amounts possible. Therefore, a Slight Risk was
introduced with this issuance.

Campbell

$$
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FOUS30 KWBC 161558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1058 AM EST Sat Nov 16 2024

Day 1
Valid 16Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF THE PACIFIC NORTHWEST...

Conventional GOES satellite imagery along with ALPW data shows an
offshore atmospheric river beginning to gradually impinge on the
Pacific Northwest. An offshore warm front will gradually begin to
overspread the region by this evening which will coincide with
strong warm air advection and moisture transport. The latest 12Z
HREF guidance suggests a 40 to 50+ kt low-level jet arriving with
an orientation gradually becoming more orthognal to the coastal
terrain of western WA including the Olympic Peninsula and extending
southward down into northwest OR. A cold front will then begin to
approach and gradually cross western WA by 12Z/Sunday. This
atmospheric river event will be progressive, but there will be a
corridor of IVT values locally exceeding 500 kg/m/s tonight across
these coastal ranges based off the 06Z GFS which seems reasonable
considering the elevated CIRA-LVT parameters offshore. Some
rainfall rates associated with the combination of strong warm air
advection and orographic forcing should reach into the 0.25" to
0.50"/hour range between 00Z and 12Z, and this will help facilitate
some rainfall totals of 2 to 3 inches by 12Z for the windward
slopes of the coastal ranges. Given the expected rates and totals
by early Sunday morning, and thus earlier arrival of this event, a
Marginal Risk of excessive rainfall has introduced for this period.

Orrison


Day 2
Valid 12Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

A southern stream cutoff low is expected to eject into the Plains
and be accompanied with very impressive IVT, PWs and forcing as it
moves across Texas and Oklahoma by Sunday evening into the
overnight hours. This should be enough to overcome the limited
forecast instability and result in deep convection and heavy
rainfall rates across portions of the Southern Rockies and the
adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain, but would expect isolated swaths as high as 4-6"
within this setup. The overall progressive nature of the system and
limited instability will cap the upper bound of rainfall
totals...but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw a minor expansion to reflect the latest trends
and WPC QPF and spans from northern Texas/panhandle to western
Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Campbell


Day 3
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained with minor adjustments for this
issuance.

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi.

Campbell
$$
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FOUS30 KWBC 171556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1056 AM EST Sun Nov 17 2024

Day 1
Valid 16Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

1600Z Update...

The 12Z HREF guidance along with 06Z RRFS/REFS output continues to
support a locally heavy rainfall event across areas of the southern
Plains by this evening and extending into the overnight period. There
has been some shift farther west with some of the swaths of heavy
rainfall potential across west TX as some of the individual CAMs
are supporting the heavier rainfall/convection along the immediate
cold front and in close proximity to the deepening surface low
that will be evolving across the TX High Plains tonight. However, a complication to the rainfall forecast is the downstream evolution
of convection along the front ahead of the surface low involving
areas of northwest TX through southwest to central OK. This is
where strong moisture convergence and forcing may foster a threat
for some training areas of convection that become aligned in a
southwest to northeast fashion. Overall, still expecting some
swaths of 2 to 4 inch rainfall totals, with isolated potential for
heavier amounts and especially across northwest TX into southwest
OK where the better convective training threat will exist. Given
the latest CAM guidance, the Slight Risk area has been expanded
somewhat to account for the varying modes of convective evolution
that are expected this evening and overnight.

Elsewhere, no changes have been made to the previous D1 outlook
including the Marginal Risk area across western OR where there
continues to be some lingering, but modest atmospheric river
activity.

Orrison

Previous Discussion...

...Southern Plains...

A southern stream cutoff low is expected to eject into the Plains
today with very impressive IVT and accompanying anomalous PWs and
forcing as it moves across Texas and Oklahoma by this evening into
the overnight hours. This looks to be enough to overcome the
limited forecast instability and result in deep convection and
heavy rainfall rates across portions of the Southern Rockies and
the adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain (per latest HREF and WPC QPF), but would expect
isolated swaths as high as 4-6" within this setup (indicated by
some of the heaviest CAMs). The overall progressive nature of the
system and limited instability will cap the upper bound of rainfall
totals, but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw only minor adjustments to reflect the latest
trends and WPC QPF and spans from northern Texas/panhandle to
western Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Churchill/Campbell


Day 2
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained once again with only minor
adjustments for this issuance (not enough confidence for 2"+ totals
to introduce a Slight Risk).

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi (changed very little from inherited with 3" localized
totals possible).

Churchill/Campbell


Day 3
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND CENTRAL GULF COAST...

...Central Gulf Coast and Southeast...

By Tuesday significantly more tropical moisture will likely advect
northward through the Gulf of Mexico in association with the
remnants of Tropical Storm Sara; which will increase the potential
for heavy rain and localized flooding concerns along portions of
the Gulf Coast. An inherited Slight Risk area was maintained for
southern AL and surrounding portions of FL/GA/MS/LA, given the
potential for localized 3-5" totals (per ensemble guidance and WPC
QPF).

...Coastal Washington, Oregon, and northwest California...

Another weak/moderate atmospheric river is expected for region for
Day 3. Total rain of 2-4" (similar to Day 1) are expected with
hourly rainfall rates possibly approaching 0.5" once again suggest
that some minor flooding issues could arise along coastal
areas/mountains. An inherited Marginal Risk area remains in effect
along the coastline.

Churchill
$$
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FOUS30 KWBC 180756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Mon Nov 18 2024

Day 1
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
LOUISIANA INTO FAR SOUTHWEST MISSISSIPPI...


An axis of much above average tropical PW values expected to be
drawn rapidly northeastward from the remnants of Sara, across the
central Gulf of Mexico and into the Central Gulf coast ahead of
the negatively tilted closed low ejecting northeastward from the
Southern Plains into the Upper Mississippi Valley day 1. These
anomalous tropical PW values, 3 to 4+ standard deviations above the
mean will enhance convection along and ahead of the cold front
pushing eastward across the Gulf coast, primarily during the second
half of day 1 from southeast Louisiana into southern Mississippi.
A slight risk area was maintained where the latest HREF
neighborhood probabilities are highest for 2 and 3"+ amounts. The
simulated hi res radars are in good agreement in showing potential
for a period of training of cells in this anomalous PW axis in the
0000-1200 UTC Tuesday time period across the slight risk area of
southeast LA into southern MS. In areas of training, hourly
rainfall amounts of 1-2"+ possible, resulting in flash flooding,
especially in the urbanized areas of New Orleans.

Across the Southern to Central Plains toward the Upper MS Valley...there
is fairly good agreement on an axis of heavy rain from northern
OK, across much of KS, eastern NE, northwest MO, southeast SD, much
of IA, southern MN into southwest WI in a region of strong
isentropic lift ahead of the deepening surface low moving quickly
northeastward across these areas. While widespread heavy amounts
are likely, hourly rates will are mostly depicted as .25-50"+ in
the latest hi res guidance. This should keep the threat of any
runoff issues at marginal. The previous marginal risk area was
trimmed on the eastern end, taking out much of IA, MO and eastern
OK. The current marginal risk coincides well with the axis of the
highest HREF 2"+ day 1 probabilities.

Oravec

Day 2
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
LOUISIANA, ACROSS FAR SOUTHERN MISSISSIPPI, FAR SOUTHERN ALABAMA
INTO THE WESTERN FLORIDA PANHANDLE...

The convection that is expected to enhance along the Central Gulf
coast late day 1, will continue into day 2 as the surface front
continues to push east along the Gulf Coast. The tropical PW
values entrained ahead of this front day 1, will continue to push
across central to eastern Gulf coast day 2. Training of cells in a
southwest to northeasterly direction likely to continue Tuesday
morning from southeast LA into far southern MS, far southern AL and
the western FL Panhandle. There is some timing differences as to
whether heavy rains will continue in the vicinity of New Orleans
early Tuesday, or have pressed just to the east. The New Orleans
metro area was kept in the slight risk given potential for an
overlap of day 1 and day 2 heavy rain areas. The latest HREF 12
hour neighborhood probabilities ending 00Z Wed show the highest
probabilities for 2 and 3"+ amounts remaining across far southeast
LA, far southern MS, far southern AL and the western FL Panhandle.
This was the region depicted in a slight risk for day 2, with the
previous risk area trimmed on the northern end across south central
AL after collaboration with WFO MOB. The slight risk area
corresponds with the hi res consensus for the greatest hourly
totals of 1-2"+ in regions of training.

...Coastal Washington, Oregon and far northwest California...

Models are in good agreement with the rapid cyclogenesis event
across the northeast Pacific off the Pacific Northwest coast. The
low level southerly flow along and ahead of the associated cold
front will become very anomalous Tuesday night into early
Wednesday, with 850-700 mb moisture flux anomalies of 3-4 standard
deviations above the mean impacting the Pac NW/far northwest CA
coast. A well defined area of frontal precip likely to push inland
across coastal Pac NW/far northwest CA after 0000 UTC. Hourly
rainfall rates in the .25-50"+ range possible, especially across
southwest Oregon into far northwest CA where the strong southerly
low level flow will have a better upslope component. For areas of
northwest California into far southwest Oregon, this will be the
beginning of a multi day Atmospheric River event. No changes made
to the marginal risk area from the previous issuance.

Oravec


Day 3
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS FAR
Southwest OREGON INTO NORTHWEST CALIFORNIA...

The multi day atmospheric river event the began late day 2 for far
southwest Oregon into northwest California will continue day 3. The
rapidly deepening day 2 low over the northeast Pacific will anchor
a nearly stationary upper vortex off the Pac NW coast day 3. Strong
persistent deep layered west southwest flow on the south side of
this vortex will impact northwest California into far southwest
Oregon day 3. with IVT values of 500-800 km/m-1 s-2. and 850-700 mb
moisture flux anomalies of 2-4 standard deviations above the mean.
There is fairly good model agreement with heavy precip totals of
3-5" primarily over northwest California, with isolated totals of
6-8" possible. No significant changes made to the previous slight risk area.

Oravec

$$
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FOUS30 KWBC 190828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Tue Nov 19 2024

Day 1
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL GULF COAST AND FOR PORTIONS OF NORTHWEST CALIFORNIA AND
SOUTHWEST OREGON...

...Central Gulf Coast...

Latest satellite imagery is showing enhancing convection to the
south of southeast LA associated with the anomalous tropical
moisture surge ahead of the cold front moving east across the
Central Gulf Coast. Trends in the latest hi res guidance suggest
this offshore enhancing convection will become the heaviest
frontal/pre-frontal rains as organized convection currently along
the front farther to the north begin to weaken after 1200 UTC. Hi
res guidance is in fairly good agreement that the heaviest rains
with the offshore enhancing convection will remain just to the
south of far southern MS, far southern AL and the western FL
Panhandle, but still affecting far southeast LA. HREF neighborhood probabilities show that the axis of the highest probabilities for 2
and 3"+ amounts will remain offshore with the northern edge of
these higher probabilities affecting areas from far southeast LA
and along the immediate far southern MS, far southern AL and
western FL Panhandle. Changes to the previous outlook were to trim
the northern extent of the slight risk area by approximately 20 nm
and to trim the western extent of the slight risk by
approximately 40nm to account for the hi res trends. At the
moment, the consensus on timing of convection supports removing the
New Orleans metro area from the slight risk, with the heaviest
rains likely to the east and southeast prior to 1200 UTC Tuesday.

...Coastal Washington, Oregon and far northwest California...

Latest models continue to be in good agreement on the rapid
cyclogenesis event across the northeast Pacific off the Pacific
Northwest coast, signaling the beginning of a multi day Atmospheric
River event for northwest CA into far southwest OR. Anomalous low
level southerly flow expected along and ahead of the associated
cold front Tuesday night into early Wednesday, with 850-700 mb
moisture flux anomalies of 3-4 standard deviations above the mean
impacting the coastal Pac NW, south into far northwest CA coast. A
well defined area of frontal precip likely to push inland across
coastal Pac NW/far northwest CA after 0000 UTC Wednesday. The
strong southerly low level flow will be mostly parallel to the
terrain along the Pac NW coast, limiting hourly rainfall rates to
.10-.25"+. Greater hourly rates likely across far southwest Oregon
into coastal NW CA where a more significant upslope component to
the low level flow is likely. Hi res guidance is fairly similar in
showing hourly rates of .25-.50"+ in the 0600 to 1200 UTC Wednesday
period across far southwest OR into far NW CA. HREF .50"/hr
probabilities subsequently are high across this area, with 1"/hr
probabilities much lower and primarily offshore. No changes made to
the previous slight risk area, which corresponds well to the above
mentioned axis of high .50"+/hr HREF neighborhood probabilities.

Oravec


Day 2
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The multi day atmospheric river event the began late day 1 for far
southwest Oregon into northwest California will continue day 2,
primarily across northwest California. The rapidly deepening low
over the northeast Pacific day 1 will anchor a nearly stationary
upper vortex off the Pac NW coast day 2. Strong persistent deep
layered west southwest flow on the south side of this vortex will
impact northwest California day 2, with IVT values of 500-800
km/m-1 s-2. and 850-700 mb moisture flux anomalies of 2-4+
standard deviations above the mean. There is fairly good model
agreement with heavy precip totals of 3-5" primarily over northwest
California, with isolated totals of 6-8" possible. The previous
moderate risk area was adjusted slightly southward by approximately
10-40 nm to account for model max axis consensus. Hourly rainfall
amounts of .25-.50"+ likely through the day 2 time period across
northwest CA with increasing flooding risks with time as soils
become saturated and streams and river levels rise. The HREF 1 hour probabilities for .50"+ totals for the first half of day 2 when
the data is available, remain high across northwest CA.

Oravec

Day 3
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 221 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

The deep northeast Pacific Vortex remains nearly stationary day 3
off the Pacific Northwest coast, supporting the continuation of the
multi day Atmospheric River event for northwest CA into far
southwest OR. Anomalous deep layered southwesterly flow will
continue on the south side of this vortex. 850 to 700 mb moisture
flux anomalies will continue to be 2 to 4+ standard deviations
above the mean with IVT values also maintained in the 500 to 800 km/m-1
s-2 range across northwest CA into far southwest OR. There is a
likelihood of very heavy rains again falling day 3 over similar
regions from the day 2 period. Hourly rates of .25-.50"+ likely to
continue at times supporting additional 24 hour rainfall amounts
of 3 to 6 inches and isolated additional 24 hour totals of 6-8"
across northwest CA into southwest OR. Storm total amounts of 12 to
16+ inches possible across portions of northwest California. Given
this day 2-3 heavy rain overlap potential, a small high risk area
was introduced over northwest CA, across the west central portions
of the EKA's CWA. Flood risks will continue to increase day 3 along
with rock and land slides as soils become saturated and streams
and rivers continue to rise and overflow. There will be a
northward shift back into far southwest Oregon day 3 for the heavy
rain potential as the upper flow back and heights rise. The rising
heights will also keep much of the day 3 precip as rain, increasing
overall runoff.


Oravec
$$
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FOUS30 KWBC 200809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 AM EST Wed Nov 20 2024

Day 1
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The latest model guidance remains consistent with the overall
evolution of the strong multi day atmospheric river event into
northwest California. The low that deepened rapidly over the
northeast Pacific Tuesday, will continue to anchor deep layered
west southwest flow on the south side of this system, impacting
northwest California day 1 with IVT values of 500-800 km/m-1 s-2.
and 850-700 mb moisture flux anomalies of 2-4+ standard deviations
above the mean. There is fairly good model agreement with heavy
precip totals of 3-5" primarily over northwest California and the
northern Sierra, with isolated totals of 6-8" possible. HREF
neighborhood probabilities are very high for 5"+ amounts across
northern California, with most areas showing 80-90%+, while 8"+
probabilities are 40-80% along the northwest CA coast and 40-60%+
over the northern Sierra. Hourly rainfall amounts of .25-.50"+
likely through the day 1 time period across northwest CA with
increasing flooding risks with time as soils become saturated and
streams and river levels rise. The HREF 1 hour probabilities for
.50"+ totals remain high in the max inflow band across northwest
CA for all of day 1. This band does continue to sink south for the
first few hours of day 1, but then become nearly stationary for the
remainder of the day 1 time period. No significant change to the
previous outlook. The slight risk was extended approximately 75
miles farther south in the lee of the northern Sierra where 2-4"
rainfall totals are possible.

Oravec


Day 2
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

The strong Atmospheric river event into northwest California will
continue into day 2 as deep layered southwesterly flow continues
to the south of the northeast Pacific upper vortex. The anomalous
deep layered southwesterly flow will continue to support 850 to 700
mb moisture flux anomalies of 2 to 4+ standard deviations above
the mean and IVT values in the 500 to 800 km/m-1 s-2 range across
northwest CA. During the second half of day 2, a strong vort
rotating around the base of the northeast Pacific vortex will
support another rapidly deepening low moving northeast
approximately 200 miles off the Pacific Northwest coast. While this
second rapid cyclogenesis event will not be as strong as the
Tuesday event, it will help to begin to push the best onshore flow
axis back to the north during the second half of day 2, returning
heavy precip into southwest Oregon. There is a likelihood of very
heavy rains again falling day 2 over similar regions from the day 1
period. Hourly rates of .25-.50"+ likely to continue at times
supporting additional 24 hour rainfall amounts of 3 to 6 inches and
isolated additional 24 hour totals of 6-8" across northwest CA
into southwest OR. Storm total amounts of 12 to 16+ inches possible
across portions of northwest California. Given the 2 days of very
heavy totals in the same region, no significant changes were made
to the previous high risk area. Significant flood risks will
continue to increase day 2 along with rock and land slides as soils
become saturated and streams and rivers continue to rise and
overflow. Ahead of the next deepening low, rising heights over
northern California will keep much of the precipitation in the form
of rain. This will increase the runoff threats for areas in the
lee of the northern Sierra. Across this region, the risk area was
increased from slight to moderate from the previous issuance.


Day 3
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The multi day atmospheric rive event will begin to weaken on day 3
as the onshore southwesterly flow weakens and the best onshore flow
axis shifts southward. The heaviest totals day 3 likely into the
upslope of the northern Sierra where high heights will again favor
a majority of the precip falling as rain. A moderate risk was added
for this region where additional totals day 3 of 2-4 inches
possible across areas that received 5-8+ inches days 1 and 2. Along
the northwest California coast, additional heavy preip totals
likely day 3, but amounts will be much less than the day 1 and day
2 amounts, generally 1 to 1.50"+, with localized 2"+ totals. A
slight risk was maintained here given the effects from the day 1
and 2 amounts, which will support additional runoff issues for
lesser than normal amounts to produce runoff concerns.

Oravec
$$
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FOUS30 KWBC 220747
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
247 AM EST Fri Nov 22 2024

Day 1
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN CALIFORNIA...

The long duration, multi-day atmospheric river event that has been
impacting northern California will finally begin to become more
progressive and start to wane in intensity as a cold front just
offshore of the West Coast advances inland and drives the axis of
deeper layer Pacific moisture transport south and eastward.

However, there will still be one more day of locally very heavy
rainfall totals which are expected to be excessive and capable of
driving locally significant flooding impacts. The 00Z HREF guidance
supports some additional 5 to 10 inch rainfall totals for the
24-hour period ending 12Z/Saturday, with the heaviest totals
expected to be over the windward slopes of the northern Sierra
Nevada, and perhaps locally a few of the coastal ranges just north
of the Bay Area where there has already been very heavy rainfall
totals over the last couple of days.

IVT magnitudes along and just south of the cold front will be rather
high at the start of the period this morning, with magnitudes over
coastal areas of northwest California upwards of 750+ kg/m/s, and
these values should decrease slowly and settle southward toward
00Z. However, as the front gets into the Bay Area and the Central
Valley by early this evening, there is some suggestion in the
guidance that the front will slow down again just a bit. Coinciding
with that will be arrival of a shortwave and associated upper-
level jet streak (130+ kts) rounding the base of the deep offshore
trough. This should foster a temporary resurgence/strengthening of
the IVT values in the 00Z to 06Z time frame across the Bay Area and
extending inland across portions of the Central Valley and
foothills of the Sierra Nevada. Thus expectations are that heavy
rains will overspread all of these areas along with some localized
increase in rainfall rates this evening. In general, the rainfall
rates will be capable of reaching 0.50" to 0.75"/hour across the
coastal ranges around the Bay Area and also into the upslope areas
of the northern Sierra Nevada. Given the additional rainfall
amounts, and considering the antecedent conditions at this point,
the Moderate Risk area is maintained across the northern Sierra
Nevada, with a separate Moderate Risk introduced just north of the
Bay Area involving Sonoma and Napa Counties. Other adjustments
include extending the Marginal and Slight Risk areas a bit farther
south to include the coastal ranges a bit south of the Bay Area and
also more areas of the Central Valley. Flooding impacts will again
be rather widespread and locally significant, with additional
concerns for debris flows, rock and landslide activity, and flash
flooding involving burn scar locations. In fact, the Park Fire burn
area involving parts of Tehama and Butte Counties will need to be
very closely monitored today for enhanced impacts given the amount
of rainfall that is forecast here.

Orrison


Day 2
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

At 12z Saturday the cold front and associated moisture plume will
be dropping south across the central Sierra Nevada. By this time
rainfall intensity will be decreasing, and the axis of better
moisture will be dropping south of the areas hardest hit over the
past few days. Thus the flood risk should be pretty low Saturday
morning along this southward dropping front.

Some weak post frontal instability is forecast which could allow for
locally heavy showers in the wake of the front over the western
slopes of the central Sierra Nevada. Not expecting anything too
intense or widespread, however there is at least a chance that a few
cells could drop a quick 0.5" or so of rain. This would be falling
over increasingly saturated ground, and so a localized flood risk is
possible. Thus we will maintain the small Marginal risk where a few
heavier post frontal showers are possible Saturday.

Chenard


Day 3
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
will provide plentiful low level moisture convergence, and mid level
shortwave energy should provide some additional lift as well.
Moisture is a limiting factor for heavy rainfall with the better
moisture plume situated well south and not really getting drawn into
this system. Thus this is not expected to be a significant
atmospheric river, and normally would not expect a system of this
nature to pose a flood risk. However given the significant rainfall
over the past few days, this additional rainfall may be enough to
cause some localized additional flooding as the area will be more hydrologically sensitive than normal. Still some model differences
in the exact evolution of this system and thus rainfall amounts this
period are uncertain...but current indications suggest localized
amounts of 2-3" will be possible.

Chenard

$$
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FOUS30 KWBC 230748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 23 2024

Day 1
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

A cold front and the remnants of the long duration, multi-day
atmospheric river that has been impacting the West Coast will be
dropping south across the central Sierra Nevada early this morning.
As the deeper layer moisture flux parameters/IVT magnitudes
continue to weaken and thin out, the rainfall rates will be
continuing to drop off. However, there still may be a sufficient
level of upslope flow/orographic ascent in the foothills of the
central Sierra Nevada for a few more hours early this morning of
spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal
instability is also still forecast which could allow for some
scattered pockets of heavy shower activity going through midday to
impact some of the western slopes of the central Sierra Nevada.
This could result in some isolated quick- hitting 0.25" to 0.50"
rainfall amounts, but these convective cells are forecast to be
progressive. Overall, the additional storm total amounts based off
the 00Z HREF guidance may reach as high as 1 to 2 inches.

Given the very wet antecedent conditions, the additional rains may
foster some additional localized runoff/flooding concerns. Thus,
the Marginal Risk area over portions of the Sierra Nevada will be
maintained.

Orrison


Day 2
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
and a weak low to mid level wave riding along it will support a
modest uptick in IVT and plentiful low level convergence. Moisture
is a limiting factor for heavy rainfall with the better moisture
plume situated well south and not really getting drawn into this
system. Thus this is not expected to be a significant atmospheric
river, and normally would not expect a system of this nature to
pose a flood risk. However given the significant rainfall over the
past few days, this additional rainfall may be enough to cause some
localized additional flooding as the area will be more
hydrologically sensitive than normal.

We are still seeing some spread in model QPF output over this
period. If some of the more aggressive solutions pan out then we
could see 1-3" of rainfall is spots with hourly totals peaking
around 0.3-0.4" in the typically more favored upslope areas.

Chenard


Day 3
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON...

Showery conditions will likely continue across portions of northern
CA and southwest OR Monday into Monday night as a deep layered low
and shortwave energy moves inland. Certainly not a classic setup
for flooding with limited moisture and dropping IVT
values...however this area remains more hydrologically sensitive
than normal following the significant atmospheric river event the
past few days. Additional rainfall over far northwest CA into
southwest OR is generally 1-3" in the guidance, with isolated
totals of this magnitude also possible over the western slopes of
the northern Sierra Nevada.

A more significant plume of moisture/IVT will actually move into
portions of southern/central CA...generally from portions of San
Luis Obispo and Monterey counties into the western slopes of the
southern Sierra Nevada. QPF has been trending up with this system,
although still seeing some model spread. The latest model consensus
suggests a possible 2-3" of rain along the coastal ranges and the
southern Sierra below snow levels. Not seeing any instability with
this system and large scale forcing is only modest, so hourly
rainfall rates are generally not expected to exceed 0.5". Given
this, and the fact that most of this area has not seen significant
rainfall recently, will continue with no areas in the excessive
rainfall outlook for now.

Chenard

$$
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FOUS30 KWBC 240816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Sun Nov 24 2024

Day 1
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible today and especially tonight
over portions of northern CA into far southwest OR as an offshore
shortwave and attendant wave of low pressure arrives near the
coastal ranges. Generally the IVT magnitudes out ahead of this
surface wave will be modest with values increasing to as high as
250 to 400 kg/m/s. However, with a front nearby and favorable
upslope flow/orographic ascent involving the coastal ranges, there
will likely be some concentrations of heavier shower activity with
rainfall rates capable of occasionally reaching into the 0.25" to
0.50"/hour range. Some pockets of heavier rain may also make it
into the far northern Sacramento Valley and adjacent foothills of
the northern Sierra Nevada and Shasta/Siskiyou ranges. The 00Z HREF
guidance supports some spotty 1 to 3 inch rainfall amounts going
through early Monday morning, and thus this is not expected to be a particularly heavy event. However, given the significant rainfall
and lingering hydrological impacts from the most recent atmospheric
river, these additional rains may result in at least some
localized runoff problems and flooding.

Orrison


Day 2
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CALIFORNIA AND SOUTHWEST OREGON...

Showery conditions will likely continue across portions of
northern CA and southwest OR Monday into Monday night as a deep
layered low and shortwave energy moves inland. Certainly not a
classic setup for flooding with limited moisture and dropping IVT
values... however this area remains more hydrologically sensitive
than normal following the significant atmospheric river event the
past few days. Additional rainfall over far northwest CA into
southwest OR is upwards of 1-3" in the guidance, with isolated
totals of this magnitude also possible over the western slopes of
the northern Sierra Nevada.

A more significant plume of moisture/IVT will move into portions of southern/central CA...generally from portions of San Luis Obispo and
Monterey counties into the western slopes of the southern/central
Sierra Nevada. IVT values are forecast to be around 500-600 kg/ms,
which at this latitude is starting to approach climatological max
values for late November. PWs are also forecast to increase to
around the 99th percentile for late November, and thus expecting
plenty of moisture and upslope flow to allow for decent rainfall
totals. Not expecting any instability, so rainfall intensity will be
solely driven by the aforementioned moisture transport and upslope enhancement...and these factors alone are unlikely to support
rainfall rates high enough for true flash flooding. However we will
have an extended period of time with elevated IVT values, and so a
prolonged steady rainfall could eventually result in some localized
minor flood concerns. Overall expecting 1-3" of rain over the
coastal ranges in San Luis Obispo and Monterey counties (with
localized totals towards 4" possible)...with totals ranging from
near 1" in the eastern valley locations to 2-4" over the western
Sierra below snow levels.

Chenard


Day 3
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN SIERRA NEVADA...

The atmospheric river event described in the day 2 discussion
continues into Tuesday over portions of the central and southern
Sierra Nevada. There is a bit more model spread by this time with
regards to the persistence of the IVT and strength of the mid/upper
forcing. A majority of the 00z deterministic models have trended a
bit weaker and faster with the shortwave energy pushing across CA
and into NV...and this trend would result in less additional QPF
over central CA on day 3. However the 00z GEFS and recent NBM
probabilities suggest there is still a chance for a slower/stronger
wave resulting in more robust rainfall amounts.

This will be the 2nd consecutive day of steady rainfall for these
areas, and so do expect some increased hydrologic sensitivity that
could result in at least some localized minor flooding issues. When
summing the two days of rainfall together, the latest WPC
deterministic QPF is calling for around 2" in Fresno to Visalia,
increasing to 3-6" in the foothills/western slopes of the southern
and central Sierra Nevada. Given the magnitude of IVT forecast even
even forecasting 1-2" spilling over to the east of the crest towards
Bishop and Independence. These forecast values average around a 2yr
ARI for 48 hr amounts, also supportive of a localized minor flood
threat. As mentioned above, there is both higher and lower end
potential with this forecast rainfall. The higher end scenarios
could even result in Slight risk level impacts, while the lower end
outcomes would be a minimal flood threat. So will need to continue
to monitor trends, and for now still think the Marginal risk is
appropriate.

Chenard

$$
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FOUS30 KWBC 250757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Nov 25 2024

Day 1
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN SIERRA NEVADA...

Moderate to locally heavy showers are expected to continue across
portions of northern CA and southwest OR today and tonight as a
deep layered low and associated shortwave energy moves inland. The
deep layer cyclonic flow coupled some steepening of the mid-level
lapse rates and orographic ascent/upslope flow over the coastal
ranges should favor some occasional 0.25" to 0.50"/hour rainfall
rates. Some of these heavier showers will also spread inland
across the northern Sierra Nevada. Overall, the additional rainfall
amounts here should be relatively modest, but portions of the
coastal ranges of northwest CA and far southwest OR may see some
1-3" rainfall totals going through early Tuesday morning. Given the wet/saturated soil conditions and high streamflows, these
additional rains may foster some renewed runoff problems and
localized flooding. A Marginal Risk will be maintained for these
areas.

Farther south into the upslope areas of the central and especially
southern Sierra Nevada, and also the coastal ranges of Monterey,
San Luis Obispo and Santa Barbara Counties, a moderately strong
atmospheric river event will be arriving today and continuing
through tonight and early Tuesday. This will be associated with the
arrival of a southern stream shortwave impulse which will be
embedded within the transport of deeper layer subtropical moisture
extending well offshore of southern CA and upstream over the
eastern Pacific. The GFS and ECMWF support increasing IVT
magnitudes especially by 00Z across the coastal ranges and into
higher terrain of the southern Sierra Nevada with magnitudes rising
to 500 to 700+ kg/m/s. The orientation of this will be generally
orthogonal to the terrain, and this should yield enhanced
orographic ascent that will be conducive for heavier rainfall
rates.

The 00Z HREF guidance favors some occasional 0.50"+/hour rainfall
rates with the higher probabilities of this focused over the
foothills of the Sierra Nevada involving Fresno and Tulare
Counties, and a small portion of northern Kern County. A consensus
of the 00Z HREF guidance supports 24-hour rainfall totals
(generally below 9000 feet/snow level) of 3 to 6 inches, with a
couple of models indicating some spotty 6 to 8+ inch amounts.
Meanwhile, over the coastal ranges from Monterey County down
through San Luis Obispo and Santa Barbara Counties, some 1 to 3+
inch rainfall totals are supported in the guidance. One key factor
in general that is supporting these higher rainfall totals, aside
from the upslope flow into the terrain, is the fact that much of
the moisture will be of subtropical origins, and this will yield PW
anomalies as high as 2 to 4 standard deviations above normal for
this time of the year. Given the forecast rainfall amounts and
associated rainfall rates, there may be some flooding concerns that materialize, and this will include at least a localized threat for
some burn scar flash flooding/debris flow impacts. As a result, a
Slight Risk for excessive rainfall has been introduced for portions
of the southern Sierra Nevada, with some localized expansion of
the prior Marginal Risk area to account for rainfall over the
coastal ranges and parts of the San Joaquin Valley.

Orrison


Day 2
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND SOUTHERN SIERRA NEVADA...

Model differences remain in the 25/00Z guidance with respect to
the timing of a shortwave moving inland from the California/Oregon
border with the ECMWF being faster to bring the shortwave trough
across the Sierra Nevada range and into the Great Basin by
27/00Z...although most of the 00Z suite of guidance...including the GFS...trended trended faster and a bit weaker. With the exception
of the Canadian GEM and Regional...the 25/00Z GFS remained heaviest
across the central and southern Sierra Nevada for the 24 hour
period ending Wednesday morning. Still expect some increased
hydrologic sensitivity that could result in at least some localized
minor flooding issues given that this will be the second
consecutive day of steady rainfall for these areas. Even though the
non-25/00Z GFS/Canadian consensus is for 2 to 4 inches across the central/southern Sierra Nevada over the 24 hour period, the amounts
tend to be more in the 1 to 3 inch range below the expected snow
levels (approx 9000 ft per the WPC Winter Weather Desk forecaster)
which may mitigate some of the concern for flooding. Even so...the
concern remains that those values may be too low at least locally
considering the coarser resolution of the available guidance at
longer lead times so an upgrade may still be needed later. Rainfall
rates and areal coverage of rainfall should decrease late in the
period as the flow of moisture becomes disrupted and the better
dynamics pull away from the area.

Bann


Day 3
Valid 12Z Wed Nov 27 2024 - 12Z Thu Nov 28 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann
$$
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FOUS30 KWBC 300630
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
130 AM EST Sat Nov 30 2024

Day 1
Valid 12Z Sat Nov 30 2024 - 12Z Sun Dec 01 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pereira

Day 2
Valid 12Z Sun Dec 01 2024 - 12Z Mon Dec 02 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pereira

Day 3
Valid 12Z Mon Dec 02 2024 - 12Z Tue Dec 03 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pereira

$$
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FOUS30 KWBC 050824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Tue Nov 5 2024

Day 1
Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER TO MID MISSISSIPPI AND LOWER OHIO VALLEYS...

A cold front tracking east across the middle of the country is
expected to stall out generally along the Mississippi and Ohio
Rivers late tonight. Southerly flow off the Gulf will continue
pumping plentiful moisture into the front. Meanwhile, in the upper
levels, a large longwave trough over the west will establish itself
as a positively tilted trough by tonight as it digs southwestward
over the Southwest. Before it does so, the last potent shortwave of
the "old" trough will race northeastward into the Great Lakes as
the leading edge of the trough becomes a roughly straight line
southwesterly from the Southwest right through into eastern
Canada.

The potent shortwave and associated surface low will both race
northeastward through the period. While they're over the upper
Mississippi Valley, they will move the front currently stalled from
Texas through Michigan eastward a bit to align with the Slight and
Marginal Risk areas along the Mississippi and Ohio Rivers. This
will give the hard hit northwestern Arkansas region a break as the
heaviest rain shifts east. However, by tonight, both features will
be well into the Great Lakes and Canada, so the forcing pushing
them east will quickly diminish, resulting in another stall-out of
the front. With continuous Gulf moisture, another round of training thunderstorms is expected up the Mississippi and lower Ohio Rivers
tonight. Given both the prior history of this front stalling as
well as urban concerns in the Memphis area especially, a Slight
Risk area was introduced with this update.

Despite the stalled out front keeping the rainfall largely confined
to a narrow corridor, the upper level forcing quickly weakening
tonight should hold rates and coverage in check much better tonight
as compared to the ongoing storms over Texas, Arkansas and Missouri
right now. Thus, forecast rainfall amounts are considerably lower
today and tonight than yesterday. The Slight is considered a
"lower-end Slight" with any flash flooding impacts expected to be
largely confined to low-lying flood prone areas and urban areas.
Like areas further west, this area has also been very dry prior to
this rainfall, so it will still take a prolonged period of heavy
rainfall to result in flash flooding.

Wegman


Day 2
Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
GEORGIA AND SOUTH CAROLINA...

A large plume of deep tropical moisture will overspread all of
Florida and move into Georgia and South Carolina on Wednesday and
Wednesday night. This same plume is home to T.S. Rafael down in the
Caribbean. PWATs will increase to as high as 2.25 inches, meaning
the storms expected to form will have plenty of moisture to convert
to heavy rainfall. The front over the Ohio Valley is unlikely to
play too much of a role in the storms over Georgia and South
Carolina. Rather, the guidance suggests a secondary front will from
at the leading edge of the moisture plume that will likely be
enhanced by the right entrance region of the upper level jet
streak, as well as possibly some upslope component (especially in
South Carolina). Regardless of the primary lifting mechanism, there
is good agreement in the guidance for an area of 3 to 5 inches of
rain (though likely much higher amounts will be embedded) in the
Slight Risk area, which is a small increase in amounts from the
previous forecast.

As with areas further west the past several days, this area has
also been incredibly dry lately, with many areas not having seen
appreciable rain in a month. For the swampy and sandy areas, this
will mean it will take a lot of rain before runoff converts to
flooding. Further north across interior SC and GA, the clay soils
are likely quite hydrophobic due to the recent dry weather, which
will notably increase runoff once the heavy rain gets started.
Thus, locally increased impacts are possible. An internal higher-
end Slight area was introduced for the peak rainfall area of east-
central Georgia and west central South Carolina for this potential.

Since this plume is home to T.S. Rafael, this would be considered a
predecessor rain event (PRE) if Rafael were to head towards the Big
Bend and GA/SC. However, since the latest forecasts keep Rafael
tracking westward in the Gulf this rainfall event is not considered
a PRE.

Wegman


Day 3
Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AS WELL AS FOR PORTIONS OF THE SOUTHERN PLAINS...

...Southeast...

The cold front over the Appalachians that will drive the Day
2/Wednesday rainfall event will move southward to coastal South
Carolina and southern Georgia Thursday and Thursday night. The
southerly flow of deep tropical moisture ahead of the front will
abate somewhat in favor of a more neutral easterly flow. This will
keep periods of heavy rain going...but both less widespread, less
persistent, and less heavily. The core of heaviest rain will shift
to the coast of SC/GA and north FL, but since most of this area is
likely to not have seen substantive amounts of rain on Wednesday,
and the coastal swamps can usually handle a lot of rainfall with
few problems, have opted to keep the area in a Marginal Risk for
this update. However, recent model trends have been for the
heaviest rain to increase with time as the CAMs move into this
period of time. Should that happen a Slight Risk will likely be
necessary.

...Southern Plains...

A potent upper level low will begin to eject out of the Southwest
and into the southern Plains on Thursday. A strong cold front will
develop as a result with plentiful snow into much of northeastern
New Mexico. This area was trimmed from the Marginal Risk with this
update. For the rest of the Marginal Risk area, some Gulf moisture
will be captured and advected northward across north Texas and
Oklahoma, but amounts will be somewhat limited. With only broad
forcing from an upper level jet streak but few coherent features
away from the strong cold front, the rainfall footprint will be
broad and disorganized. This area has seen plentiful rainfall over
the past few days, so the additional rainfall, while not much, may
still cause isolated instances of flash flooding.

Wegman

$$
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FOUS30 KWBC 060823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Wed Nov 6 2024

Day 1
Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST...

In coordination with CAE/Columbia, SC and FFC/Peachtree City, GA
forecast offices, a Moderate Risk upgrade was introduced with this
update.

A plume of deep tropical moisture, indirectly associated with
Hurricane Rafael will continue overspreading portions of the
Southeast today. The moisture plume will run into a developing
front over Georgia and South Carolina. An upper level jet streak
will add lift to the atmosphere. Late today into this evening, the
nocturnal low level jet's strengthening will kick the moisture
advection into high gear. Frontogenesis occurring over Georgia and
South Carolina will act to focus the heavy rain over the narrow
Moderate Risk corridor. Expect training convection to develop
during the evening which will persist through the overnight and
while slowly weakening in the pre-dawn hours, is likely to continue
into western South Carolina into Day 2/Thursday.

HREF probabilities show a 20 to 25 percent chance of a 100 year
rainfall event in the Moderate Risk area. FFGs are very high in the
area due to recent dry conditions with only very light rain. This
has likely made some of the clay soils quite hydrophobic. The
hydrophobic soils will likely increase the runoff from the heavy
rain, which will likely quickly exceed FFGs as the heavy rain
likely persists for multiple hours.

Wegman


Day 2
Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHEAST AS WELL AS FOR MUCH OF THE TEXAS PANHANDLE AND WESTERN OKLAHOMA...

...Southeast...

Heavy rain will be ongoing across Georgia and especially South
Carolina at the start of the period Thursday morning. While the
rainfall will be diminishing in intensity, it's expected after the
much heavier rains of Wednesday night, that the lighter totals
forecast for Thursday will still be influential since antecedent
conditions will be radically different by then as compared to where
they are now. The Slight Risk threat largely is for the Thursday
morning period with lingering rainfall from Wednesday night. Expect
the rain to continue diminishing through the day as the moisture
plume shifts westward with Hurricane Rafael.

...Texas Panhandle and Western Oklahoma...

A positively tilted longwave trough and associated upper level
energy in the form of jet streaks and shortwave energy will support
the development of a strong low over the southern Plains. To the
west, a significant amount of the low's moisture will fall in the
form of snow. Where the precipitation will remain as rain, expect
steady heavy rain across the Texas Panhandle and western Oklahoma.
While rates are not generally expected to be notably heavy, the
flood threat will be primarily focused on the long duration of
steady rain, as well as the fact that much of this area was hit
very hard with heavy rain a few days ago, so soils and rivers
remain elevated in this region. Thus, low FFGs support the Slight
Risk upgrade.

Wegman


Day 3
Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL AND SOUTHERN PLAINS...

The upper level low over the Southwest will eject into the Plains
on Friday. There will be significant upper level energy associated
with the low, which combined with strong frontogenesis, plentiful
Gulf moisture advecting up the Plains on the low level jet, and a
strengthening surface low...expect heavier rainfall amounts across
Oklahoma and Kansas on Friday as compared with Wednesday. While
north Kansas didn't see as much rain a few days back compared with
areas further south...much of the rest of the Slight risk area does
overlap with both the heavy rain from a few days ago but continued
rainfall from Day 2/Thursday. Thus, added an internal higher-end
Slight for western Oklahoma into south Kansas. Depending on how
much rain the area gets on Day 2, there is some potential for a
Moderate Risk upgrade in this area with future updates.

Wegman


$$
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FOUS30 KWBC 070820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Thu Nov 7 2024

Day 1
Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHEAST AS WELL AS PORTIONS OF WEST TEXAS AND WESTERN OKLAHOMA...

...Southeast...

Heavy rain continues across Georgia and South Carolina this
morning. A nearly stationary front across the region is acting as a
focus for that rainfall, which due to an influx of tropical
moisture moving into the front from the Atlantic remains stuck. The
moisture running into the front is allowing for convective
development along the front in Florida and Georgia, which then
advect northeastward into South Carolina. While convective coverage
has been slowly diminishing, along with the storms with the
heaviest rainfall, it's likely that the rain will continue into the
day today. The inherited Slight and Marginal risks were largely
left unchanged with this update, as the rainfall is generally
behaving as expected. Since there will be a very slow drift of the
heaviest rainfall towards the South Carolina coast, which due to
the swampy terrain is generally much better equipped to handle
heavy rainfall rates, a Slight Risk is sufficient to cover the
flash flooding threat, rather than a continuation of the Moderate.

...West Texas and Western Oklahoma...

A strong upper level low over the Southwest will interact with an
LLJ of Gulf moisture and a developing surface low forming ahead of
the upper level low. A large area of rain will develop from the
Texas and Oklahoma Panhandles south and east over portions of the
Cap Rock of Texas and much of Oklahoma west of Oklahoma City.
Instability will be a significant limiting factor on short-term
rainfall rates and the strength of any convection, but the fact
that all of those synoptic systems just described will be very slow
moving should increase the duration of steady rainfall. Since much
of this region was hit hard with heavy rain a few days ago, FFGs
remain low enough that they should be overcome in widely scattered
instances in the Slight Risk area, and in isolated instances in the
Marginal Risk region.

Wegman


Day 2
Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS...

The upper level low over the Southwest and its attendant surface
low will both continue moving northeast across the Plains on
Friday. With continued influx of Gulf moisture, the typical comma
shaped precipitation footprint will develop from Texas to Oklahoma.
Out west in Colorado and New Mexico the precipitation will be
mostly in the form of snow. Where it remains all rain, the heaviest
rain will be from north Texas across Oklahoma and into south
central Kansas. As today, instability will remain the primary
limiting factor for heavy rain in most areas, as MUCAPE values
generally stay under 1,000 J/kg. For the Slight Risk area, there
has been a general eastward/faster shift in the guidance, but that
will at least be somewhat offset by the rainfall expected today
over the Panhandles and western Oklahoma.

The rationale for the Slight is two-fold. While somewhat lesser
amounts of rain are expected overall for southern Oklahoma and
Texas...greater instability there will favor at least some storms
capable of heavy rainfall in that area. Meanwhile, north of
Oklahoma City, expect a much longer duration rainfall as the
surface low takes its time getting its act together. So while
instantaneous rates will likely be lower than further south, it
will be made up for by the longer duration of the rain.

Wegman


Day 3
Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

The upper level low that was driving the heavy rainfall the
previous two days will move faster towards the northeast as it
continues its return to the jet stream over the northern tier of
states. The area will still have plenty of moisture from the Gulf
to work with as Rafael (or what's left of it) remains over the
central Gulf but the plume of moisture its embedded in continues to
be drawn north ahead of the surface low's strong cold front. The
front will likely be the dominant forcing for storms in this
region. As on Friday, the southern areas of Louisiana and
Mississippi will have the greater moisture and instability, while
further north FFGs are much lower due to recent heavy rainfall.
Thus, once again the Slight Risk is a two-fold combination of
factors that individually increase the flooding risk. The front
should still be slow enough moving that training storms tracking
north along the front could still cause flooding problems,
especially in Louisiana.

Wegman

$$
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FOUS30 KWBC 080758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Fri Nov 8 2024

Day 1
Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS...

The upper level low over the Southwest and its attendant surface
low will continue moving northeast across the Plains on Friday.
With a continued influx of Gulf moisture with precipitable water
values of 1.5-2.25" expected near the cyclone's cold front, the
typical comma shaped precipitation footprint will develop farther
up the warm conveyor belt from Texas to Oklahoma which should occur
with precipitable water values at or above an inch. Out west in
Colorado and New Mexico the precipitation will be mostly in the
form of snow. Where it remains all rain, the heaviest rain will be
from north Texas across Oklahoma and into south-central Kansas.
Instability should be sufficient for heavy rain-related issues.

While somewhat lesser amounts of rain are expected overall for
southern Oklahoma and Texas...greater instability there as high as
1500 J/kg of MU CAPE will favor at least some storms capable of
heavy rainfall in that area, with hourly rain totals to 2.5" and
local amounts as high as 6" where cells train. Meanwhile, north of
Oklahoma City, expect a much longer duration rainfall as the
surface low takes its time getting its act together. So while
hourly rain totals will likely be lower than further south, perhaps
1" where slantwise convection/frontogenesis aloft forces greater
precipitation efficiency, it will be made up for by the longer
duration of the rain.

Roth/Wegman


Day 2
Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

An upper level low will move faster towards the northeast. As
Rafael moves west over the central Gulf, a plume of moisture with
precipitable water values of 1.5-2" gets drawn north ahead of the
surface low's strong cold front. The front will likely be the
dominant forcing for storms in this region. As on Friday, the
southern areas of Louisiana and Mississippi will have the greater
moisture. MU CAPE rises towards 1000 J/kg across parts of LA/MS/TN.
Areas further north have soils which are more saturated/flash
flood guidance is lower. The front should progress slow enough that
training storms tracking north along the front could still cause
flooding problems, especially over portions of LA, MS, and TN.
Hourly rain totals to 2.5" and local amounts of around 6" appear
likely. Lingering disagreement on exactly where the maxima are
expected kept the risk level from being raised to Moderate, though
an upgrade remains possible in future updates should this placement
issue resolve itself and model QPF magnitudes don't back off.

Roth


Day 3
Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST, LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...
AND UPPER OHIO VALLEY...

Central Gulf Coast northeast towards southwest Pennsylvania...
Moisture and instability from the Gulf gets advected from the
vicinity of Raphael northeast across portions of the Central Gulf
Coast through the Tennessee and Upper Ohio Valleys. The forward
progress of the front across much of the East should keep any
excessive rainfall concerns minimal -- some of the risk area from
eastern AR to the east-northeast just south of the OH river
accounts for recent rains/some soil saturation. The latest NCEP
and CMC guidance suggests that Raphael gets much closer to the Gulf
Coast than the NHC forecast, which led to avoiding use of the GFS,
NAM, and Canadian guidance explicitly. Since the distance of the
storm from the region at this time frame is unclear, it's also
unclear how high precipitable water values get, though they should
rise above 1.5", particularly in the southern portion of the area
near the coast, which should be enough for heavy rain concerns. It
appears that enough instability could be available along the
immediate Gulf Coast for convection with heavy rainfall. Hourly
rain totals to 2" with local totals to 5" appear achievable along
or near the immediate Gulf coast. Whenever better agreement
amongst the guidance on the future of Rafael as well as the
placement any rainfall maximum near the coast occurs, a Slight
Risk could prove useful at that later time. To the north, hourly
totals up to ~1" with local totals to 2" appear to be the maximum
potential.

Pacific Northwest...
Precipitable water values rise to 0.75-1" with a front moving into
the area. Inflow from the Pacific rises to 50-60 kts from the
south-southwest near the front. In the wake of the front,
instability moves in due to colder air aloft. Hourly rain totals
exceeding 0.5" appear likely here, which would be most problematic
in burn scars. Left the Marginal Risk area intact though simplified
the area's appearance.

Roth

$$
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FOUS30 KWBC 090748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 9 2024

Day 1
Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN AND NEAR IMPERIAL
CALCASIEU IN LOUISIANA...

An upper level low will move faster towards the northeast. As
Rafael moves west over the central Gulf, a plume of moisture with
precipitable water values of 1.5-2" gets drawn north ahead of the
surface low's strong cold front. The front will likely be the
dominant forcing for storms in this region as it stalls, which have
already begun to evolve in recent radar imagery. The southern
areas of Louisiana and Mississippi will have the greater moisture.
MU CAPE rises to 1000 J/kg or so across parts of LA/MS/TN. Areas
further north have soils which are more saturated/flash flood
guidance is lower. The front should progress slow enough that
training storms tracking north along the front could still cause
flooding problems, especially over portions of LA, MS, and TN.
Hourly rain totals to 2.5" and local amounts of around 8" due to
cell training appear likely based on the 00z HREF probabilities of
8" totals. There is much better agreement on the placement when
compared to this time yesterday. Portions of Central LA have
received 300%+ of their average seven day rainfall, so soils should
have some sensitivity. Coordination with the LCH/Lake Charles LA
forecast office led to the increase to a High Risk.

Roth


Day 2
Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST, LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...
AND UPPER OHIO VALLEY...

Central Gulf Coast northeast towards southwest Pennsylvania...
Moisture and instability from the Gulf gets advected from the
vicinity of Raphael northeast across portions of the Central Gulf
Coast through the Tennessee and Upper Ohio Valleys. The forward
progress of the front across much of the East should keep any
excessive rainfall concerns minimal -- some of the risk area from
eastern AR to the east-northeast just south of the OH river
accounts for recent rains/some soil saturation. The guidance
remains unclear as to whether heavy rainfall gets ashore or not.
Precipitable water values should rise above 1.5", particularly in
the southern portion of the area near the coast, which should be
enough for heavy rain concerns. It appears that enough instability
could be available along the immediate Gulf Coast for convection
with heavy rainfall. Hourly rain totals to 2" with local totals to
5" appear achievable along or near the immediate Gulf coast.
Whenever better agreement amongst the guidance on the future of
Rafael as well as the placement any rainfall maximum near the coast
occurs, a Slight Risk could prove useful at that later time. To
the north, hourly totals up to ~1" with local totals to 2" appear
to be the maximum potential.

Pacific Northwest...
Precipitable water values rise to 0.75-1" with a front moving into
the area. Inflow from the Pacific rises to 50-60 kts from the
south-southwest near the front. In the wake of the front,
instability moves in due to colder air aloft. Hourly rain totals
exceeding 0.5" appear likely here, which would be most problematic
in burn scars. Left the Marginal Risk area intact though simplified
the area's appearance.

Roth


Day 3
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST & CENTRAL GULF COAST...

Central Gulf Coast...
Precipitable water values north of Rafael are expected to remain in
the 1.5-2" range in the vicinity of a slowly progressive front near
the Central Gulf Coast. While much of the guidance keeps rainfall
amounts minimal, the 00z NAM showed local amounts of 3"+, though
it gets there by allowing Rafael to get much closer to the Gulf
Coast than other model guidance. Considering that heavy rainfall
is possible over the previous couple of days, figured a new
Marginal Risk was a sensible precaution in case the NAM is close to
correct and/or soils are saturated by that time.

Pacific Northwest...
Precipitable water values of 0.5-0.75" and MU CAPE of a few hundred
J/kg are expected in a post-frontal environment. Inflow off the
Pacific is expected to remain around 40 kts. Hourly rain totals
exceeding 0.5" remain possible, which would be most problematic in
burn scars. Left the inherited Marginal Risk area intact though
simplified the area's appearance.

Roth

$$
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FOUS30 KWBC 100924
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EST Sun Nov 10 2024

Day 1
Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL AND
SOUTHWEST LOUISIANA...

Southwest and Central Louisiana...
Moisture and instability from the Gulf gets advected from the
vicinity of Raphael northwest to north across portions of the
Central Gulf Coast through the Tennessee Valley. A slowly
advancing convective complex early this morning across southwest LA
is drifting eastward, showing hourly rain totals to 3" and
isolated totals to 13" thus far over portions of Allen Parish.
There is concern that this complex could hold together past 12z,
which is part of the reason for the upgrade to a Slight Risk, after coordinating with the LCH/Lake Charles LA forecast office.
Additional activity could form behind it closer to a surface
boundary in TX (could be forming at the present time) which is
expected to advect eastward during daytime heating. Moderate Risk
level impacts cannot be ruled out should such thunderstorm activity
persist over recently saturated soils into this afternoon.

Southeast Louisiana...
The guidance broadly remains unclear as to whether heavy rainfall
gets too far ashore southeast LA while mesoscale guidance shows the
complex to the west fizzling as it moves into the region.
Precipitable water values of 1.5-2" Sunday morning across southeast
LA advect offshore with time, which should limit heavy rainfall
concerns. Still, hourly rain totals to 2" and local totals to 4"
are possible on an isolated basis should storms backbuild or train.

Interior Southeast/Mid-South/Tennessee Valley...
The forward progress of the front should keep any excessive
rainfall concerns minimal in areas a bit north of the Gulf Coast
-- some of the risk area from eastern AR to the east-northeast just
south of the OH river accounts for recent rains/some soil
saturation. In this more northern portion of the Marginal Risk
area, hourly totals up to ~1" with local totals to 2" appear to be
the maximum potential. A decent amount of the area within the
Marginal Risk would find the forecasted rainfall welcome.

Changes to continuity were to shave off some of the northeast
portion of the Marginal Risk area, using 00z HREF probabilities of
0.5"+ as a guide, and include some of far southeast Texas to
account for the limited QPF signal there as soils are a bit more
sensitive after the past 24-36 hours of heavy rainfall.

Roth


Day 2
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Roth


Day 3
Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR
WESTERN WASHINGTON...

Precipitable water values of 0.75-1" advect in as a cold front
approaches, perhaps moves ashore, the Washington and northern
Oregon coast Wednesday morning. Inflow from the Pacific rises to
60 kts at 850 hPa out of the south-southwest with limited
instability available near and behind the front. The ingredients
appear to be there for hourly rain totals of 0.5"+, which would be
most problematic in burn scars. Over the full day, local amounts
of 3-4" are advertised for much of the guidance, which would be
most problematic along the Skokomish river in the Olympic
Peninsula. Enough was there in the guidance for the inherited
Marginal Risk area to remain from continuity; simplified its shape.

Roth

$$
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FOUS30 KWBC 110751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Mon Nov 11 2024

Day 1
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

A non-zero chance for flash flooding exists within the far
Southeast Parishes of LA and along the immediate AL/MS coastal
plain as lingering moisture on the northern periphery of the
remnant low from Rafael will persist within the area mentioned
above. Frontal progression to the northwest will slowly sink
southeast with some CAMs initiating a period of stronger boundary
layer convergence that would capable of north-south alignment of
heavier rainfall situated over the Southeast-most Parishes in LA.
The factor that will likely limit the threat will be the lower end
rates between 1-2"/hr max that fall well-below the current FFG
markers located over the proposed impact area. Totals of 2-4" with
locally as high as 6" are depicted within a few of the CAMs, but
it's not a full agreement in the suite neighborhood probabilities
dropping off precipitously for accumulations >3". Decided to retain
the current nil ERO nationally, but wanted to point out the low-
end probability (<5%) for isolated flood concerns in the zone
encompassing the above Parishes to the immediate coastline across
both AL/MS.

Kleebauer


Day 2
Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND NORTHWEST OREGON...

The next Atmospheric River will impact the PAC Northwest with an
IVT pulse between 500-800 kg/ms signaling a modest moisture
advection regime that historically leads to marginal flash
flooding concerns along the immediate WA/OR coasts, the terrain of
the Olympics and along the foothills of the Cascades where the
orographic enhancement pattern is defined. PWAT anomalies between
1-1.5 deviations above normal will enter the region by the second
half of the period with the heaviest precip likely coinciding with
the initial moisture surge into the coastal plain. Some modest
instability will be focused along the immediate coast as defined by
the some of the hi- res guidance and mean SBCAPE output within the
ensemble blend. Rates will be generally ~0.5"/hr within the
heavier bands, but some 0.75-1.25"/hr rates are plausible within
the Olympics in Northwest WA state, as well the immediate coast of
WA down into Northwest OR. This is the primary axis of the IVT
pulse with a distinct west to southwest orientation of the mean
flow. This initial surge will be more of a primer for the following
day as the pattern spills into Wednesday, but some isolated flash
flood concerns are plausible within the initial surge of the AR.
The previously inherited MRGL risk was maintained with only a minor
adjustment a touch further south in Oregon to reflect the latest
trends in the mean QPF depicting >2" along the OR coast. Totals
will be generally between 1-3" with isolated to 4" into the Olympic
Peninsula, much of the precip falling within the 00-12z window on
Wednesday.

Kleebauer


Day 3
Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND OREGON, NORTHWEST CALIFORNIA, AND OVER THE
SOUTHEASTERN U.S...

...Pacific Northwest...

Atmospheric River from the D2 period continues through much of the
D3 time frame with a waning signal towards Wednesday night into
Thursday. IVT core will come ashore between Astoria down through
the King Range of Southwestern OR with a signal for an additional
2-4" of rainfall with locally as high as 5.5" within the orographic
enhancement in-of the Klamath Mountains. Rates will maintain a
steady- state of 0.25-0.5" over a span of several hours, but the
primary core of the IVT pulse will likely ramp up rates >0.5"/hr
with some 1-1.5"/hr rates plausible across those elevated coastal
areas of OR and Northwestern CA. Rainfall will continue up into WA
state with the heaviest rain confined to the Olympics with an
additional 2-4" likely occurring northwest of Olympia. Two day
totals over the coastal areas of WA/OR will settle between 3-6"
with a max of 8" confined to both the Klamath-Siskiyou and Olympic
Ranges thanks to some orographic enhancement providing localized
QPF maxima that is customary from AR orientations extending from
southwesterly flow.

Additional flash flood opportunities will exist over portions of
Northwest CA to areas south of Mount Shasta where remnant burn
scars will act as an area of heightened concern with local FFG
indices much lower than surrounding areas. Totals will be between
2-4" in this area with the max of 5" occurring over the Northwest
tip of CA north of Eureka where heavier rainfall longevity will
enhance totals locally. A MRGL risk was maintained for the threat
with much of Western WA/OR encompassed, as well as Northwest CA
with an extension inland to south of Mount Shasta to cover for the
burn scar areas that could see >1.5" of rainfall on Wednesday.

...Southeast U.S...

A sprawling surface ridge east of the Mississippi will quickly
migrate eastward allowing for a strong return flow pattern across
the Southeastern U.S along the the western flank of the ridge.
Lingering moisture stemming from the remnants of Rafael will
fester in the Northern Gulf before being ushered poleward around
that clockwise circulation from the high pressure controlling the
pattern across the Mississippi Valley and points east. PWATs will
elevate up close to +2 deviations lending credence to a suitable
airmass for maintaining convective activity. Further west, a
shortwave trough will migrate into the Central Plains with surface
low pressure forming downstream across the Southern Plains/Mid-
Mississippi Valley creating a strengthening convergence pattern
along a cold front that develop on the tail end of the low.
Progressive flow within the setup will alleviate some of the
concerns of lingering precip that will develop from the Lower
Mississippi Valley to points north and east.

The jury is still out on the expected intensity of the precip that
develops, but considering the convective prospects given the
theta-E advection pattern anticipated along with a source of
surface based convergence along and ahead of the forecasted cold
front, some stronger cores capable of rates >1-2"/hr and totals
breaching 3" will be possible, as noted by some of the current
deterministic. Ensemble bias corrected QPF output indicates some
2-3" amounts across parts of the Deep South signaling the threat.
UFVS First Guess Field has a MRGL risk encompassing parts of LA and
MS where the heavy rainfall from recent days has really dropped the
FFG indices across parts of the area. The antecedent conditions
over Southwest and Central LA will be privy to a heightened flash
flood threat leading a MRGL risk extending back into those areas.
The MRGL encompasses much of the Lower Mississippi Valley over into
AL extending up into TN, coinciding with the forecasted meridional
extent of the theta-E progression which would lead to convective
precipitation modes. Will assess as we get closer, but this is the
proxy for where the expected heavier rainfall threats could occur.

Kleebauer

$$
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FOUS30 KWBC 120751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Tue Nov 12 2024

Day 1
Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND NORTHWEST OREGON...

The previous forecast remains steadfast with the expected totals
and evolution of our next disturbance progged to enter the PAC
Northwest. The next Atmospheric River is currently maneuvering
through the Northeast Pacific with a well-defined IVT pulse located
around the base of a strong, closed upper low and associated
surface cyclone churning over the North Pacific as indicated via
the latest CIMSS MIMIC-TPW satellite derived product. PWAT
anomalies between 1-1.5 deviations above normal will enter the
region by the second half of the period with the heaviest precip
likely coinciding with the initial moisture surge into the coastal
plain, forecast between 21-03z Wednesday (Late-afternoon and early
evening PST). Some modest instability will be focused along the
immediate coast as defined by the hi- res guidance and mean SBCAPE
output within the ensemble blend (HREF/NBM). Rates will be
generally ~0.5"/hr within the heavier bands, but some 0.75-1.25"/hr
rates are plausible within the Olympics in Northwest WA state, as
well the immediate coast of WA down into Northwest OR. This is the
primary axis of the IVT pulse with a distinct west to southwest
orientation of the mean flow. This initial surge will be more of a
primer for the following day as the pattern spills beyond Wednesday
morning, but some isolated flash flood concerns are plausible
within the initial surge of the AR. The previously inherited MRGL
risk was maintained coinciding with the higher (>50%) 12-hr
probabilities of exceeding 2". Totals will be generally between
1-3" with isolated to 5" into the Olympic Peninsula.

Kleebauer


Day 2
Valid 12Z Wed Nov 13 2024 - 12Z Thu Nov 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
WASHINGTON AND OREGON, NORTHWEST CALIFORNIA, AS WELL AS PORTIONS OF THE SOUTHEAST AND OHIO VALLEY ...

...Pacific Northwest...

Minimal changes were necessary from the previous forecast issuance
as guidance maintains relevant continuity. Atmospheric River from
the D1 period continues through much of the D2 time frame with a
waning signal towards Wednesday night into Thursday. IVT core will
come ashore between Astoria down through the King Range of
Northwest CA with a signal for an additional 2-4" of rainfall with
locally as high as 5.5" within the orographic enhancement in- of
the Klamath Mountains. Rates will maintain a steady- state of
0.25-0.5" over a span of several hours, but the primary core of the
IVT pulse will likely ramp up rates >0.5"/hr with some 1-1.5"/hr
rates plausible across those elevated coastal areas of OR and
Northwestern CA. Rainfall will continue up into WA state with the
heaviest rain confined to the Olympics with an additional 3-5"
likely occurring northwest of Olympia. Multi-day totals over the
coastal areas of WA/OR will settle between 3-6" with a max of 8"
confined to both the Klamath- Siskiyou and Olympic Ranges thanks to
some orographic enhancement providing localized QPF maxima that is
customary from AR orientations extending from southwesterly flow.

Additional flash flood opportunities will exist over portions of
Northwest CA to areas south of Mount Shasta where remnant burn
scars will act as an area of heightened concern with local FFG
indices much lower than surrounding areas. Totals will be between
2-4" in this area with the max of 5" occurring over the Northwest
tip of CA north of Eureka where heavier rainfall longevity will
enhance totals locally. A MRGL risk was maintained for the threat
with much of Western WA/OR encompassed, as well as Northwest CA
with an extension inland to south of Mount Shasta to cover for the
burn scar areas that could see >1.5" of rainfall on Wednesday.

...Southeast U.S and Ohio Valley...

A sprawling surface ridge east of the Mississippi will quickly
migrate eastward allowing for a strong return flow pattern across
the Southeastern U.S along the the western flank of the ridge.
Lingering moisture stemming from the remnants of Rafael will
fester in the Northern Gulf before being ushered poleward around
that clockwise circulation from the high pressure controlling the
pattern across the Mississippi Valley and points east. PWATs will
elevate up close to +2 deviations lending credence to a suitable
airmass for maintaining convective activity. Further west, a
shortwave trough will migrate into the Central Plains with surface
low pressure forming downstream across the Southern Plains/Mid-
Mississippi Valley creating a strengthening convergence pattern
along a cold front that develop on the tail end of the low.
Progressive flow within the setup will alleviate some of the
concerns of lingering precip that will develop from the Lower
Mississippi Valley to points north and east.

There has been a growing consensus on the expansion of convective
activity across the Deep South into the Ohio Valley with a
formidable heavy rain signature located across the Gulf coast where
the best instability footprint is located, and across the Southern
Ohio Valley where ascent will be maximized within the diffluent
area of the upstream shortwave trough in tandem with the
approaching cold front. QPF off the recent 00z HREF blended mean
in the final 12-hrs signifies some potential for significant totals

2" with a few CAMs depicting as much as 6" across portions of LA

and MS with more of a 2-4" plateau across the Ohio Valley. Ensemble
means are generally stable at 1-2" across a large area, but might
be smoothing out the higher totals as is the bias of the ensemble
blend. In any case, the pattern is certainly favorable for at least
isolated prospects for flash flooding, especially in South-Central
LA where recent QPE from 48 hrs prior sat at 5-10" with locally
higher, so the threat in this locale is greatly enhanced just due
to the antecedent soil moisture anomalies within the past few days.
Further north is not nearly as robust in the soil moisture
anomalies, however some areas within TN and Western KY do have
some lingering top-layer soil moisture from the same event that
occurred in LA. The previous MRGL risk was maintained with a minor
adjustment north into Southern IN as reflected within the latest
QPF trends and expected northern edge of the heaviest precip
Wednesday night into Thursday. Discrepancy in the exact placement
of heavier convection across the Gulf states deterred an additional
upgrade over the region, however the threat for at least one risk
category higher is in play as we move closer to the event. For now,
a blanket MRGL risk was sufficient with the potential for an
upgrade plausible in the next few forecast updates.

Kleebauer


Day 3
Valid 12Z Thu Nov 14 2024 - 12Z Fri Nov 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

$$
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FOUS30 KWBC 020710
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Mon Dec 2 2024

Day 1
Valid 12Z Mon Dec 02 2024 - 12Z Tue Dec 03 2024

Onshore flow across far south Texas will maintain a moist/unstable
airmass that could conditionally support heavy rainfall near the
Brownsville vicinity and adjacent coastal areas especially from the
afternoon onward. This risk is too isolated/conditional for
inclusion of a Marginal risk area at this time but will be
monitored through the day. The probability of rainfall exceeding
flash flood guidance is less than 5 percent.

Cook


Day 2
Valid 12Z Tue Dec 03 2024 - 12Z Wed Dec 04 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

The Marginal Risk area for portions of eastern Texas and much of
western Louisiana that was inherited was removed with this update.
A fairly robust 30-40 kt LLJ will advect Gulf moisture into much of
the Mississippi Delta region and west into eastern Texas on
Wednesday. PWATs may exceed 1.75 inches, especially along the Gulf
coast. However, this is the only ingredient that is sufficient for
flooding rains. Instability, while non-zero...will be meager,
struggling in most areas to get to 500 J/kg of MUCAPE. This will be
the primary limiting factor against flooding, as a lack of
instability should keep most of the rain as stratiform. Secondly,
much of the eastern half of the U.S., including eastern Texas and
Louisiana have been very dry lately, so stream and creekbeds are
running low. The drawn out period of mostly light rain in this
area will therefore mostly be beneficial. While embedded convective
cells could briefly cause locally heavy rain, the dry conditions
and high FFGs should mitigate any flooding. Finally, at the tail
end of the event late Wednesday night into Thursday, cold frontal
passage will push any heavy rain south and out of where the
heaviest rains are expected over central Louisiana. This too should
diminish the flooding potential.

As always, increases in the guidance for rainfall amounts in the
next couple days or greater instability could require a reissuance
of the Marginal, but for now the rainfall in the guidance likely
falls in the sub-Marginal 0-5% chance category area-wide.

Wegman

$$
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FOUS30 KWBC 030729
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
229 AM EST Tue Dec 3 2024

Day 1
Valid 12Z Tue Dec 03 2024 - 12Z Wed Dec 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH TEXAS...

A moist airmass (characterized by 1.4+ inch PW values) will migrate
slowly northwestward through the day along a surface trough located
near the south Texas coast. Convergence along that trough axis
will support areas of deep convection through the day, with
kinematic profiles supporting slow storm motions even as mid-level
flow weakens during the forecast period. Isolated spots of 2-4 inch
rainfall amounts appear possible per latest CAMS in the Marginal
Risk area. Isolated flash flood potential will exist in this
regime.

Cook


Day 2
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 040721
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Wed Dec 4 2024

Day 1
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. Strengthening warm/moist advection across east Texas
and Louisiana will result in widespread shower and thunderstorm
activity most of the day through early Thursday. Several areas of
2-4 inch rainfall totals are expected especially from southeast
Texas into central Louisiana. The rain will fall in areas that have
not experienced appreciable rainfall over at least the past week,
and models are inconsistent with depiction of any focused corridors
of heavier and/or training convection that could result in larger-
scale flash flood concerns. FFG thresholds are also relatively
high. <5% probabilities are maintained for this outlook with the
expectation that any flash flood concerns should be isolated at
best.

Cook


Day 2
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 050738
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Thu Dec 5 2024

Day 1
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto


Day 2
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
EASTERN TEXAS...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move northeastward out of New Mexico and into the central Plains. PWATs
along the upper Texas coast are expected to increase to above 1.75
inches by Saturday night. While shower and light rain activity is
expected especially over western portions of the Marginal Risk area
during the day Saturday, the heaviest rain will be on Saturday
night due to the typical nocturnal strengthening of the LLJ.
Instability will be the primary limiting factor as MUCAPE values
struggle to even get above a couple hundred J/kg. Portions of the
Marginal Risk area will get some rain Friday and Friday night, such
that this much stronger second round may train over some areas
that due to previous rain may have more saturated soils. Isolated
flash flooding is possible, especially in urban and flood prone and
low lying areas.

Wegman

$$
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FOUS30 KWBC 061533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024

Day 1
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Far Deep South Texas...
The slow sagging frontal zone is nearly flat west to east across
the Northern Gulf of Mexico and a weak surface to boundary layer
cyclone has developed along the upstream edge near the southern tip
of Texas. Enhanced deep layer moisture resides along and north of
the boundary across this section of the front with total PWat
values ranging from 1.5-1.75". Warm sea surface and proximity to
the front allow for some weak instability in proximity to that low
with values of 750-1000 J/kg available along/offshore. So there
will remain some solid potential for thunderstorm activity across
far south Texas. However, the vast majority of guidance continues
to suggest best overall convergence and strongest updrafts will
remain offshore or at the near coast. It is possible some weakening
showers or rouge thunderstorm may make an appearance onshore in
proximity to the urban corridor of the lower Rio Grande Valley,
making the risk for flash flooding non-zero, but not rising to high
enough probability/coverage to reach 5% threshold for delineating
a Marginal Risk at this time.

Gallina


Day 2
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN TEXAS AND WESTERN LOUISIANA...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move northeastward out of New Mexico and into the central Plains. PWATs
along the Upper Texas coast are expected to increase to 1.5"+ and
instability should remain minimal (and mostly confined to the
coast). The heaviest rain is forecast to be on Saturday night due
to the typical nocturnal strengthening of the low-level jet. Soils
have become a bit more saturated in this region due to the recent
rainfall, and some rainfall is expected on Friday. Isolated flash
flooding is possible, especially in areas of more saturated soils,
urban areas, and flood prone/low lying areas.

Churchill/Roth


Day 3
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST...

Continued moisture flux transport via a moderately strong low-
level jet is expected to result in increasing coverage of flash
flooding on Sunday, as the bulk of the forcing and jet dynamics
via the aforementioned strong shortwave trough will not arrive
until midday (along with maximized instability). Localized training
is expected to result in 2-4" totals, suggesting a lower-end
Slight Risk based on the probability spectrum (which ranges from
15-40%). Given that most of the region is experiencing very dry
antecedent soil conditions (10th percentile or lower for much of
MS/AR/LA, per NASA SPoRT-LIS 0-100 cm soil moisture), any flooding
should be relegated to flood-prone areas (with the greatest risk
for any localized significant flash flooding over west-central LA,
where antecedent soil conditions and streamflows are locally higher).


Churchill
$$

d
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FOUS30 KWBC 070820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Dec 7 2024

Day 1
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

The Slight Risk has been maintained along and near the central
Gulf Coast for Sunday and Sunday night, confined to where more
appreciable deep-layer instability exists to support a higher
probability of more intense short-term rainfall rates (2"+/hr).
Still, this is considered a lower-end Slight Risk (based on the
wide 15-40% probability spectrum), suggesting widely scattered
instances of flash flooding at best (given expected localized
totals of 2-4" are near the corresponding 3-hr FFGs of 3-4").
Also, much of the Slight Risk area, particularly from Jasper TX
east-northeast to Alexandria LA, have received heavy rainfall over
recent days, and as a result the antecedent soil conditions and
streamflows are a bit higher compared to areas farther north (where
NASA SPoRT-LIS 0-100 cm soil moisture in the bottom 10th percentile
for much of MS/AR/LA). In addition to drier antecedent conditions
farther north and east across the Mid-South, very limited elevated
instability (MU CAPE ~200 J/kg at best) should prevent rainfall
rates from climbing too high (generally 1.5"/hr or less, driven by
impressive forcing via the shortwave and associated jet dynamics).
This suggests that a Marginal Risk should suffice with any flash
flooding being highly localized, despite areal average rainfall
amounts near 2" (expected to be spread out over 6+ hours).

Churchill/Hurley


Day 3
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally under 1") to much of the
Northeast. Farther south along the central Gulf Coast, there are
hints of another (weaker) shortwave trough within the southern
stream that may enhance heavy rainfall along a trailing cold front.
Given the slowing of the front, the opportunity for training areas
of heavy rainfall will be greater on Day 3, best depicted by the
00z ECMWF and ECENS (depicting localized banded totals of 2-5" and probabilities for 2" exceedance as high as 30%). This risk appears
greatest from southeast LA to southern AL and the tip of the FL
Panhandle, though the EC model system is the most extreme and
farthest east of all the models. Maintained the inherited Slight
Risk area with some adjustments, limiting the eastern extent a bit
given the lack of agreement from the other models. Uncertainty is
still relatively high, but subsequent outlooks should get a better
handle on things as these details begin to enter the CAM period.

Churchill
$$
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FOUS30 KWBC 080805
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Sun Dec 8 2024

Day 1
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

The inherited Marginal Risk area was maintained for this update,
suggesting that any flash flooding should remain localized today,
as rainfall rates will generally remain capped below 1"/hr (with
lacking instability being the primary limiting factor). The
heaviest rains for the period are expected in the middle of the
Marginal Risk area over northern Mississippi, where an interaction
with a front in that area will lead to greater forcing than areas
farther south along the Gulf Coast. Where the front adds additional
forcing, localized convective bursts may be embedded within the
broader rainfall shield.

Hydrologically, most if not all of the soils in the are are at or
below average for moisture. Low/dry creek beds should easily
handle most of the light to briefly moderate rainfall rates. Any
isolated flooding will be in any urban areas, as well as other low-
lying or more flood-prone areas.

Churchill/Wegman


Day 2
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN LOUISIANA AND COASTAL MISSISSIPPI...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally well under 1") to much
of the Northeast. Farther south along the central Gulf Coast, a
weaker shortwave within the southern stream may locally enhance
heavy rainfall along a trailing cold front. The inherited Slight
Risk in this area was maintained, mainly from the I-10 & I-12
corridors from Baton Rouge east to Biloxi, where the flooding
potential appears greatest in urban and more flood-prone areas
(given relatively dry antecedent conditions and streamflows normal
to below normal). There is still some question as to how far east
the heavy rainfall threat will extend (as the ECMWF and ECENS have
been persistent outliers suggesting an eastward extension into the
FL Panhandle and surrounding southeast AL and southwest GA), but
the signal for localized totals of 2"+ are maximized near the
Slight Risk (and confined to where instability and forcing are
greatest).

Churchill


Day 3
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST AND CENTRAL GULF COAST...

Large scale upper trough will be digging into the central US by
Tuesday, setting the stage for more organized heavy rainfall into
portions of the Southeast. Initially positively tilted, the trough
looks to quickly take on a more neutral tilt over the course of
the course of the day, as an associated 250 mb jet streak
strengthens from ~110 kts to ~170 kts over 24 hours (with our area
of interest for the Slight Risk located near the right-entrance
region). As with prior days, instability once again looks to be the
primary limiting factor, and this is particularly the case farther
north into the Carolinas. While the GFS/GEFS depict 2-4" localized
totals into northern GA and western SC, the ECMWF/ECENS largely
limit this threat (for 2" and 3" exceedance) a bit farther south
into central GA, southeast AL, and the FL Panhandle (where the
inherited Slight Risk was maintained and adjusted a bit based on
the new model data). Given relatively dry antecedent conditions and
associated 3-6 hr FFGs generally ranging from 3.0-4.0", the Slight
is on the lower-end of the 15-40% probability distribution,
suggesting widely scattered instances of flooding (at best).

Churchill

$$
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FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 9 2024

Day 1
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

Overall pattern evolution will continue to point to a benign
instability axis (MUCAPE ~500-1000 J/kg) being confined closer to
the Gulf with sufficient deep layer shear and moisture presence
(PWATs b/w +1 to +2 deviations) allowing for a small area of
convective development capable of locally enhanced rainfall rates
for flash flood potential. Given the overall antecedent dry
conditions prior, the prospects of flash flooding will likely be
relegated to more urban settings along the Central Gulf coast,
including places such as New Orleans, Biloxi, and Mobile. A few
other Parishes within Southeastern LA also contain areas that
experience a bit easier ability to flash flood given some of the
above urbanization factors, so decided to maintain the previous
SLGT risk inheritance with only some minor modifications made to
the overall risk area.

Latest 00z HREF neighborhood probabilities for >3" of rainfall are
within the low-end 20-25% range with highest prospects likely
between 1-2" when taking relevant CAMs into account. This is
similar to the previous forecast prob fields from the 12z HREF
iteration, and with coordination from the local WFOs along the Gulf
Coast earlier for the SLGT risk, there was no need to deviate from
what was proposed in the last update. Best chance will reside from
training echoes on persistent west-southwest flow during the
diurnal instability maximum, carrying into the evening before the
setup decays prior to the next period of relevant convection for
D2.

Kleebauer


Day 2
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
ALABAMA TO UPSTATE SOUTH CAROLINA...

As the previous shortwave trough kicks to the northeast, a stronger
mid-level trough will dig quickly out of the Rockies into the
Southern Plains by Tuesday morning with sights downstream on areas
east of the Mississippi. Increasing meridional flow ahead of the
digging trough will aid in the advection of deep moisture poleward
with the initial surge propagating north out of the Gulf through
the Southeast by Tuesday afternoon, eventually spreading northward
into the Southern and Central Mid Atlantic the second half of the
period. The advection pattern will be sufficient in increasing
regional mid-level instability as profile moisture saturates deeper
into the profile across the areas above. PWAT anomalies on the
order of 2-3 deviations above normal will be common over parts of
GA spreading up through the neighboring Carolinas, eventually
surging into the Central Mid Atlantic and Northeastern U.S by the
end of the D2 cycle. This will prime several areas for heavy
rainfall prospects as the final ingredients come to together for a
locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into Tuesday afternoon and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through Central GA, aligning with the cold front as it moves
through the region. The eventual heavy rain footprint will spread
northeastward into the Southern Mid Atlantic with locally heavy
rainfall impacting areas as far north as Virginia with some lighter
precip making headway into areas further north. Places across
Southwestern VA down to the I-40 corridor in NC will see anywhere
from 1-2" of rainfall with locally higher further south through the
zone. 2-3" with local totals to 4" will be plausible for the area
extending from Southeastern AL through Central GA into Upstate SC,
an area more prone to flash flood prospects thanks to the
complexity of the local terrain.

This signal has been prevalent over the past series of
deterministic model outputs and is now firmly within the ensemble
mean as bias corrected ENS now supports 2.5-3" through much of the
area outlined above in the 2-3" forecast. The saving grace in this
setup is the time of year allowing for lower surface based
instability parameters and very dry antecedent conditions leading
into the event. This allowed for a SLGT risk to be sufficient for
the setup, a carry over from the previous forecast. The MRGL risk
still extends into VA where the northern periphery of the heavy
rainfall is expected.

Kleebauer


Day 3
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

As the longwave pattern evolves further, the neutral-tilted trough
from previous D2 time frame will enhance further with the mean
trough taking on a more negative tilt allowing for enhanced surface
low strengthening as the surface low progresses northward into New
England and eventually Quebec. PWAT anomalies on the order of 2-3
deviations above normal will quickly progress northward
encompassing much of New England by Wednesday afternoon, even
getting up towards +4 deviations as we move into Wednesday night
over Northern New England from NH through ME. This pattern is
textbook for a corridor of heavy rainfall to spread from the Mid
Atlantic through all of the Northeast with the primary areas of
interest location along and ahead of the cold front that will sweep
eastward during the forecast period. Portions of Western NY state
and Northern PA will eventually change to winter precipitation
allowing for a decay in flash flood concerns, however areas
downstream will be under a respectable deluge of rainfall prior to
the rain threat ceasing in wake of the front.

Totals of 1.5-2.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME
as we close out the forecast period. 1-2" is forecast for points
further west, but those areas also have the added snow melt factors
that will play into the flooding concerns as high dewpoints and
heavy rain will promote rapid snow melt for places in the interior
over VT/NH and the neighboring Berkshires in MA. The expectations
are for some creeks and smaller streams to become problematic in
the setup and could cause localized flood concerns during the peak
of the heavy rainfall. The rivers across New England can thankfully
take a decent surge of moisture after a very dry fall, so the
prospects of significant flooding are very low at this point.

The setup is still worthy of a large SLGT risk encompassing much
of New England with the best threats likely over urbanized areas in
LI and Southern New England and over those zones in the interior
where rapid snow melt has historical precedence for localized
flash flood concerns. A MRGL risk will encompass the SLGT risk
across Northeast with an extension down into Central and Eastern NC
as the early portion of the forecast period will still see
moderate to heavy rainfall Wednesday morning before pulling
northward.

Kleebauer
$$
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FOUS30 KWBC 100756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Tue Dec 10 2024

Day 1
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL ALABAMA
THROUGH PORTIONS OF THE WESTERN CAROLINAS...

The overall synoptic pattern remains on track to deliver a swath of
heavier rainfall to portions of the Southeast and Southern Mid
Atlantic during the period. A strong mid- level trough is currently
analyzed over the Four Corners region, digging quickly to the
southeast with sights on the Southern Plains by later this morning.
As the mean trough pivots across the Southern Plains near OK/TX,
increasing meridional flow ahead of the digging trough will aid in
the advection of deep moisture poleward with the initial surge
propagating north out of the Gulf through the Southeast by the
afternoon, eventually spreading northward into the Southern and
Central Mid Atlantic the second half of the period. The advection
pattern will be sufficient in increasing regional mid- level
instability as moisture saturates deeper into the profile across
the areas above. PWAT anomalies on the order of 2-3 deviations
above normal will be common over parts of GA spreading up through
the neighboring Carolinas, eventually surging into the Central Mid
Atlantic and Northeastern U.S by the end of the D1 cycle. This
will prime several areas for heavy rainfall prospects as the final
ingredients come to together for a locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into the afternoon hours and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through central GA into portions of east-central AL, aligning
with the cold front as it moves through the region. The eventual
heavy rain footprint will spread northeastward into the Southern
Mid Atlantic with locally heavy rainfall impacting areas as far
north as southwestern Virginia with some lighter precip making
headway into areas further north.

Recent 00z HREF output remains steadfast with the prospects of a
widespread 1-3" of rainfall across a large chunk of the Southeast
into the Highlands of the Western Carolinas with the northern
extension of any maxima peeking into southwestern Virginia where
some of the terrain will make use of the southeasterly upslope
component to the wind field. Probability fields in both the
neighborhood and EAS realm are pretty solid when it comes to the
potential with the latter of which indicating a well-defined axis
of 30-50% for at least 2", a signal prevalent historically for
favored SLGT risks or higher for the current impact areas.
Neighborhood probabilities are of course higher in percentile given
how the prob field is calculated, however the most notable output
is the very high 60-80% coverage for >3" and low-end probs of
20-25% for up to 5" across the area southeast of Birmingham up
through the Atlanta metro. This was a small shift northward in
terms of the heaviest QPF axis from previous forecasts and align
well with the latest ECMWF and GFS ML outputs, a testament to
increasing agreement within the deterministic on where the heaviest
rainfall will occur. The saving grace from this setup being a more
prolific outcome is the lack of sufficient surface based
instability that tends to add a greater convective element to the
rainfall rates that shift the signal to a higher-end risk threat.
Thankfully, that is not the case here, so the threat does remain a
bit capped on the upper bound of potential.

The previous SLGT risk was adjusted a bit further northwest across
AL/GA to reflect the change within the axis of heaviest rainfall
putting the Atlanta metro squarely within the SLGT risk forecast.
The SLGT remains firmly in-place across Upstate SC with a general
northward extension into the Appalachian front of southwestern NC
as well as small eastward expansion into the Charlotte metro to
atone for the higher QPF being signaled as of the latest 00z
guidance (All in coordination with the local WFO in
Greenville/Spartanburg). The MRGL risk remains generally in a
similar place compared to previous forecast, but some minor
adjustments to the northwest across AL and northward a bit further
into western VA with the most notable inclusion of the Birmingham
metro where the urban flash flood threat will lie right on the edge
where guidance is depicting the heaviest rainfall.

Kleebauer


Day 2
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

The previous forecast remains largely unchanged as the synoptic
regime anticipated for mid-week continues to trend favorable for
heavy rainfall across much of the Northeast and Mid Atlantic. The
neutral- tilted trough from previous D1 time frame will enhance
further with the mean trough taking on a more negative tilt
allowing for enhanced surface low strengthening as the surface low
progresses northward into New England and eventually Quebec. PWAT
anomalies on the order of 2-3 deviations above normal will quickly
progress northward encompassing much of New England by Wednesday
afternoon, even getting up towards +4 to +5 deviations as we move
into Wednesday night over Northern New England from NH through ME.
This pattern is textbook for a corridor of heavy rainfall to spread
from the Mid Atlantic through all of the Northeast with the
primary areas of interest located along and ahead of the cold front
that will sweep eastward during the forecast period. Portions of
Western NY state and Northern PA will eventually change to winter
precipitation allowing for a decay in flash flood concerns, however
areas downstream will be under a respectable deluge of rainfall
prior to the rain threat ceasing in wake of the front.

Totals of 2-3.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME as
we close out the forecast period. 1-2" with locally higher amounts
are forecast for points further west, but those areas also have
the added snow melt factors that will play into the flooding
concerns as high dewpoints and heavy rain will promote rapid snow
melt for places in the interior over VT/NH and the neighboring
Berkshires in MA. The expectations are for some creeks and smaller
streams to become problematic in the setup and could cause
localized flood concerns during the peak of the heavy rainfall. The
rivers across New England can thankfully take a decent surge of
moisture after a very dry fall, so the prospects of significant
flooding are lower than normal at this point.

The previously inherited forecast was largely unchanged as the
setup and subsequent forecast is still worthy of a large SLGT risk
encompassing much of New England with the best threats likely over
urbanized areas in LI and Southern New England and over those
zones in the interior where rapid snow melt has historical
precedence for localized flash flood concerns. A MRGL risk will
encompass the SLGT risk across Northeast with an extension down
into Central and Northeastern NC as the early portion of the
forecast period will still see moderate to heavy rainfall Wednesday
morning before pulling northward.

Kleebauer


Day 3
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer
$$
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FOUS30 KWBC 110803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Dec 11 2024

Day 1
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

Potent longwave trough is currently aligned over the Mississippi
Valley with a tilt closer to neutral as of the latest UA analysis.
A vigorous 160kt speed max at 250mb was analyzed over the Ohio
Valley on the leading side of the trough with sampled winds from
the 00z KILN sounding indicating those 160kt winds are verified.
This signals a very potent upper level evolution with an
anticipated holding of strength the next few hrs before a further
strengthening is anticipated later this morning and afternoon as
the trough axis begins tilting more negative allowing for a jet
buckling downstream as the speed max enters New England and
Quebec. At the surface, multiple waves of low pressure are centered
north to south from the eastern Great Lakes, down through the
Southern Appalachians as they lie within the elongated cold front
extending down from the northern low to the Central Gulf coast. The
forecast is for these waves of low pressure to propagate northward
during the next 3-6 hrs before the surface low over the lakes
becomes the dominant SLP feature, strengthening rapidly as the
trough buckles and the low pressure centered gets positioned firmly
within the right entrance region of the very powerful jet over
northeast North America.

The evolution above will lead to a large swath of rainfall to
encompass much of the area along and east of the Appalachian front
with a trailing axis of convection situated over the southern half
of the Carolinas down into FL as the precip aligns along and just
ahead of the surface cold front. Frontal progression will lead to
rainfall shifting eastward overtime, however the environment ahead
of the front will be significantly enhanced with deep layer
moisture advecting poleward well ahead of the mean trough. PWAT
anomalies are already over +2 deviations across a good portion of
the Northeast U.S down through the Southeast with anomalies
expected to climb closer to +3/+4 deviations as me move into the
late-morning afternoon hours over the Northern Mid Atlantic into
New England. Assessment of local bufr soundings across Southern New
England from Central LI up through CT/MA/NH/ME signal a deep, uni-
directional wind field with modest theta_E advection occurring
during the morning and afternoon time frame as the warm-conveyor
points directly through the area with a well-defined warm front
bisecting the area by 18z.

The time frame between 18-00z will be the most significant time
frame with regards to heavy rainfall potential across much of New
England as the cold front begins racing eastward and the warm front
tilts southwest to northeast from Western CT up through Southern
ME. This alignment will become a focus of a strong, cold rain band
that is textbook in these evolving inland synoptic pressure falls
leading to a period where rainfall rates will be a solid 0.5-1"/hr
extending within that convergence zone. The line will advance
eastward eventually as the cold front approaches and kicks the
convergence axis downstream leading to the heavy rain threat
shifting more towards the coastal regions with sights on Eastern
MA/NH/ME towards the 00-03z time frame. Backed flow off the
Atlantic will aid in maintaining the convergent field along and
ahead of the advancing front with heavy rains lingering until the
front back finally pass through and the winds shift west to
northwesterly ushering in colder air and stabilizing the mid-levels
enough to limit the heavier precip. Prior to this occurring, a
solid 2-3" with locally as high as 4" of rainfall are anticipated
in the zone encompassing Central and Eastern LI, much of MA/NH and
the western half of ME. Rapid snow melt will be a problem for
areas located in the Northern Berkshires and Worcester Hills
towards the MA/NH border extending northward as NOHRSC snow depth
analysis pins a solid 2-4+" on the ground currently within those
zones with over a foot in parts of NH/VT/Western ME. This will
allow for potential small stream cresting and ice jam concerns in
the interior that would exacerbate flood concerns during the
heaviest of the precip time frame later this afternoon.

An expanse of Flood Watches are positioned across much of the
above areas, including all of VT as complex terrain and higher snow
depths in the Green Mountains and surrounds leading to higher than
normal flood potential, despite the heaviest rain footprint likely
a bit further east. Considering the above variables and in
coordination with local WFOs, the inherited SLGT risk was
relatively unchanged with the only shift being an extension a bit
further southwest into CT to cover for urban flood concerns within
the axis of expected heavy rainfall during the cold frontal band
being forecasted across much of the 00z CAMs suite. The MRGL risk
still extends around the SLGT and further south into the Central
Mid Atlantic and Carolinas with the primary concerns likely
isolated urban flood potential, especially in the larger metro
corridors like Raleigh, DC/Balt, and Philadelphia.

Kleebauer


Day 2
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

Fairly robust disturbance over the Pacific will translate
southeastward towards the North-Central CA coast by the end of the
week with a persistent IVT pulse oriented southwest to northeast
into the coastal plain over the course of the forecast period
leading to waves of moderate precipitation to impact areas north of
the Bay area. Current progs indicate precip totals on the order of
2-4" with locally higher embedded within the coastal ranges
spanning north of Santa Rosa. Current rates anticipated to be
modest with upwards of 0.5-0.75"/hr at peak intensity lending to a
lower end threat for flash flooding, however totals are significant
enough to warrant a low-end threshold MRGL, mainly for the coastal
urban areas of Northwest CA up through Southwestern OR where
totals will be on the order of 2-4", especially into the windward areas
of the Klamath Range. MRGL was generally maintained from previous
forecast with only some minor modifications to align with the
latest guidance.

Kleebauer
$$
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FOUS30 KWBC 120805
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Thu Dec 12 2024

Day 1
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

The overall forecast maintained a general continuity in the
expected synoptic evolution of a weak atmospheric river transpiring
across the Northwest CA coast, extending northward into Southwest
OR. IVT signatures between 700-1000 kg/ms will advect into the
coastal plain with some protrusion inland on the initial surge as
the southwest to northeast alignment of the pulse relegates a
majority of the moisture into the coast and adjacent coastal ranges
that will garner the greatest QPF just given the upslope component
of the wind field. A widespread 2-4" with local max of 5-5.5" is
forecast over the period with the general maxima likely within the
coastal King Range as is customary with IVT pulses exhibiting a
southwestern angle of approach across Northern CA. Rainfall rates
on the order of 0.25-0.5"/hr will be most common within these bands
of precipitation, however a few instances of >0.5"/hr might be
realized during the period between 00-12z Saturday when the primary
surge is forecast amongst the 00z deterministic suite. This
correlates well with the anticipated IVT maximum making headway
into the coastal plain which could lead to localized flooding
concerns in the expected zone(s) of impact. As of this juncture,
the heaviest rainfall will lie north of Santa Rosa with the primary
areas of interest likely positioned from the King Range up US101 to
the southern half of the Siskiyou in southwestern OR. With this
alignment, very little change was necessary from the previous MRGL
risk issuance, thus the forecast was relatively unchanged as the
MRGL risk was sufficient in consensus.

Kleebauer


Day 3
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA INTO THE CENTRAL SIERRA FOOTHILLS...

Atmospheric River setup from the previous period will translate
into D3 with a greater inland protrusion of the IVT pulse as the
upper level trough and associated surface low pivot onshore with
an expected wind shift that will mute heavier rain prospects along
the coast and eventually inland as we move into the second half of
the period. The threat is currently very marginal in nature within
the confines of the inland topography with the eastern foothills
of the Sierra's being the furthest east expanse of any heavy rain
threat, but even that is bordering on the lower end of a risk
threshold. This period is more of the transition point in the
pattern as the event will see a general decay in total rainfall and
overall rates as the system moves inland. There is a further south
push of the IVT pulse in the early portions of Saturday with some
modest dynamical element involved as the 500mb vorticity evolution
does bring some significant forcing towards the North- Central CA
coastal plain with impacts to places like Santa Rosa down into the
Bay area. Thankfully the pattern is progressive, but a quick 1-2"
is within the realm of possibility for a mainly urban zone which
could bring about some localized flooding prospects before the
disturbance moves inland and we get a quick end to the chances.

A MRGL risk continues for a good portion of Northwest CA with the
risk area now extending down into the Bay area compared to the
previous D2 alignment. A MRGL risk is also located at the base of
the Sierra foothills for locations below 5500ft.

Kleebauer
$$
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FOUS30 KWBC 130826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 AM EST Fri Dec 13 2024

Day 1
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

Maintained the Slight Risk area over portions of northwest
California with only minor adjustments needed based on latest
deterministic and ensemble guidance. Rainfall should be growing in
both areal coverage and intensity today as a plume of moisture is
directed from the eastern North Pacific into northwest California.
Event total rainfall amounts through early Saturday morning are
still forecast to be in the 3 to 5 inch range across the Slight
Risk area. By late tonight/early Saturday morning the heavier rain
will shift southward as a strong mid level shortwave approaches the
region. This feature will act to increase large scale ascent,
while also cooling the mid levels enough to allow for some weak
instability to develop within the IVT axis. The 13/00Z HREF
neighborhood probabilities of rainfall exceeding 0.5" in an hour
are over 50% across most of the Slight risk...both across the most
favored upslope areas and also within a narrow southward shifting
axis of stronger convergence. This does not appear to be a high end
event given the progressive nature of the system...although some
creek and stream flooding and a few mudslides will be possible.

Bann


Day 2
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

Rainfall associated with an atmospheric river will be on-going as
the Day 2 period begins at 14/12Z...with the plume of deepest
moisture continuing to be directed in to parts of northwest
California although the axis of heaviest rainfall rates/amount will
be gradually be easing eastward in time with the main shortwave
while being shunted southward ahead of the surface cold front.
southward with time. The higher rates should quickly diminish with
the departure of the shortwave moves...with the risk of heavy rain
over by 18z. Thus this Slight risk is really only for the first few
hours of this day 2 period.

Bann

Day 3
Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.|

Bann
$$
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FOUS30 KWBC 140829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Dec 14 2024

Day 1
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
COASTAL CALIFORNIA FROM SANTA ROSA INTO SAN Francisco AND SAN JOSE...

Even though a much of the areal coverage of moderate to locally
heavy rainfall associated with an atmospheric river has already
moved on shore...maintained the Slight risk area mainly along the
coast from Santa Rosa southward into San Francisco and San Jose
given an expected up-tick in rainfall rates as a compact shortwave
trough approaches from the west. Short term guidance from the
convective allowing mesoscale guidance shows several hundred Joules
per kilogram of CAPE moving into that portion of California
immediately ahead of the shortwave impulse. The 14/00Z HREF
maintained a nearly 100 pct chance of half-inch per hour rainfall
rates and nearly 40 percent chance of 1 inch per hour rates over
and near the San Francisco Bay area during the early- to mid-
morning hours. The combination of the rainfall rates and the
urbanization of the area still points to an elevated risk of flood
impacts. The overall impacts should be limited by the fairly short
duration of heaviest rainfall...which looks to taper off quickly in
the 15Z to 18Z period.

Bann


Day 2
Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Mon Dec 16 2024 - 12Z Tue Dec 17 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent

Bann
$$
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FOUS30 KWBC 170751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Tue Dec 17 2024

Day 1
Valid 12Z Tue Dec 17 2024 - 12Z Wed Dec 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA...

Model consensus is for increasingly active showers late Tuesday
afternoon into Tuesday night/early Wednesday along the east central
to southeast coast of Florida. Low level easterly to southeasterly
flow in an axis of PW values 1.5 to 2+ standard deviations above
the mean expected to persist into the east central to southeast
Florida coast, with models showing an axis of defined boundary
layer convergence in this low level inflow axis parallel to the
east central to southeast coast. There will be potential for very
slow moving cells along the coastal regions, supporting locally
very heavy rainfall amounts and an urban flash flood threat. HREF
neighborhood probabilities are high (50-90%) for 3"+ amounts from
Cape Canaveral, south to Miami, while the 5"+ neighborhood
probabilities are high (60-90%) from near West Palm Beach, south to
Miami. The best hi res overlap, evident in the greatest EAS
probabilities for 2"+ amounts, is from near West Palm, south to
just north of Miami.

Oravec


Day 2
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 3
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec
$$
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FOUS30 KWBC 180823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Wed Dec 18 2024

Day 1
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF THE
OHIO AND TENNESSEE VALLEYS ALONG WITH THE SOUTHEAST FLORIDA COAST...

...Parts of the Mid-South to Lower Ohio Valley...
Maintained the Marginal Risk area across parts of western and central
Tennessee into southern Kentucky as models still advertise an
upper-level trough amplifying to the west which allows for a more
curved upper level jet streak on the lee side of the trough. That
configuration may result in some enhanced rainfall rates...an idea
supported by the 18/00Z HREF neighborhood probabilities showing
potential for rainfall rates exceeding an inch per hour between
12-18Z today (mainly in Tennessee). With rainfall blossoming to
the west of the outlook area in the pre-dawn hours this morning
helping to prime the soils a bit more and a general slowing of the
eastward progression across the Tennessee Valley compared with
earlier forecasts...opted to extend the Marginal risk area a bit
farther westward than previously indicated. The HREF probability of
exceeding flash flood guidance remained low but did reflect
somewhat higher probabilities west of the same guidance from the
17/12Z and 17/18Z runs. The decision to expand westward slightly
was also driven by the observed rainfall during the past day or two
over parts of northwest Tennessee and southwest Kentucky which has
resulted in the latest 1-hour and 3- hour flash flood guidance
coming down and the 0-40km soil moisture percentiles per NASA SPoRT
have climbed to 60-80% for most areas.

...Southeast FL...
Post frontal easterly low level flow off the Atlantic will linger
allowing for a possible repeat of convective clusters over and
near the southeast Florida coastline today. The 00Z runs of the
high-res CAMs continued to show onshore low-level moisture
flux/convergence and sufficient deep-layer instability to support
localized downpours. The 18/00Z HREF supports rates of 2+ inch per
hour peaking around 10 percent during the afternoon with and a
50-70 percent 40km neighborhood probabilities of at least 5 inches
of additional rain and 30-50 percent probabilities of 8+ inches.
Given that these numbers are comparable with the 17/12Z and 17/18Z
runs of the HREF...saw little reason to make too many changes other
than nudges to the boundary of the previously issued Marginal risk
area.


Bann

Day 2
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Fri Dec 20 2024 - 12Z Sat Dec 21 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann
$$
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FOUS30 KWBC 210756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Sat Dec 21 2024

Day 1
Valid 12Z Sat Dec 21 2024 - 12Z Sun Dec 22 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next, more prominent atmospheric river is forecast to impact
the NorCal coast into southwestern OR by the end of the D2 into D3
time frame with a better aligned IVT pulse anticipated for areas
that saw significant rainfall totals over the past 7-10 days. The
heaviest rain will be confined to the immediate coast with a
southwest to northeast oriented moisture flux expected leading to
upslope flow a bit orthogonal to the coastal terrain extending from
the King Range up through the Siskiyou in southwest OR. Current
forecasted totals within the ensemble suite are between 2-4" with
some deterministic output between 4-5" at max. This is generally
within the confines of a low-end MRGL risk threshold when assessing
historical precedence for these types of events. The main
difference between this event and the previous was the primary
hydrometeor will come in the form of rainfall, even inland as lower
heights will be confined offshore and a flood of warmer air in the
boundary layer will protrude inland. Only the highest elevations of
the Northern Sierra and Mount Shasta will have a higher threat for
winter ptypes, so this points some of the more sensitive areas of
northern CA into play where burn scar remnants are located.

The inherited MRGL from the D4 was relatively maintained for the
areas outlined in CA, but the prospects for flash flooding were
pretty low given the QPF signature for locations north of
southwestern OR, so decided to pull back the northern extension
along the OR coast. The MRGL extends inland to include those more
sensitive areas within the complex terrain, even extending to the
foothills of the Northern Sierra where heavier precip will make its
way by the end of the forecast cycle and likely carrying beyond to
the D4 period.

Kleebauer
$$
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FOUS30 KWBC 220759
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sun Dec 22 2024

Day 1
Valid 12Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

There is a non-zero opportunity for isolated flash flood concerns
across portions of the eastern FL coastline, mainly along the Space
Coast near Coco Beach up to Cape Canaveral and Titusville. A
developing surface trough off the coast will nose into the
coastline later this evening with flow turning more perpendicular
to the coast allowing for increased frictional convergence in a
small zone within the trough. HREF neighborhood probabilities are
hovering around 20-30% at peak for >3" in any spot within the area
above, very much on the lower end of any flash flood threshold, and
mainly below the current FFG indices for 1/3/6 hr time frames.
More robust deterministic has closer to 4.5-5" over a short period
of time the back end of the forecast period, but most of the CAMs
maintain the heaviest precip offshore. The prospects are very low,
but wanted to make mention that threat is non-zero.

Kleebauer


Day 2
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

Consensus continues to grow for a quick hitting weak to borderline
moderate atmospheric river aimed at southwestern OR down into
northern CA by late Monday into Tuesday morning. Recent ensemble
depictions are in agreement on an IVT pulse centered over the
northern CA coast, protruding inland through the end of the period
with warm air advecting well into the boundary layer and above
yielding a majority rainfall signal across even some of the higher
terrain inland. NAEFS output for PWAT anomalies indicates a tongue
of 2-3 deviations above normal moisture plume being ushered in on
deep layer southwest flow thanks to a shortwave trough undercutting
a broad upper level cyclone located off the PAC Northwest coast.
This look is fairly textbook for a period of prevailing
southwesterly flow running orthogonal to some of the adjacent
coastal ranges that are situated from southwest OR down through
northern CA. The zones most impacted will lie at the foothills of
Mount Shasta and the northern Sierra Nevada for the inland portion,
as well as the King and Siskiyou Ranges that align from OR down
through northwestern CA. Rainfall totals on the order of 3-5" with
locally higher in the highest elevations of the above ranges are
forecast during the time frame beginning 21z Monday through 12z
Tuesday with some carryover into D3 before the main shortwave moves
inland. 2-4" will be common within the coastal plain with locally
higher in areas south of Eureka. Localized flooding prospects are
higher than normal given the stronger IVT pulse being depicted,
also impacting areas that saw decent rainfall as of a week to 10
days prior. The saving grace is the time frame is not very long in
terms of impact, so that helps limit a greater flash flood threat.

The previously inherited forecast was generally maintained outside
some fine tuning across the Sierra Foothills and the northern
periphery of the MRGL risk into OR.

Kleebauer


Day 3
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D3 time
frame with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central
Sierra, less so for areas back into the coast due to negative
vorticity advection. Locally enhanced rainfall will be plausible
the initial portion of the period with the heaviest likely aligned
along those Foothill regions of the Northern Sierra, mainly within
elevations below 8000ft, although snow levels will fall below
7000ft towards the end of the more significant precipitation time
frame Tuesday morning. Additional totals of 1-3" are possible
within a 6-10 hour window prior to the precip ending leading to a
low-end potential for flash flood concerns just outside the
Valley that extends from Redding down to Sacramento. Higher runoff
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood
concerns despite this being an event that doesn't maintain a more
prolonged precipitation signature. Considering limited deviation
from run-to-run amongst guidance with considerable agreement in
timing and magnitude of precipitation, continuity from the previous
forecast was maintained with very little in the way of adjustment
in the inherited MRGL risk across the Sierra Foothills.

..Arklatex and Lower Mississippi Valley..

Surface ridge to the northeast will slowly drift further to the
northeast allowing for a more broad return flow regime to affect
areas across the Western Gulf into the Lower Mississippi Valley.
Southerly flow will advect a bit more unstable air northward with
areal theta_E averages on the increase when assessing the ensemble
means and initial D3 cluster analysis with heavy weight towards the
GEFS and ECENS output. Aloft, a shortwave will dig southeast with
enhanced mid-level energy accompanying aiding in better regional
forcing as we work into Tuesday afternoon and evening. This will
help setup a period of convective initiation across Eastern TX and
points north with the primary axis of heaviest precip likely
aligned southwest to northeast across the Arklatex down into the
eastern Hill Country located east of I-35. Weak jet coupling will
help with the expansion of the precip field across the above areas
with those embedded convective signatures likely situated along and
ahead of an advancing cold front as surface cyclogenesis in-of the
Red River will become a focus for activity in question. Locally
heavy rainfall will be forecast within those stronger convective
cores, however the PWAT anomalies and available surface based
instability will be modest, at best with regards to the setup. This
will likely lead to more spotty instances of >3" of total precip
in any one location with the best areas of interest mentioned above.

The previous MRGL risk was maintained, but did trim some of the
eastern flank of the risk area as probabilities off the blend are
pretty meager and agree with the current ML output basing best
chances to the northwest of where the risk area was drawn. A small
extension to the southwest was done out of prospects for the
traditional southwestern bias in heavier QPF within these types of
setups. This was featured within a few of the regional
deterministic and works well with the proposed elevated SBCAPE
anticipated in that area.

Kleebauer
$$
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FOUS30 KWBC 221925
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Sun Dec 22 2024

Day 1
Valid 16Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

Update...

The 12Z guidance did not suggest any significant changes were
required, therefore made only minor adjustments to the previous outlook area.

Previous Discussion...

Consensus continues to grow for a quick hitting weak to borderline
moderate atmospheric river aimed at southwestern OR down into
northern CA by late Monday into Tuesday morning. Recent ensemble
depictions are in agreement on an IVT pulse centered over the
northern CA coast, protruding inland through the end of the period
with warm air advecting well into the boundary layer and above
yielding a majority rainfall signal across even some of the higher
terrain inland. NAEFS output for PWAT anomalies indicates a tongue
of 2-3 deviations above normal moisture plume being ushered in on
deep layer southwest flow thanks to a shortwave trough undercutting
a broad upper level cyclone located off the PAC Northwest coast.
This look is fairly textbook for a period of prevailing
southwesterly flow running orthogonal to some of the adjacent
coastal ranges that are situated from southwest OR down through
northern CA. The zones most impacted will lie at the foothills of
Mount Shasta and the northern Sierra Nevada for the inland portion,
as well as the King and Siskiyou Ranges that align from OR down
through northwestern CA. Rainfall totals on the order of 3-5" with
locally higher in the highest elevations of the above ranges are
forecast during the time frame beginning 21z Monday through 12z
Tuesday with some carryover into D3 before the main shortwave moves
inland. 2-4" will be common within the coastal plain with locally
higher in areas south of Eureka. Localized flooding prospects are
higher than normal given the stronger IVT pulse being depicted,
also impacting areas that saw decent rainfall as of a week to 10
days prior. The saving grace is the time frame is not very long in
terms of impact, so that helps limit a greater flash flood threat.

The previously inherited forecast was generally maintained outside
some fine tuning across the Sierra Foothills and the northern
periphery of the MRGL risk into OR.

Pereira/Kleebauer


Day 3
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

Update...

The 12Z guidance did not suggest any significant changes were
required, therefore made only minor adjustments to the previous outlook areas.

Previous Discussion...

..Sierra Foothills..

Atmospheric river from previous period will
bleed into the D3 time frame with the primary shortwave trough
quickly propagating inland with increased forcing upon arrival into
the Northern and Central Sierra, less so for areas back into the
coast due to negative vorticity advection. Locally enhanced
rainfall will be plausible the initial portion of the period with
the heaviest likely aligned along those Foothill regions of the
Northern Sierra, mainly within elevations below 8000ft, although
snow levels will fall below 7000ft towards the end of the more
significant precipitation time frame Tuesday morning. Additional
totals of 1-3" are possible within a 6-10 hour window prior to the
precip ending leading to a low-end potential for flash flood
concerns just outside the Valley that extends from Redding down to
Sacramento. Higher runoff capabilities due to terrain orientation
and soil moisture anomalies running closer to normal will present
some potential for flood concerns despite this being an event that
doesn't maintain a more prolonged precipitation signature.
Considering limited deviation from run-to-run amongst guidance with considerable agreement in timing and magnitude of precipitation,
continuity from the previous forecast was maintained with very
little in the way of adjustment in the inherited MRGL risk across
the Sierra Foothills.

..Arklatex and Lower Mississippi Valley..

Surface ridge to the northeast will slowly drift further to the
northeast allowing for a more broad return flow regime to affect
areas across the Western Gulf into the Lower Mississippi Valley.
Southerly flow will advect a bit more unstable air northward with
areal theta_E averages on the increase when assessing the ensemble
means and initial D3 cluster analysis with heavy weight towards the
GEFS and ECENS output. Aloft, a shortwave will dig southeast with
enhanced mid-level energy accompanying aiding in better regional
forcing as we work into Tuesday afternoon and evening. This will
help setup a period of convective initiation across Eastern TX and
points north with the primary axis of heaviest precip likely
aligned southwest to northeast across the Arklatex down into the
eastern Hill Country located east of I-35. Weak jet coupling will
help with the expansion of the precip field across the above areas
with those embedded convective signatures likely situated along and
ahead of an advancing cold front as surface cyclogenesis in-of the
Red River will become a focus for activity in question. Locally
heavy rainfall will be forecast within those stronger convective
cores, however the PWAT anomalies and available surface based
instability will be modest, at best with regards to the setup. This
will likely lead to more spotty instances of >3" of total precip
in any one location with the best areas of interest mentioned above.

The previous MRGL risk was maintained, but did trim some of the
eastern flank of the risk area as probabilities off the blend are
pretty meager and agree with the current ML output basing best
chances to the northwest of where the risk area was drawn. A small
extension to the southwest was done out of prospects for the
traditional southwestern bias in heavier QPF within these types of
setups. This was featured within a few of the regional
deterministic and works well with the proposed elevated SBCAPE
anticipated in that area.

Pereira/Kleebauer
$$
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FOUS30 KWBC 230758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 23 2024

Day 1 Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next atmospheric river will make headway into southwest OR and
northwest CA coastal plain with a protrusion of elevated IVT inland
as we work the back end of the period. Consistency among all major deterministic, both global and hi-res indicate a widespread area of
2-4" with locally as high as 5.5" in-of the impacted areas, mainly
north of Santa Rosa up into southwest OR. Latest HREF probs for >3"
are very high (80+%) across areas like the King and Siskiyou Ranges
along with the Foothills of Mount Shasta. Despite the elevation, a
strong warm nose with this event will send snow levels spiking
upwards with the base pushing close to 8000ft MSL for the
rain/snow delineation point. This will create a better heavy
rain threat even away from the coast with areas inland also
maintaining a threat for localized flash flood concerns, especially
as we move into early Tuesday morning. The heaviest rainfall will
likely be within those coastal ranges which are some of the harder
areas to flood, so that will help limit the extensive flash flood
prospects we see with some events. The progressive nature of the AR
regime will also aid in the anticipated impacts, however the threat
is still within the low to medium end of the MRGL risk threshold
leading to a continuance of the MRGL from the previous forecast
issuance. The area(s) with the greatest potential are those that
are still dealing with burn scar aftermath with very sensitive
runoff capabilities. Those are included within the MRGL risk,
especially across northern CA.

Kleebauer


Day 2 Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D2 time
frame with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central
Sierra, less so for areas back into the coast due to negative
vorticity advection. Locally enhanced rainfall will be plausible
the initial portion of the period with the heaviest likely aligned
along those Foothill regions of the Northern Sierra, mainly within
elevations below 8000ft, although snow levels will fall below
7000ft towards the end of the more significant precipitation time
frame late Tuesday morning. Additional totals of 1-2" are possible
within a 6-10 hour window prior to the precip ending leading to a
low-end potential for flash flood concerns just outside the Valley
that extends from Redding down to Sacramento. Higher runoff
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood
concerns despite this being an event that doesn't maintain a more
prolonged precipitation signature.

The previously inherited MRGL risk was generally maintained,
however some of the risk area was cut out due to provide a gradient
between the Sierra locations that will trend to more winter
precipitation and the areas that will remain liquid through much
of, if not the entire duration of the event.

..Arklatex and Lower Mississippi Valley..

Surface ridge over the eastern CONUS will slowly drift further to
the northeast allowing for a more broad return flow regime to
affect areas across the Western Gulf into the Lower Mississippi
Valley. A steady flux of higher theta_E's will begin moving
northward out of the Western Gulf, carrying as far north as the Red
River before stabilizing as we reach into OK. Aloft, a digging
mid- level shortwave will exit the Central Rockies with sights on
the Southern Plains leading to enhanced left exit region dynamics
and surface cyclogenesis in-of the Red River Valley with a cold
front extending from the base into TX and a small warm front
lifting northward around the Arklatex. The classic mid-latitude
cyclogenesis will create a sector of modest destabilization with
the primary axis aligned from southwest to northeast across east TX
up through the Arklatex, eventually extending northward into AR
and southern MO. The current indications are the best organized
convective schemes will be situated around the small warm sector
along and ahead of the approaching cold front with relative
buoyancy and increasing upstream shear allowing for scattered
convection to develop late Tuesday afternoon through the evening.

As of this time, the deep layer moisture pattern is still
relatively meager compared to some of the more impactful events
that occur in this area of the country, however there is enough
instability and PWAT anomalies creeping between 1-1.5 deviations
above normal to constitute some isolated flash flood concerns,
mainly within those stronger cores. SPC D2 risk includes a targeted
Marginal Risk for severe weather, overlapping the inherited MRGL
risk ERO for the period. Areal rainfall averages will be between
0.5-1" across east TX and 0.75-1.5" across AR, but there is a
growing consensus among the CAMs to have scattered instances of a
quick 2-3" of rainfall within the best convective environment
during the pattern evolution with a 5-10% risk of >3" within the
tail end of the 00z HREF neighborhood probs. This threat is likely
still within the lower end of the MRGL risk threshold, but the
convective premise was enough to maintain general continuity from
the previous issuance.

Kleebauer


Day 3 Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
WASHINGTON AND OREGON...

Another atmospheric river surging off the Pacific will make an
appearance into the Pacific Northwest by Wednesday afternoon
through the end of the D3 cycle. Model guidance is keen on a surge
of moisture represented by fairly elevated IVT signatures on the
order of 600-800 kg/ms within the global ensemble blend. There is
some discrepancy between one of the main deterministic (GFS) and
the other global members carrying more of a 25th percentile outcome
in total precip due to a less robust IVT pulse comparatively. When
assessing the ensemble means from the GEFS compared to the
deterministic, the ensemble sways more in favor the scenario of
greater magnitudes into the IVT channel leading to a more
pronounced atmospheric river regime. This trend leaned more into
maintaining continuity in the inherited MRGL risk across the PAC
NW, although a few changes were made in the proposal.

The first change was to scale back on the eastern extension of the
MRGL risk due to considerably less deep layer moisture advecting
inland at this juncture for the D3 time frame. FFG indices are
still pretty high all the way towards the windward side of the
Cascades, so the prospects for FFG exceedance were pretty low and
generally fall below the 5% threshold criteria. The second change
was to cut out the higher elevations in the Olympic Peninsula due
to the primary ptype trending towards snow with more emphasis on
rainfall closer to the coast and below 4000ft MSL. The risk area
still encompasses the lower elevations surrounding the Olympics and
channels into the Olympic National Forest on the southern flank of
the Peninsula. Anticipate totals of 2-4" with locally as high as 5"
in the risk area extending from coastal southwest OR up through the
Olympic coast of WA.

Kleebauer
$$
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FOUS30 KWBC 261951
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Thu Dec 26 2024

Day 1
Valid 16Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EAST TEXAS
THROUGH THE ARKLATEX AND ADJACENT AREAS OF THE LOWER MISSISSIPPI VALLEY...

...16z update...
Recent observational trends, 12z Hi-Res CAMs and HREF
probabilities further support initial Day 1 reasoning and placement
of Slight Risk area across E TX into the Arklatex and Lower MS
Valley later today. As such, only slight westward adjustments were
needed with this update, to account for slightly earlier convective
initiation observed in the observational trends.

Additionally, southward adjustment with Marginal Risk in central
CA also appears on solid track with the arrival of the next AR wave
in the later 6-12 hours of the Day 1 period.

Gallina

~~~Prior Discussion~~~

..East Texas...ArkLaTex...Lower Mississippi Valley..

GOES satellite imagery shows a strong shortwave trough digging
southeastward across the southern Rockies which will be ejecting
and amplifying out across the southern Plains this afternoon and
the Lower MS Valley tonight. The mid to upper-level trough is
expected to become negatively tilted this evening as it crosses
through the ArkLaTex and moves downstream, and there will be a
substantial amount of upper-level jet energy/forcing in place via
the arrival of a 90+ kt jet streak and associated jet couplet which
the guidance has been quite consistent in adverting. Coinciding
with this will be surface cyclogenesis and the northward advance
of a warm front into the Lower MS Valley which will foster the
northward advance of moisture and instability. The 00Z HREF
consensus favors a convergent low-level jet increasing to the
order of 50 to 70 kts this evening across areas of far eastern TX
through western and northern LA and much of southern AR and western
MS. Strong convection will be developing over areas of central and
especially eastern TX this afternoon and impacting much of the
broader ArkLaTex region and the Lower MS Valley this evening and
overnight. Multicell and supercell thunderstorm activity is likely
given the available moisture and instability transport and the
enhanced shear profiles. The convection should tend to become more
progressive in time with a QLCS evolution expected tonight, but
concerns with cell-merger activity and some periodic cell-training
will support rainfall rates reaching 1 to 2 inches/hour with some 2
to 4+ inch storm totals possible. Scattered areas of flash
flooding, especially around the more sensitive urban locations,
will be possible. This will also be aided by the fact that at least
locally there has been recent rainfall that has moistened the soil
conditions and increased the streamflows. Therefore, the Slight
Risk area has been maintained and locally expanded to account for
the heavy rainfall threat and antecedent conditions.

...Pacific Northwest through Northern California...

The arrival of a new Pacific shortwave trough and associated
atmospheric river surge will bring a new round of heavier rainfall
to the coastal ranges of the Pacific Northwest on down through
northern CA later today through Friday morning. The shortwave
energy will be quite vigorous and will be amplifying which will
drive strong cyclogenesis just offshore of western WA/OR tonight.
Enhanced IVT magnitudes around the southern flank of the low center
are expected to impact especially southwest OR and northwest CA
with values increasing to as much as 750 to 1000 kg/m/s. This
strong moisture transport and related warm air advection will
facilitate rainfall rates increasing to as much as 0.50" to
0.75"/hour at least briefly as the core of the stronger IVT arrives
and with aid from orographic ascent over the coastal ranges.
Additional rainfall amounts by Friday morning with this next
atmospheric river surge are expected to be on the order of 3 to 6
inches, with the heaviest totals likely focused over southwest OR
and northwest CA. Given the level of additional rains and
wet/saturated soil conditions, there will be increasing concerns
for more flooding impacts. The Marginal Risk area will be
maintained across the region, except it has been expanded somewhat
farther south in northern CA to include the San Francisco Bay
metropolitan area given expectations for heavy rain arriving here
by early Friday morning along with some urban flooding concerns.

Orrison


Day 2
Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF MISSISSIPPI, ALABAMA AND
TENNESSEE...

...1930 UTC Update...
Minor tweaks made to both Marginal Risk areas, based on the latest guidance/trends (now within the high-res CAM windows). Still
expecting the risk areas to remain sub-Slight, based on the degree
and duration of favorable moisture transport, also supported by the
latest CSU ERO first guess fields.

Hurley

...Previous Discussion...

...Southeast...

A Marginal risk was maintained across portions of MS, AL and TN with
this update. Convection will likely be ongoing at 12z Friday along
this corridor as a front pushes eastward. The front slows and starts
to wash out during the day Friday as large scale forcing weakens.
This weakening of the front and forcing decreases confidence in
convective evolution, as these factors could result in decreased
convective coverage/organization and little to no flash flood risk.
However a corridor of modest convergence will likely still exist
near the fading front, and some chance outflow from earlier
convection could locally enhance this convergence. There will be a
supply of instability upstream, and so if convection is able to
persist then there is a potential for a few training segments to
evolve within the Marginal risk area. Given the dry antecedent soil
conditions and streamflows, and only a conditional threat of a few
training segments, this is generally considered a lower end Marginal
risk. However given this risk was only recently introduced, and the
continued non-zero risk of localized training resulting in 3"+
amounts, we will hold on to the Marginal for now and continue to
monitor trends. The risk area was shifted a bit off to the southeast
to better match where the latest models depict the best chance of
these localized higher rainfall amounts.

...Northwest...

After what should be a relative lull in rainfall to start the
period, another round of stronger IVT should move into the Pacific
Northwest Friday night. 24 hour rainfall totals through 12z Saturday
will generally not be too significant. Most areas are looking at 1-
2", with peak values around 3" possible from far northwest CA into
far southwest OR. These amounts themselves would typically not pose
much of a flood concern. However it has been a wet recent stretch
over this area, and thus soul saturation and streamflows are running
well above average. Given this increased sensitivity, the uptick in
rainfall coverage and intensity Friday night may continue to pose
some flood risk.

Chenard
$$
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FOUS30 KWBC 270800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Fri Dec 27 2024

Day 1 Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF THE MID-SOUTH...

...Northwest...

After what should be a relative lull in rainfall for most of the
day, there will be another round of stronger IVT/atmospheric river
activity arriving this evening and continuing overnight across the
Pacific Northwest with an emphasis on southwest OR and northwest
CA. This will be driven by the arrival of another Pacific shortwave
trough and attendant surface low that will be yielding an eastward
advance of strong low to mid-level flow. The latest GFS/ECMWF
solutions suggest IVT values here surging upwards of 750+ kg/m/s in
the 00Z to 12Z time frame tonight across especially southwest OR
and far northwest CA. A combination of strong warm air advection,
enhanced moisture transport and orographic ascent over the coastal
ranges will support 0.25" to 0.50"/hour rainfall rates with these
rates likely exceeding a 0.50"/hour at least occasionally across
areas of Coos, Curry and Del Norte Counties. More broadly across
the region, the additional rainfall amounts should reach 1 to 2
inches by early Saturday morning, but the latest HREF guidance
suggests some of the favored windward slopes seeing as much as 2 to
4 inches where the rainfall rates will be highest in conjunction
with the stronger IVT values. Given the continuation of the very
wet pattern across the region and with streamflows already running
high, these additional rainfall totals may pose concerns for
increasing runoff problems and flooding. Thus, a Marginal Risk for
excessive rainfall remains in place for this period.

...Mid-South...

A Marginal Risk remains depicted across portions of MS, AL, TN and
has been tweaked to include a small part of northwest GA for this
update. A few broken bands of convection will be crossing through
central and northern MS this morning and gradually advancing
downstream across areas of central and northern AL, middle TN and
far northwest GA going through the afternoon and early evening
hours. The activity will be associated with the northeast ejection
of a strong mid-level shortwave trough out of the Lower MS Valley
early this morning which will cross the Mid-South and lift into the
OH Valley. Convection should remain focused in close proximity to
a trailing front, but instability along the boundary should be
quite modest and this coupled with the stronger forcing lifting
away off to the northeast should favor the activity being quite
disorganized overall with fairly modest rainfall rates. A low-
end, very modest threat for some runoff problems will exist if the
broken pockets of convection near the front can train over the same area.

Orrison

Day 2 Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

A stalling front and IVT plume will allow for a prolonged period of
moderate to locally heavy rain Saturday into Saturday night across
portions of northwest CA into southwest OR. The magnitude of the
event should peak Saturday night as IVT values likely exceed 750
J/kg/m^2 and large scale forcing increases ahead of an approaching
mid level trough. Not expecting there to be any instability to work
with, but the increasing IVT and forcing should allow for rainfall
rates around a half inch per hour, and the stalling of the front/IVT
axis will allow these higher rates to persist. There are some model
solutions that even suggest 5-7" of rain is a possibility.

Even though this area is used to seeing rainfall of this magnitude
without significant impacts, in this case this rain will be falling
after what has already been a prolonged wet period. So saturated
soil conditions and elevated stream and river levels will likely
increase the susceptibility to flooding. For this reason think we
will see a notable increase in the flood risk over this area by
Saturday night, justifying the Slight risk. The inland extent of the
heavier rain could also end up greater than normal, with 1-3"
possible even in inland areas of southwest and central Oregon.

Given that setup, in coordination with MFR/Medford, OR forecast
office, the inherited Slight Risk was expanded northeastward to
include much of Coos, Douglas, and Jackson Counties in Oregon. An
internal higher end Slight was introduced to include Coos & Curry
Counties in OR and Del Norte County in CA. The lion's share of the
rainfall expected in this Day 2 period will be from the evening
through much of the overnight. Since the AR will remain relatively
stationary through this period, expect widespread 4-6 inch rainfall
amounts. This will be added to the rainfall expected today into
tonight and well-above-average river levels already established in this area.

...Lower Mississippi Valley into the Southeast...

A swath of heavy rain is likely across portions of the Southeast
into the TN valley on Saturday into Saturday night. A strong
shortwave at the base of the longwave trough will eject eastward
into the area, accompanied by a strong upper jet. These two features
support robust divergence aloft and support a widespread convective
threat. The system as a whole should remain progressive, however as
low level moisture transport increases there does appear to be a
window for some training/backbuilding of convection. Overall this
looks to be a widespread 1-2" event, however localized swaths of
heavier amounts are likely. Given the magnitude of mid/upper forcing
in place, combined with PWs increasing over the 90th percentile for
late December, and sufficient upstream instability forecast...it
seems plausible that we could see localized swaths of 3-4".

As yesterday, it appears with some instability and potential for
training convection that there remains a reasonable likelihood that
somewhere in the Marginal Risk area that Slight Risk impacts will
materialize. Unfortunately, guidance remains all over the map as to
where that potential will be realized, from the mountains of the
western Carolinas to central Louisiana. Given last evening's
rainfall and several Flash Flood Warnings, it seemed prudent to
expand the inherited Marginal south and west to cover much of
Louisiana. While the heaviest totals remain most likely over
northern MS/AL and TN, the dry antecedent soil conditions should
preclude more than isolated Slight Risk impacts...so the large
Marginal Risk remains for this update.

Wegman/Chenard

Day 3 Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

The A.R. moving into the Northwest will weaken through Sunday
morning. Thus, rainfall rates should diminish with time. Since the
bulk of the Day 2/Saturday A.R. will occur overnight Saturday night,
much of the impacts of the A.R. will be felt with the diminishing
rainfall on Sunday morning. Thus, for consistency both in messaging
and the timing of the worst impacts, a small Slight Risk area was
introduced with this update for the southwest corner of Oregon and
the northwest corner of California. This area will be the hardest
hit area through Saturday night, so the continued rainfall into
Sunday morning will have the greatest impacts in this area.
Otherwise, expect continued rainfall all up and down the Pacific
Northwest, with high elevation snow.

...Mid-Atlantic...

A mature low over the Midwest will track north up the Mitten of
Michigan and into Ontario by Monday morning. Ahead of the low's cold
front, a plume of subtropical moisture will stretch up the entire
Eastern Seaboard. The combination of the moisture and much warmer
temperatures will support widespread light to moderate rain over
much of the Mid-Atlantic. While much of the Mid-Atlantic has been
dry, especially south of the Mason Dixon line, some upslope may help
wring out the moisture along the I-81 corridor through New York and
the I-95 corridor from Richmond through NYC. Any localized training
may cause widely scattered instances of flash flooding.

Wegman
$$
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FOUS30 KWBC 280815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sat Dec 28 2024

Day 1
Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF EASTERN
TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH...

...Northwest...

A front will be in the process of gradually stalling out in a west
to east fashion this morning across areas of southwest OR and
northwest CA as the deeper layer Pacific flow becomes parallel to
the boundary. This will maintain atmospheric river conditions today
across the coastal ranges which will be rather modest at least for
this morning. However, the last of a series of offshore shortwaves
embedded within the deeper layer Pacific flow should approach
later today and cross the region tonight which will yield a strong
resurgence of IVT magnitudes that should approach and locally
exceed 750 kg/m/s. This will be driven by shortwave amplification
and the development of a new area of surface low pressure which
will move into the Pacific Northwest by early Sunday morning.
Rainfall rates are expected to increase once again as the strong
warm air advection pattern ahead of the shortwave combine with the
stronger low to mid-level moisture transport into the terrain. The
00Z HREF guidance supports rainfall rates rising to locally over a
0.50"/hour, but they will tend to peak along and just ahead of the
arrival of the offshore cold late tonight. Some stronger convective elements/heavy showers are suggested in the HREF guidance with
passage of the cold front closer to 12Z/Sunday and this may foster
some spotty rainfall rates peaking into the 0.75" to 1.0"/hour range.

This latest atmospheric river will then weaken as the front moves
inland, but additional rainfall amounts of as much as 3 to 6+
inches are expected, with the heaviest amounts likely tending to be
in vicinity of Coos, Curry and Del Norte Counties involving the
favored coastal ranges of southwest OR and northwest CA. Lesser
amounts are expected to advance farther south down the coast, with
some locally heavy rains likely making it as far south as the north
side of the Bay Area Sunday morning along with the northern Sierra
Nevada. Overall, given the wet antecedent conditions and high
streamflows, these additional rains are likely to increase the
flooding threat on area streams and tributaries and thus the Slight
Risk area is maintained for the coastal ranges. The Marginal Risk
area for this update was tweaked and adjusted a bit farther south
to account for some heavy rainfall getting closer to the
aforementioned Bay Area.

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

A well-defined and organized outbreak of strong convection is
expected to begin this morning across portions of central and
especially eastern TX as a strong shortwave trough amplifies
across the southern Plains and drives cyclogenesis from west to
east while interacting with an increasingly strong southerly low-
level jet. A substantial pool of warm-sector instability with
MLCAPE values of 1500 to 2500 J/kg has already pooled across south-
central to southeast TX and stretching over into southern LA. This
will be lifting north through midday as a warm front lifts north
ahead of the approaching surface low. Strong convection with
widespread concerns for supercell thunderstorms will evolve by
midday and through this afternoon across eastern TX with impacts
overspreading the Arklatex and broader Lower MS Valley by this
evening. The latest HREF model consensus supports the low-level jet
increasing to as much as 40 to 60+ kts across far eastern TX, LA,
southern AR and into western and northern MS by 00Z. As this
unfolds, the aforementioned warm front attempting to lift north
ahead of the surface low should become a focus for widespread
convection with potentially multiple rounds of very heavy rainfall
impacting the same area. Given the level of moisture and
instability transport coupled with strong shear, the stronger and
more organized convective cells will be capable of producing
rainfall rates of 1.5 to 2.5 inches/hour. Concerns for cell-mergers
and cell-training will exist and by this evening, some regional
swaths of 2 to 4 inches of rain with isolated 5+ inch totals will be possible.

Going into the overnight hours, the strong shortwave energy should
take on a bit of a negative tilt as it approaches the Mid-South and
aims for the OH Valley. This will drive the strong low-level jet
up through large areas of MS, AL, TN and nosing up toward the OH
River by early Sunday morning. Strong bands of heavy showers and
thunderstorms with a large degree organization should impact the
region, with a QLCS evolution expected to generally unfold across
much of the Mid-South going through the overnight hours given
enhanced deep layer forcing/shear and robust moisture and
instability transport. Localized swaths of 2 to 4 inches of rain
will also be possible across the Mid-South where any cell-training
can take place.

Given the wet antecedent conditions and elevated streamflows
across much of eastern TX and the Lower MS Valley, scattered to
locally numerous areas of flash flooding are expected to evolve,
and some of the flash flooding may be locally significant. The
flash flood threat will be a bit more conditional downstream across
the Mid-South and especially areas farther north into the OH
Valley, but given the heavy rainfall potential, the threat of flash
flooding will certainly exist. Given the wetter trend in the 00Z
hires model CAMS and broader convective outbreak that is expected,
a much more expansive Marginal Risk has been accommodated along
with a large Slight Risk area involving eastern TX, the Lower MS
Valley and basically all of the Mid-South. It cannot be ruled out
that a need for a targeted Moderate Risk upgrade will exist with
the next cycle update across some portions of the Slight Risk area.

Orrison

Day 2
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

The A.R. moving into the Northwest will weaken through Sunday
morning. Thus, rainfall rates should diminish with time. Since the
bulk of the Day 1/Saturday A.R. will occur overnight Saturday night,
much of the impacts of the A.R. will be felt with the diminishing
rainfall on Sunday morning. Thus, for consistency both in messaging
and the timing of the worst impacts, the Slight Risk area was
continued with this update with few changes for the southwest corner
of Oregon and the northwest corner of California. Otherwise, expect
continued rainfall all up and down the Pacific Northwest, with high
elevation snow.

...Mid-Atlantic...

A mature low over the Midwest will track north up the Mitten of
Michigan and into Ontario by Monday morning. Ahead of the low's cold
front, a plume of subtropical moisture will stretch up the entire
Eastern Seaboard. The combination of the moisture and much warmer
temperatures will support widespread light to moderate rain over
much of the Mid-Atlantic. While much of the Mid-Atlantic has been
dry, especially south of the Mason Dixon line, some upslope may help
wring out the moisture along the I-81 corridor through New York and
the I-95 corridor from Richmond through NYC. Any localized training
may cause isolated instances of flash flooding, especially should
any training of heavier rain occur over flood-sensitive and urban areas.

Wegman

Day 3
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 281950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sat Dec 28 2024

Day 1 Valid 16Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF EASTERN
TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH...

...16Z update...

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

Complex and messy evolution of convection is underway across
northeastern TX into the Lower MS Valley this morning. Increasing
low level winds ahead of an approaching potent mid-level shortwave
trough will allow for increasing instability from the TX/LA border
into and across the Lower MS Valley during the day. The
orientation of a warm front from northeastern TX into northern MS
(SW to NE) matches mean steering flow from the SW which will
promote areas of training heavy rain. Forcing for ascent will
continue to increase across these same areas ahead of the shortwave
trough and with increasing upper level jet divergence/diffluence.
Storm scale evolution remains a bit uncertain with the latest 12Z
HREF in disagreement with the exact placement of a stripe of 3 to
6+ inches of rain from northeastern TX into northern LA and
northern MS, which precludes an upgrade to Moderate but within that
zone of expected training, Moderate-type impacts could be
observed, especially given overlap with potentially sensitive
grounds due to recent rainfall and/or urban environments.

...Northwest...
No significant changes were made to the Marginal and Slight Risks
in place across the West Coast with similar thinking in the
previous discussion still holding true. Ongoing steady rain into
western OR and northwestern CA will continue during the day with an
expected increase in precipitation intensity near 00Z ahead of an
approaching Pacific cold front. The main area of concern is across
the northern CA/southern OR coast where heavy rain has been
observed over the past week or so and adding an additional 3 to 5
inches through 12Z Sunday (locally higher possible), where what is
essentially a higher end Slight Risk is in place.

Otto

...previous discussion follows...

...Northwest...

A front will be in the process of gradually stalling out in a west
to east fashion this morning across areas of southwest OR and
northwest CA as the deeper layer Pacific flow becomes parallel to
the boundary. This will maintain atmospheric river conditions today
across the coastal ranges which will be rather modest at least for
this morning. However, the last of a series of offshore shortwaves
embedded within the deeper layer Pacific flow should approach
later today and cross the region tonight which will yield a strong
resurgence of IVT magnitudes that should approach and locally
exceed 750 kg/m/s. This will be driven by shortwave amplification
and the development of a new area of surface low pressure which
will move into the Pacific Northwest by early Sunday morning.
Rainfall rates are expected to increase once again as the strong
warm air advection pattern ahead of the shortwave combine with the
stronger low to mid-level moisture transport into the terrain. The
00Z HREF guidance supports rainfall rates rising to locally over a
0.50"/hour, but they will tend to peak along and just ahead of the
arrival of the offshore cold late tonight. Some stronger convective elements/heavy showers are suggested in the HREF guidance with
passage of the cold front closer to 12Z/Sunday and this may foster
some spotty rainfall rates peaking into the 0.75" to 1.0"/hour range.

This latest atmospheric river will then weaken as the front moves
inland, but additional rainfall amounts of as much as 3 to 6+
inches are expected, with the heaviest amounts likely tending to be
in vicinity of Coos, Curry and Del Norte Counties involving the
favored coastal ranges of southwest OR and northwest CA. Lesser
amounts are expected to advance farther south down the coast, with
some locally heavy rains likely making it as far south as the north
side of the Bay Area Sunday morning along with the northern Sierra
Nevada. Overall, given the wet antecedent conditions and high
streamflows, these additional rains are likely to increase the
flooding threat on area streams and tributaries and thus the Slight
Risk area is maintained for the coastal ranges. The Marginal Risk
area for this update was tweaked and adjusted a bit farther south
to account for some heavy rainfall getting closer to the
aforementioned Bay Area.

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

A well-defined and organized outbreak of strong convection is
expected to begin this morning across portions of central and
especially eastern TX as a strong shortwave trough amplifies
across the southern Plains and drives cyclogenesis from west to
east while interacting with an increasingly strong southerly low-
level jet. A substantial pool of warm-sector instability with
MLCAPE values of 1500 to 2500 J/kg has already pooled across south-
central to southeast TX and stretching over into southern LA. This
will be lifting north through midday as a warm front lifts north
ahead of the approaching surface low. Strong convection with
widespread concerns for supercell thunderstorms will evolve by
midday and through this afternoon across eastern TX with impacts
overspreading the Arklatex and broader Lower MS Valley by this
evening. The latest HREF model consensus supports the low-level jet
increasing to as much as 40 to 60+ kts across far eastern TX, LA,
southern AR and into western and northern MS by 00Z. As this
unfolds, the aforementioned warm front attempting to lift north
ahead of the surface low should become a focus for widespread
convection with potentially multiple rounds of very heavy rainfall
impacting the same area. Given the level of moisture and
instability transport coupled with strong shear, the stronger and
more organized convective cells will be capable of producing
rainfall rates of 1.5 to 2.5 inches/hour. Concerns for cell-mergers
and cell-training will exist and by this evening, some regional
swaths of 2 to 4 inches of rain with isolated 5+ inch totals will be possible.

Going into the overnight hours, the strong shortwave energy should
take on a bit of a negative tilt as it approaches the Mid-South and
aims for the OH Valley. This will drive the strong low-level jet
up through large areas of MS, AL, TN and nosing up toward the OH
River by early Sunday morning. Strong bands of heavy showers and
thunderstorms with a large degree organization should impact the
region, with a QLCS evolution expected to generally unfold across
much of the Mid-South going through the overnight hours given
enhanced deep layer forcing/shear and robust moisture and
instability transport. Localized swaths of 2 to 4 inches of rain
will also be possible across the Mid-South where any cell-training
can take place.

Given the wet antecedent conditions and elevated streamflows
across much of eastern TX and the Lower MS Valley, scattered to
locally numerous areas of flash flooding are expected to evolve,
and some of the flash flooding may be locally significant. The
flash flood threat will be a bit more conditional downstream across
the Mid-South and especially areas farther north into the OH
Valley, but given the heavy rainfall potential, the threat of flash
flooding will certainly exist. Given the wetter trend in the 00Z
hires model CAMS and broader convective outbreak that is expected,
a much more expansive Marginal Risk has been accommodated along
with a large Slight Risk area involving eastern TX, the Lower MS
Valley and basically all of the Mid-South. It cannot be ruled out
that a need for a targeted Moderate Risk upgrade will exist with
the next cycle update across some portions of the Slight Risk area.

Orrison
$$
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FOUS30 KWBC 290802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Sun Dec 29 2024

Day 1
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF SOUTHWEST OREGON AND NORTHWEST CALIFORNIA ALONG WITH THE
FOOTHILLS OF THE NORTHERN SIERRA NEVADA...

...Western Oregon and Northern California...

The tail end of a fairly strong atmospheric river will be impacting
especially southwest OR and northern CA this morning as a shortwave
trough and associated wave of low pressure advances into the
Pacific Northwest with a trailing cold front extending well to the
south and gradually moving inland across northern CA. IVT
magnitudes along and just ahead of the cold front will be on the
order of 750 to 1000 kg/m/s across the northwest CA coastal ranges
including Del Norte and Humboldt Counties. Lesser magnitudes will
be advancing inland with the front into the Shasta/Siskiyou ranges
and the northern Sierra Nevada. The IVT values should come down by
18Z, but sufficient levels of onshore flow, convergence along the
trailing front, and orographic ascent over the terrain should favor
areas of heavy rain lingering well into the 18Z to 00Z time frame
across the foothills of the northern Sierra Nevada. The 00Z HREF
guidance favors 0.50"+/hour rainfall rates at least occasionally
impacting these areas, and the threat of heavy rain will also
envelope the Bay Area where the coastal ranges here may also see
some occasionally enhanced rainfall at least briefly with rates
exceeding a 0.50"/hour before the front clears the area. For the
period, some additional 2 to 3 inch rainfall amounts will be
possible locally across the coastal ranges of southwest OR and
northwest CA, and also separately in the foothills of the northern
Sierra Nevada by tonight. A Slight Risk of excessive rainfall has
depicted across these areas, and is an introduction in the case of
the northern Sierra Nevada foothills. Wet antecedent conditions
along with high streamflow will encourage there being at least a
small stream and urban flooding threat, and there may be a low-end
flash flood threat involving the more sensitive burn scar locations
where these heavier rains persist. Please consult MPD #1204 for
more details concerning the near-term heavy rainfall/flooding
concerns across the region.

...Central/Southern Appalachians into the Southeast...

An early-morning QLCS will be crossing through the eastern Gulf
Coast region and the broader interior of the Southeast as a strong
southerly low-level jet of 40 to 60 kts drives enhanced moisture
transport along with a nose of favorable instability just ahead of
an advancing cold front. This will be occurring as a surface low
and associated mid to upper-level trough rides northeastward up
into the Great Lakes region. The QLCS activity should at least clip
portions of the southern Appalachians and then advance across the
Piedmont of the Southeast and eventually the coastal plain. Some
of the more urbanized locations that see this QLCS passage may see
a quick 1 to 1.5 inches of rain in less than an hour, and this may
result in some urban runoff problems with a highly isolated threat
of flash flooding. In general, some of the moist flow into the central
and southern Appalachians this morning should favor at least some
locally heavy rainfall totals that may reach upwards of a couple
inches before the cold front clears the region. A Marginal Risk of
excessive rainfall has been depicted across all of these areas.

...South Florida and the Keys...

The latest GOES-E IR satellite imagery shows a cluster of very
heavy showers and thunderstorms over the southeast Gulf of Mexico
in association with a low-level warm front that is attempting to
advance northeastward toward the southern FL Peninsula. The
convection has been focusing in close proximity to a well-defined
instability gradient and there has been some well-organized
convective cells with some mesocyclone characteristics evolving
northwest of the lower FL Keys over the last few hours as seen in
the Key West radar. Deep moisture and elevated instability has
already been favoring very high rainfall rates, but the challenge
for today will be whether or not any of this organized convection
can advance bodily inland across South FL. The 00Z HREF model
consensus, 00Z NSSL MPAS guidance, and 06Z HRRR suggests Collier
and Monroe Counties will see heavy rainfall this morning as this
warm front pushes eastward at least toward the coast while also
lifting up through the FL Keys. It is possible that Miami-Dade
County could get into at least some peripheral bands of showers and thunderstorms as well, some of which will be capable of producing
heavy rainfall rates of 1 to 2 inches/hour. This would certainly
introduce concerns for some urban flash flooding. Given the
mesoscale nature of this setup, there is a concern for locally very
heavy rainfall totals (possibly 5+ inches over parts of Collier
and Monroe Counties and portions of the Keys). Given the level of
uncertainty with the coverage and the amounts, a Marginal Risk will
be depicted for the time being, but it would not be a stretch that
a targeted upgrade to a Slight Risk will be needed this morning to
address the threat for at least some areal flooding and
potentially urban flash flooding.

Orrison


Day 2
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Tue Dec 31 2024 - 12Z Wed Jan 01 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 310802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 31 2024

Day 1
Valid 12Z Tue Dec 31 2024 - 12Z Wed Jan 01 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA...

A strong front associated with a compact low will approach and move
into the Pacific coast of Oregon and California late Thursday
night. IVT values from the GEFS peak at about 600 kg/ms around 12Z
Friday. While river levels have come down a bit from the recent
rainfall and atmospheric river event, expected rainfall on Day
2/Wednesday should bring levels back up and resaturate the soils.
Thus, with this next round Thursday night, the expected 2-4 inches
of rain could still cause isolated instances of flash flooding.
The front does not have a true connection to the tropics, so this
round of rain is expected to be of lighter intensity and lesser
duration than the previous A.R. event.

Wegman
$$
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FOUS30 KWBC 010752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Wed Jan 1 2025

Day 1
Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA...

The 00Z HREF guidance has come in with a somewhat wetter footprint
for today across areas of southwest Oregon and northwest California
as a new round of offshore shortwave energy and proximity of a
front helps to bring in a new round of moderate to heavy rainfall.
The GFS and ECMWF solutions suggest IVT values increasing to near
500 kg/m/s along the immediate coast from Curry County, Oregon down
through Del Norte and Humboldt Counties in northwest California.
Offshore CMORPH2/microwave data shows heavier rainfall rates
associated with this next modest surge in atmospheric river
activity that will be arriving, and the HREF guidance suggests
rainfall rates will generally peak in the 0.25" to 0.50"/hour range
with perhaps an occasional instance or two of 0.50"+/hour rates.
Going through 12Z/Thursday, the HREF consensus suggests some
24-hour rainfall amounts of 2 to 4 inches with a couple of spotty
5+ inch amounts possible, and especially for northwest California
where some of the higher IVT values will persist a bit longer. The
antecedent conditions are wet across the region, and these rains
may cause some isolated runoff concerns. As a result, a Marginal
Risk of excessive rainfall has been depicted for this area.

Orrison


Day 2
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

Multiple impulses of upper level energy racing along a 150 kt WSW
jet will move into the Oregon and California coast near the state
line during this Day 2/Thursday period. The first is a front that
will have already moved into the area tonight, and will diminish in
intensity and slowly shift north through the early morning. A second
impulse moves in Thursday afternoon, resulting in a much more
concentrated area of rain about 50 miles or so to either direction
of the state line. Finally, a third and the strongest impulse will
move in late Thursday night and Friday morning. Each of these 3
impulses of energy will cause rainfall rates to pick up on the
Oregon and far northern California coast. None of them are expected
to advect in much instability, so upslope will likely be the primary
forcing mechanism for rainfall. With nearly continuous rainfall,
albeit to varying intensity with each impulse over already very
saturated ground, runoff and small stream flooding will be
possible. Since the heavier/steadier rainfall will be ongoing at
the start of the period over northern California, and guidance has
been creeping up as far as total rainfall amounts go, the Marginal
Risk extends south into Humboldt County to account for continuing
rainfall and isolated runoff problems that may be ongoing from the
Day 1/Wednesday period.

Wegman


Day 3
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A strong shortwave and associated surface low will plow into the
coast of California and Oregon on Friday. Locally heavy rain with
rates around 1/2 inch per hour will be ongoing Friday morning along
the coast. The shortwave and low will move into the coast Friday
evening and through the overnight. This will shift the heaviest
rains both southward and inland, bringing northern portions of the
Sacramento Valley, which has been largely missed with the previous
days' rains into the potential for heavy rain and higher elevation
snow. The Marginal Risk inherited was left unchanged with this
issuance. The heaviest rains are likely to be Friday morning as the aforementioned shortwave and surface low approach. Once they move
into the coast Friday evening and overnight, rainfall rates should
diminish quite a bit as the associated moisture moves inland. Any
flooding from the Day 2/Thursday period will likely continue into
Friday morning along the Oregon and California coasts...with the
flooding for the Sacramento Valley more likely Friday afternoon.

Wegman
$$
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FOUS30 KWBC 020705
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
205 AM EST Thu Jan 2 2025

Day 1
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

A relatively weak, long duration atmospheric river will continue
to impact portions of the Pacific Northwest coast today, shifting
focus from northwest California into far southwest Oregon. While
there is a distinct lack of instability to fuel rainfall rates
beyond 0.25"/hr, tropospheric moisture is still rather elevated
with precipitable water values of 1.0-1.2" (2.0-2.5+ standard
deviations, near the 95th percentile). Two distinct rounds of
precipitation (one from late this morning through mid afternoon,
and another round from late evening through the overnight) are
expected to bring additional rainfall of 3-5" (primarily for Del
Norte county, CA and Curry county, OR). While rapid onset flooding
is generally not expected (given the muted rainfall rates), these
areas (and surroundings) have already received 1-2" over the past
24 hours with USGS streamflow data indicating widespread above
normal to much above normal streamflows (with some even at record
territory for the date), suggesting additional rainfall may lead to
localized flooding impacts.

Churchill


Day 2
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A strong shortwave and associated surface low will plow into the
coast of California and Oregon on Friday. Locally heavy rain with
rates around 1/2 inch per hour will be ongoing Friday morning along
the coast. The shortwave and low will move into the coast Friday
evening and through the overnight. This will shift the heaviest
rains both southward and inland, bringing northern portions of the
Sacramento Valley, which has been largely missed with the previous
days' rains, into the potential for heavy rain and higher
elevation snow. The Marginal Risk inherited was left unchanged with
this issuance. The heaviest rains are likely to be Friday morning
as the aforementioned shortwave and surface low approach. Once they
move into the coast Friday evening and overnight, rainfall rates
should diminish quite a bit as the associated moisture moves
inland. Any flooding from the Day 2/Thursday period will likely
continue into Friday morning along the Oregon and California
coasts...with any minor flooding for the Sacramento Valley more
likely Friday afternoon.

Wegman


Day 3
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 030830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Jan 3 2025

Day 1
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A long duration atmospheric river has resulted in 2-4" rainfall
totals over portions of far northwestern CA over the past several
days, with 1-3" occurring over the past 48 hours (extending into
portions of far southwestern OR as well). A final push of low-level
moisture transport is beginning to reach the coastline early this
morning, as a potent shortwave and associated surface cyclone
brings the most significant source of uplift yet (as well as
elevated instability with the HREF indicating MU CAPE of 250-500
J/kg in association with height falls aloft). The shortwave and
low will move into the coast this evening and through the
overnight shifting, shifting the heaviest rainfall both southward
and inland with time (bringing northern portions of the Sacramento
Valley, which has been largely missed with the previous days'
rainfall, into the Marginal risk for excessive rainfall). Areas
most at-risk for rainfall rates exceeding 0.25"/hr (which can
become problematic for particularly sensitive terrain, such as
burn scars) are along the upslope portions of the Coast Range,
Klamath Mountains, Cascade Range, into the northern Sierra Nevada
(per HREF Ensemble Agreement Scale, 10-100 km, neighborhood
probabilities of 60-90% for 1-hr, 0.25" exceedance). Additional
24-hr totals of 1-3" are expected (with a slight majority of the
rainfall occurring prior to 00z).

Churchill


Day 2
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 3
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone is anticipated to develop over
the central US into Day 3 (originating from the aforementioned
shortwave impacting the Pacific Northwest on Day 1), as strong DPVA
via a digging shortwave (cutting off into an upper low) interacts
with the right-entrance region of a broad phased jet structure
(~130 kt jet streak @ 250 mb over the Mid-Atlantic). A substantial
low-level jet (850 mb of 40-50 kts) is expected to form as a
result, ushering in highly anomalous (for this time of year)
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While
downscaled global guidance (GFS/ECMWF) suggests only localized 1-2"
totals at this stage, the impressive dynamics of this system
(along with the anomalous moisture) suggest the potential for this
guidance to be exceeded (as hi-res CAMs will shed a light on as
the event gets closer in range). That said, both the progressive
nature of the system and a distinct lack of instability should be
limiting factors for organized heavy rainfall. Maintained an
inherited Marginal risk area for much of the Southeast (with some
expansion towards the central Gulf Coast, where instability will be
more abundant) encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad
QPF exists, as this precipitation will mostly occur in the form of
ice and snow).

Churchill

$$
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FOUS30 KWBC 040815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sat Jan 4 2025

Day 1
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone will develop over the central US
into Day 2, originating from a potent shortwave trough currently
digging into the Four Corners region. The trough is expected to
cut-off into an upper-low as it emerges into the central/southern
Plains with strong DPVA interacts with the right-entrance region
of a broad phased jet structure (~130 kt jet streak @ 250 mb over
the Mid-Atlantic, and a smaller ~100 kt jet streak near the Ozarks).
A substantial low-level jet (850 mb of 40-60 kts) is expected to
form as a result of deep cyclogenesis, ushering in highly
anomalous tropospheric moisture (for this time of year) with
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While
downscaled global guidance (GFS/ECMWF) continues to suggest only
localized 1-2" totals, the impressive dynamics of this system
(along with the anomalous moisture) suggest the potential for this
guidance to locally be exceeded (as the 00z FV3 suggests, but
additional CAMs should better capture the even with the upcoming
12z runs). That said, both the progressive nature of the system and
the limited window of time in the afternoon to take advantage of destabilization (with SB CAPE from east TX and southern LA to the
Mid-South reaches 500-2000 J/kg) are expected limit localized
totals to the 2-3" range (with as much as 1-2" in an hour with the
main line of convection). Have maintained the inherited Marginal
risk area for much of the Southeast (with some additional expansion
towards the southwest, where instability will be most abundant)
generally encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad
QPF exists, as this precipitation will mostly occur in the form of
ice and snow).

Churchill


Day 3
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 050815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sun Jan 5 2025

Day 1
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone is taking shape over the central
US early this morning, originating from a potent shortwave trough
digging through the Central Rockies over the past couple of days.
The trough is in the process of cutting off into an upper-low as
it emerges from the Rockies into the central Plains. Strong DPVA
in association with the trough will interact with the right-
entrance region of a broad phased jet structure (~140 kt jet
streak @ 250 mb over the Mid-Atlantic, and a smaller ~100 kt jet
streak near the Ozarks). A substantial low-level jet (850 mb of
40-60 kts) is expected to form as a result of deep cyclogenesis,
stretching from the Central Gulf Coast to the Mid-South, ushering
in highly anomalous tropospheric moisture (for this time of year)
with precipitable water values nearing 1.5" (between the 90th
percentile and max moving average, per JAN/BNA/BMX sounding
climatology). Most hi-res CAMs continue to suggest localized 1-2"
totals in association with combined totals from scattered
convection in the open warm sector and the following primary squall
line in association with the approaching cold front, though a
couple of solutions (FV3 and CMCreg) suggest localized streaks of
2-3" totals (from northeast LA through north/central MS into
northwest AL). While much of these totals may occur over a
relatively short period (with any flash flooding conditions more
likely due to storm mode and sub-hourly rates and totals of 1-2"),
00z HREF 40-km neighborhood probabilities for 2" exceedance over 3
hours barely peak at 20-30%. Therefore the progressiveness of this
system (and the limited window of time in the afternoon to take
advantage of destabilization, with SB CAPE from east TX and
southern LA to the Mid-South reaching 500-2000 J/kg) should largely
preclude any organized flash flood threat. Have maintained the
inherited Marginal risk area once again, generally encompassing the
expected warm sector of the system (while excluding areas farther
north where more impressive broad QPF exists, as this precipitation
will mostly occur in the form of ice and snow).

Churchill


Day 2
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 3
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 052013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 PM EST Sun Jan 5 2025

Day 1
Valid 16Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

...16z update...
Small adjustments were made to the broad Marginal Risk area based
on guidance trends and 12z Hi-Res/HREF probability output. 12z HREF
continues to show a solid high probability axis with the convective
line for 1"/hr or 1"/3hr (over 90%). However, the forward speed of
cells continues to quickly diminish the potential for high overall
totals, as 2"/3hr values only peak at 30% in west-central MS,
eventually expanding into northern MS and mainly in the 18-00z time frames.

Based on the evolution from the Hi-Res CAMs, the highest potential
is across this axis, with some lower signals further south and west
into central LA and far SE TX though higher FFG values in that
region would have lower potential of exceedance anyway.
Observational trends and rapid refresh guidance from the HRRR/RAP
denote the core of surface to 850mb moisture and flux is generally
parallel to the approaching front and about 25-50 miles downstream.
Increasing insolation and steepening lapse rates will allow for the
instability axis to align with the moisture, that pre-cursory
convection is likely to develop in the 15-18z time frame across SE
TX into central LA, ahead of the developing convection noted in W
AR/NE TX attm. Eventually, the gap between closes and
streams/ascent merge for the potential for some very short-term
enhancement of totals to 1-2" ranges. This axis does align with an
area of recent dryness compared to locations further west over the
last 7-10 days per AHPS...further diminishing the need for a Slight
Risk area; though central/northeast LA into northern MS will be
the area of greatest potential for any incidents of localized flash
flooding concerns.

Elsewhere, the warm advection across the mid-MS Valley has trended
a bit northward and some of these cells may reach SE MO/W KY where
grounds may have some ice in place. As such, the Marginal Risk was
lifted a bit northward with this update. Equally, trends with
convection across central LA into S MS and slightly slower cold
front pressing eastward after 06z; has warranted a bit further
trimming of the eastern side of the Marginal Risk in AL.

Gallina

---Prior Discussion---

An impressive deep layer cyclone is taking shape over the central
US early this morning, originating from a potent shortwave trough
digging through the Central Rockies over the past couple of days.
The trough is in the process of cutting off into an upper-low as
it emerges from the Rockies into the central Plains. Strong DPVA
in association with the trough will interact with the right-
entrance region of a broad phased jet structure (~140 kt jet
streak @ 250 mb over the Mid-Atlantic, and a smaller ~100 kt jet
streak near the Ozarks). A substantial low-level jet (850 mb of
40-60 kts) is expected to form as a result of deep cyclogenesis,
stretching from the Central Gulf Coast to the Mid-South, ushering
in highly anomalous tropospheric moisture (for this time of year)
with precipitable water values nearing 1.5" (between the 90th
percentile and max moving average, per JAN/BNA/BMX sounding
climatology). Most hi-res CAMs continue to suggest localized 1-2"
totals in association with combined totals from scattered
convection in the open warm sector and the following primary squall
line in association with the approaching cold front, though a
couple of solutions (FV3 and CMCreg) suggest localized streaks of
2-3" totals (from northeast LA through north/central MS into
northwest AL). While much of these totals may occur over a
relatively short period (with any flash flooding conditions more
likely due to storm mode and sub-hourly rates and totals of 1-2"),
00z HREF 40-km neighborhood probabilities for 2" exceedance over 3
hours barely peak at 20-30%. Therefore the progressiveness of this
system (and the limited window of time in the afternoon to take
advantage of destabilization, with SB CAPE from east TX and
southern LA to the Mid-South reaching 500-2000 J/kg) should largely
preclude any organized flash flood threat. Have maintained the
inherited Marginal risk area once again, generally encompassing the
expected warm sector of the system (while excluding areas farther
north where more impressive broad QPF exists, as this precipitation
will mostly occur in the form of ice and snow).

Churchill


Day 2
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 070732
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
232 AM EST Tue Jan 7 2025

Day 1
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

Day 2
Valid 12Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

Highly anomalous upper trough pattern across the Western CONUS will
help generate a distinct shortwave ejection evolution that will
create a litany of issues across TX into the Lower Mississippi
Valley. Upper low expected to be centered over Sonora the prior
period will open as it advances eastward with a second shortwave
digging around the backside of the mean trough creating a fairly
dynamic setup across the Southern Plains. A strong diffluent axis
of ascent ahead of the ejecting trough will create a blossoming of precipitation over TX, spreading eastward as the trough migrates
through the Lone Star State. Surface low pressure will spawn in-of
the western Gulf with the low center generally close to the TX
coastal plain, opening the door for a modest instability gradient
to align within the immediate TX coast up to southwest LA as the
pattern progresses. Convective regime will be most prominent along
the coast where forecast theta_E's will slowly climb just above
seasonal norms allowing for a focused area of surface instability
along with the already favorable mid and upper level dynamics.

A cluster of thunderstorms will likely form over the western Gulf
within the confines of the surface reflection lending a better
potential for convection to move inland with a heavy rain footprint
tied to any storm moving onshore. There will be a sharp northern
extent of how long any convective regime can last as much colder to
the north of the coast will limit surface based instability and
thwart the opportunity for convection to march inland. As of now,
the forecast MRGL risk inherited from D4 was relatively maintained,
although the northern periphery was trimmed back given the
lackluster instability fields and relatively progressive storm
motion on the northern fringe of the best instability coupled with
ascent. The most favorable locations for rates that could generate
enough support for flash flooding potential is still relegated to
the immediate Gulf coast with a small buffer further inland if any
cells can motion past the immediate coastal plain and affect some
of the urban areas extending from CRP up through HOU and LCH.

Kleebauer

$$
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FOUS30 KWBC 080803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Jan 8 2025

Day 1
Valid 12Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Large scale ascent ahead of an open, yet vigorous upper trough
will help initiate surface cyclogenesis over the western Gulf with
a modest push of warm unstable air in-of the TX coast, eventually
into southwestern LA. Current guidance is coming into agreement on
a skinny axis of heavier rainfall near the middle and upper TX
coasts propagating east-northeast into the Lower Sabine by the end
of the period with local amounts of 2-4" plausible within the
heavier rain footprint. The driving component of the potential is
still very much the increasing frictional convergence pattern on
the western flank of the surface low as easterly winds bisect the
TX coast up into LA with the core of better theta_E's remaining at
the immediate coast and over the Gulf waters. The key is the
onshore flow mechanism taking some of the convective cores offshore
and moving them inland causing some heavier rainfall to protrude
inland before running into a wall of cooler, more stable air thanks
to an Arctic push that occurred prior to the evolving surface pattern.

Recent NBM prob fields are >50% for locally 2+" of rainfall within
some of the urban footprint in coastal TX which allows the
opportunity for local flood concerns within those urbanized zones
thanks to higher runoff potential. The saving grace from this being
a higher risk is the agreement on minimal surface based
instability with the mean SBCAPE in relevant ensembles hovering
between 50-100 J/kg with some deterministic maybe exceeding that
at the immediate coast. With the cells likely to struggle being
rooted at the surface, the prospects of more widespread flash
flooding are lower than normal creating a threat that is more
localized and within the lower to mid bounds of the MRGL risk
threshold. The previous forecast was generally maintained outside
some adjustment further on the northern periphery of the risk area,
aligning with the probability and mean instability fields.

Kleebauer


Day 3
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

Our favorable longwave trough and accompanying surface pattern will
move eastward with the surface low over the Gulf moving onshore
within LA by the first half of the period. Increasing surface
convergence within the periphery of the surface low will generate
an axis of heavier precipitation aligned within the modest
frontogenetic setup within the northern periphery of the surface
circulation. There's growing support for a heavy rain footprint
over south-central LA, moving eastward into southeastern LA where
the urban corridors centered around New Orleans come into play.

Like the prior period, the instability pattern is meager,
especially at the surface with most of the ascent driven within the
top of the boundary layer and above where the area lies in the
right-entrance region of the upper jet focused to the north. There
is enough consensus on the location of heaviest rainfall within the
that frontogenetic evolution, coinciding with a strong IVT pulse
(~1000 kg/ms) being depicted within the NAEFS, a solid 4+ standard
deviation interval for the time of year. This should help mitigate
the negligible instability pattern in place and maintain a solid
heavy rain footprint in that focal area mentioned above. Another
consideration is the much colder grounds occurring prior to the
arrival of the rainfall as Arctic air will navigate southward and
allow for a period of near and sub-freezing air to harden the top
soil layer creating a slightly higher potential for runoff.

The MRGL risk from D4 was maintained and expanded to include much
of south-central LA through southeast LA and coastal MS. There is a
potential for a targeted SLGT risk in future updates, mainly within
the corridor extending along and south of I-20 and just north of
the coastal Parishes as the best flood threat will exist within the
more urban zones.

Kleebauer

$$
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FOUS30 KWBC 090751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Thu Jan 9 2025

Day 1
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Increased mid and upper level forcing is ongoing over portions of
TX thanks to the slow ejection of a potent ULL situated over
Sonora. A digging shortwave trough analyzed over the Central
Rockies will continue plunging southward with an eventual partial
phase with the primary shortwave disturbance moving east into West
TX. At the surface, a maturing surface low over the western Gulf
will slowly wander northward towards the middle TX coast with
expected moisture advection regime to be a significant player in
the prospects of locally heavy rainfall in-of the immediate TX
coast. As of this time, there is a general consensus on the
heaviest precip being focused up the coast near Port Aransas up
through the remainder of the middle and upper TX coasts, pinning a
bullseye closer to the Galveston area and points northeast. This is where
a frictional convergence regime with persistent easterly flow on
the north side of the main surface low will help initiate a period
of convection just off the immediate coast within the primary axis
of higher theta_E's located on the western flank of the surface
reflection. Convection will hug the coast with some of the heavier
precip cores moving ashore creating opportunity for rainfall rates
to reach 1-1.5"/hr at peak intensity and producing totals between
2-3" with locally as high as 4" along that immediate stretch of
coast. The longwave evolution will generate ample ascent within the
diffluent axis downstream of the potent mean trough, pulling
moisture inland and generation pockets of heavy rain just inland,
but to less of a degree of the immediate coast thanks to the local
instability maximum likely hugging the coast with near 0 CAPE just
inland.

Recent probabilities from the 00z HREF are indicative of the
locally heavy rain threat with the neighborhood probs of >3" up
between 40-70% for the stretch of the TX coast beginning from Port
Aransas up the Lower Sabine where TX/LA border along the Gulf
coast. There's non-zero 5+" probs, but a much lower factor of
probabilities comparatively (<20%) and mainly confined to a small
area near Galveston. This has been the zone of heaviest precip
potential and likely strongest low-level convergence due to the
forecast proxy of the surface low and greatest theta_E advection
regime. Considering all the above variables, there was little need
to deviate from the previous forecast. The previous MRGL risk was
maintained given the steady forecast continuity and recent
favorable probability fields.

Kleebauer


Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer


Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

$$
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FOUS30 KWBC 100038
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
738 PM EST Thu Jan 9 2025

Day 1
Valid 01Z Fri Jan 10 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Increased mid and upper level forcing is ongoing over portions of
TX caused by a potent ULL moving towards the Gulf Coast, with a
surface low deepening near the Middle and Upper TX Coasts at the
time of this discussion's writing. Warm air/moisture advection is
occurring near a coastal front to the northeast of the cyclone
which is allowing hourly rain totals to range between 0.5-1" as of
late. A smidgen of mixed layer and most unstable CAPE is present
near and ahead of the low, which should increase a little more
tonight. When combined with increasing frontogenesis along the
coastal front and a cooler air mass, there is concern for
occasional organized convection as low-level inflow/effective bulk
shear is sufficient for such should enough instability be present.
Convective elements should try to build further along and near the
coastal plain/swamp/bayous in this region. While hourly rain totals
to 1.5" and local amounts to 4" remain in the cards, recent
dryness suggests that urban areas would be most impacted. IVT
values with this system top 1,000 kg/m/s presently, which is well
above the maximum observed IVT in the CFSR database for this time
of year and implies some upward potential for rainfall efficiency
despite the expectation of some forward progress with time. Some
eastward extension to the Marginal Risk area was made to align with
10%+ chance of 5"+ noted in the 18z HREF probabilities, and the
back edge carved back due to system progression.

Roth/Mullinax/Kleebauer


Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...

The inherited Marginal Risk remains in good shape as the region
will struggle to see a dearth of instability aloft and any
storms/showers will be progressive in their easterly movement.
That said, NAEFS shows that around 12Z Friday, IVT values aimed at
the central Gulf Coast will be as high as 1250 kg/m/s, which would
be above all observed IVT values in the CFSR database for this
time of year. Similar to the Upper Texas Coast on the Day 1 ERO,
soundings are highly saturated and warm cloud layers are around
12,000ft deep. This should result in efficient warm rainfall that
may support highly localized flash flooding potential, especially
in urbanized areas that drain poorly along the I-10 corridor. But
the progressive storm motions and lack of modest instability should
keep the areal extent and severity of the flash flood threat to a
limited number of at-risk urbanized areas through the early
afternoon hours on Thursday.

Mullinax

--Previous Discussion--

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer

Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Mullinax

$$
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FOUS30 KWBC 120829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sun Jan 12 2025

Day 1
Valid 12Z Sun Jan 12 2025 - 12Z Mon Jan 13 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Louisiana Coast...
A frontal wave spawned by an upper level disturbance moving between
a Southwest U.S. upper level low and a subtropical ridge near Cuba
is expected to bring precipitable water values up to 1.5" over
portions of the central Gulf Coast as early as late this afternoon
in response to low level flow becoming southwesterly at 20-35 kts.
Despite the moisture transport into the area...the overall risk of
excessive rainfall appears to be held in check by meager
instability and the progressive nature of the system. The 12/00Z
HREF guidance maintained continuity with the 11/12Z run that show
the probability of 0.5"+ totals don't persist anywhere along or
near the Louisiana coast for more than an hour or two. There was a
slight southward shift and a subtle decrease in rainfall amounts in
the guidance which resulted in decreasing neighborhood
probabilities for 2+ inch rainfall amounts compared with the
previous runs.

Bann


Day 2
Valid 12Z Mon Jan 13 2025 - 12Z Tue Jan 14 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Jan 14 2025 - 12Z Wed Jan 15 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 160757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Thu Jan 16 2025

Day 1
Valid 12Z Thu Jan 16 2025 - 12Z Fri Jan 17 2025


The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Fri Jan 17 2025 - 12Z Sat Jan 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 3
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

PW values expected to rise to 2 to 2.5 standard deviations above
the mean Saturday into early Sunday along and ahead of the arctic
frontal boundary pushing across the northeast Gulf into the
Southeast and North Florida. The axis of strong low level west
south westerly flow along and ahead of the arctic front will slow
for a period late Saturday into early Sunday as mid level height
falls dive into the base of the broad vortex across eastern North
America. This will support potential for training of precip areas
along and just to the south of the arctic front across far southern
Georgia into North Florida. The non NCEP models...EC..CMC...CMC
GEM and UKMET...are showing a more organized axis of heavy rainfall
potential than the GFS and NAM. WPC qpf is leaning more toward the
non-NCEP guidance given the anomalous PW values and potential for
a period of training. The expected axis of heavy rains is forecast
across regions that have seen increases in relative soil moisture
over the past week from recent heavy rains that have stretched from
the central Gulf Coast into the FL Panhandle and North Florida.
Still, FFG values remain high, with the forecast additional
rainfall amounts of 1-2" likely only resulting in isolated runoff
issues.


Oravec
$$
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FOUS30 KWBC 170813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Jan 17 2025

Day 1
Valid 12Z Fri Jan 17 2025 - 12Z Sat Jan 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

The latest CAM guidance suite has some signals for heavy rainfall
in the vicinity of the Florida Keys along a quasi-stationary
frontal boundary during the day Friday, and the model consensus is
for most of this convection to remain over the Florida Straits and
the western Bahamas. Although some of these cells may affect
coastal areas of South Florida, the coverage and rainfall rates are
low enough to preclude any risk areas at this time.

Hamrick


Day 2
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

A corridor of moisture convergence in the warm sector of a
developing Southeast U.S. surface low, in the presence of anomalous
PWs for this time of year, will fuel the development of multiple
rounds of showers and thunderstorms. Some of these cells may
train over the same areas across portions of northern Florida and
into far southern Georgia, with some rainfall rates perhaps
reaching an inch per hour. The overall trend has been for slightly
lower overall QPF with this event in the latest 00Z model guidance
suite, so the existing Marginal Risk area from the prior Day 3 has
been reduced in size some. Although the UKMET was one of the
wettest solutions for its 12Z run, it is less expansive with the
area affected, more in line with the other guidance for its 00Z
run. The NAM remains one of the less impactful solutions. It is
possible that the Marginal Risk area could be removed if model
trends continue lower in future updates.

Hamrick

Day 3
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick
$$
d
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FOUS30 KWBC 180722
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
222 AM EST Sat Jan 18 2025

Day 1
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

A corridor of moisture convergence in the warm sector of a
developing Southeast U.S. surface low, in the presence of PWs on
the order of 1.5 to 1.75 inches, will fuel the development of
multiple rounds of showers and thunderstorms across northern
Florida and across the southern tier of counties in Georgia. The
majority of the heavier rainfall should happen between 00Z and 12Z
Sunday when mixed layer CAPE rises into the 250-750 J/kg range with southwesterly flow from the eastern Gulf. This could result in
rainfall rates in excess of an inch per hour with the most
persistent cells. Some of the CAM guidance indicates swaths of 2-3
inch QPF during this period, and this may lead to some minor
instances of flooding in poor drainage areas. The axis of heavy
rainfall is expected to become more progressive by 12Z Sunday and
beyond across the central Florida Peninsula.

Hamrick

Day 2
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Mon Jan 20 2025 - 12Z Tue Jan 21 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick
$$
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FOUS30 KWBC 190724
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
224 AM EST Sun Jan 19 2025

Day 1
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The broken line of convection crossing the northern Florida
Peninsula early Sunday morning is expected to become more
progressive and weaken after 12Z Sunday, and thus the potential
drops off enough to have no risk areas for the Day 1 time period.
Therefore, the probability of rainfall exceeding flash flood
guidance is less than 5 percent.

Hamrick

Day 2
Valid 12Z Mon Jan 20 2025 - 12Z Tue Jan 21 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Tue Jan 21 2025 - 12Z Wed Jan 22 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick

$$
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FOUS30 KWBC 230737
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Thu Jan 23 2025

Day 1
Valid 12Z Thu Jan 23 2025 - 12Z Fri Jan 24 2025


The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 2
Valid 12Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

A developing closed low over central California late Saturday is
expected to advect enough moisture from the Pacific to generate
scattered to numerous showers that should start around 00Z Sunday,
with most guidance indicating QPF on the order of 0.25 to 0.50 of
an inch, with most of this within the last 6 hours of Day 3.

After collaboration with WFO Los Angeles, a Marginal Risk area has
been introduced for the Transverse Ranges and the surrounding
valleys. Even though the next round of rain appears to be mainly
moderate in intensity for the 00Z-12Z Sunday time period, the new
burn scar areas are extremely sensitive to run-off, and thus a
normally beneficial and modest rainfall could become problematic
and easily lead to flooding, with potentially serious impacts if
rainfall is heavier than expected and concentrated over a burn scar
near populated areas. There is still uncertainty regarding this
event, so please monitor future forecasts and outlooks as details
become better refined.

Hamrick

$$
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FOUS30 KWBC 240825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Fri Jan 24 2025

Day 1
Valid 12Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

A developing closed low over central California by late Saturday
will foster an area of surface low pressure across the Central
Valley which will then refocus offshore of the coastal ranges
Saturday night and Sunday morning. This will help set the stage
for weak and relatively moist onshore flow to aim into the
Transverse Ranges (and eventually into the Peninsular Ranges as
well). This onshore flow coupled with DPVA and steepening mid-
level lapse rates will allow for the development and expansion of
moderate to locally heavy shower activity, and especially by Sunday
morning when some pockets of locally stronger convection may be
possible. This will include areas from Santa Barbara County
southeast through San Diego County, and also some western portions
of San Bernadino and Riverside Counties. A somewhat stronger and
wetter solution continues to be advertised by much of the latest
model guidance, and generally as much as a 0.5" to 0.75" of rain is
forecast by Sunday morning (with highly localized stronger
orographic ascent/upslope flow facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains Saturday night and Sunday morning may be heavy enough
to result in some localized debris flow and related flash flooding
activity. Therefore, the Marginal Risk has been maintained over
portions of the Transverse Ranges and the Los Angeles Basin. Some
local adjustments were made based on the addition of high-res
guidance (primarily the NAM-nest) focusing on upslope regions most
at risk for highly localized 1"+ totals.

Churchill/Orrison


Day 3
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas). An additional 0.75"
to 1.50" is generally forecast for much of the same areas as Day 2,
and concerns are greatest for isolated convection with 0.25"+/hr
rates interacting with recent burn scar areas resulting in the
potential for localized flash flooding and debris flows. While the
bulk of the rainfall should be beneficial for the region, interests
in the burn scars (and more sensitive flash flooding areas) should
stay tuned to the forecast as more high resolution model data
becomes available in the next 24-48 hours to better hone in on the
threat and specific areas.


...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast resulting in
areas of moderate to heavy rainfall by Sunday morning. While areal
average QPF is generally expected to be capped at 1"/6-hr period,
24-hr totals could add up to be locally as high as 3-4" (per
ensemble bias corrected guidance and GEFS/ECENS probabilities for
3" exceedance of 1-5%). While instability will be rather limited,
MU CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, the inherited Marginal Risk area was maintained and
adjusted based on the latest guidance (and is in line with the
GEFS-driven machine learning first guess guidance, depicting a
Marginal Risk area).

Churchill

$$
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FOUS30 KWBC 241526
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1026 AM EST Fri Jan 24 2025

Day 1
Valid 16Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

...Southern California...

A developing closed low over central California by late Saturday
will foster an area of surface low pressure across the Central
Valley which will then refocus offshore of the coastal ranges
Saturday night and Sunday morning. This will help set the stage
for weak and relatively moist onshore flow to aim into the
Transverse Ranges (and eventually into the Peninsular Ranges as
well). This onshore flow coupled with DPVA and steepening mid-
level lapse rates will allow for the development and expansion of
moderate to locally heavy shower activity, and especially by Sunday
morning when some pockets of locally stronger convection may be
possible. This will include areas from Santa Barbara County
southeast through San Diego County, and also some western portions
of San Bernadino and Riverside Counties. A somewhat stronger and
wetter solution continues to be advertised by much of the latest
model guidance, and generally as much as a 0.5" to 0.75" of rain is
forecast by Sunday morning (with highly localized stronger
orographic ascent/upslope flow facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains Saturday night and Sunday morning may be heavy enough
to result in some localized debris flow and related flash flooding
activity. Therefore, the Marginal Risk has been maintained over
portions of the Transverse Ranges and the Los Angeles Basin. Some
local adjustments were made based on the addition of high-res
guidance (primarily the NAM-nest) focusing on upslope regions most
at risk for highly localized 1"+ totals.

Churchill/Orrison


Day 3
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas). An additional 0.75"
to 1.50" is generally forecast for much of the same areas as Day 2,
and concerns are greatest for isolated convection with 0.25"+/hr
rates interacting with recent burn scar areas resulting in the
potential for localized flash flooding and debris flows. While the
bulk of the rainfall should be beneficial for the region, interests
in the burn scars (and more sensitive flash flooding areas) should
stay tuned to the forecast as more high resolution model data
becomes available in the next 24-48 hours to better hone in on the
threat and specific areas.


...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast resulting in
areas of moderate to heavy rainfall by Sunday morning. While areal
average QPF is generally expected to be capped at 1"/6-hr period,
24-hr totals could add up to be locally as high as 3-4" (per
ensemble bias corrected guidance and GEFS/ECENS probabilities for
3" exceedance of 1-5%). While instability will be rather limited,
MU CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, the inherited Marginal Risk area was maintained and
adjusted based on the latest guidance (and is in line with the
GEFS-driven machine learning first guess guidance, depicting a
Marginal Risk area).

Churchill

$$
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FOUS30 KWBC 250800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sat Jan 25 2025

Day 1
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

...Southern California...

A developing closed low over central California will foster an
area of surface low pressure across the Central Valley which will
then refocus offshore of the coastal ranges tonight into Sunday
morning. This will help set the stage for weak and relatively moist
onshore flow to aim into the Transverse Ranges (and eventually
into the Peninsular Ranges as well). This onshore flow coupled with
DPVA and steepening mid-level lapse rates will allow for the
development and expansion of moderate to locally heavy shower
activity, and especially by Sunday morning when some pockets of
locally stronger convection may be possible. This will include
areas from Santa Barbara County southeast through San Diego County,
and also some western portions of San Bernadino and Riverside
Counties. Generally as much as a 0.5" to 0.75" of rain is forecast
by Sunday morning (with highly localized stronger orographic
ascent/upslope flow possibly facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains may be heavy enough to result in some localized debris
flow and related flash flooding activity. Therefore, the Marginal
Risk has been maintained over portions of the Transverse Ranges and
the Los Angeles Basin. While these conditions are unlikely to
manifest (as the rainfall will largely be beneficial for fire
fighting activities), sub-hourly rates up to 0.25" would be the
primary driver of the threat.

Churchill/Orrison


Day 2
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas), though snow levels
will be falling and that should help to mitigate runoff issues at
higher elevations. An additional 0.75" to 1.50" is generally
forecast for much of the same areas as Day 1, and concerns are
greatest for isolated convection with 0.25"+/hr rates interacting
with recent burn scar areas resulting in the potential for
localized flash flooding and potential debris flows/mudslides
(driven primarily by sub-hourly rates of 0.25", if able to
manifest with very limited instability). While the bulk of the
rainfall should be beneficial for the region, interests in the burn
scars (and more sensitive flash flooding areas) should stay tuned
to the forecast.

...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast along with a wave
of low pressure, resulting in areas of moderate to heavy rainfall
by Sunday morning. While areal average QPF is generally expected to
be capped near 1" over a 3-6 hr period, 24-hr totals could add up
to be locally as high as 3-4" (mainly now indicated by the ECMWF
and ECENS suite). While instability will be rather limited, MU
CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, uncertainty remains high given differences in the ensemble
systems and how the deterministic solutions handle the mass fields
(with the more aggressive EC suite suggesting the potential for a
future targeted upgrade for the Houston metro region, should hi-res
guidance support this with future updates. The inherited Marginal
Risk area was maintained with no adjustments necessary, as the
guidance remains fairly consistent spatially with depiction of QPF.

Churchill


Day 3
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 260830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Sun Jan 26 2025

Day 1
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND THE
PALISADES, EATON, AND HURST BURN SCARS, GENERALLY JUST NORTH AND
WEST OF THE CENTER OF LOS ANGELES, CALIFORNIA...

...Southern California...

Shower coverage is becoming more numerous to widespread early this
morning across much of the Southern California coastline and
adjacent inland areas, as a deep layer closed low churns near the
central California coast. As the low gradually slides southward
along the coastline today, localized moderate to heavy downpours
will become more likely along upslope portions of the Transverse
and Peninsular Ranges (and to a lesser extent the adjacent Los
Angeles Basin and coastal areas). While falling snow levels should
help to mitigate runoff issues at higher elevations, there are
increasing concerns for potentially significant localized impacts
for more sensitive localities, chiefly for recent burn scars where
runoff concerns are highest. While an additional 0.5" to 1.0 of
areal average QPF is expected for the region, some hi-res CAM
solutions (including the HRRR) indicate highly localized totals of
1"+. Most concerningly, these localized totals are possible in and
around the Palisades, Eaton, and Hurst burn scars. The CAMs also
indicate the potential for sub-hourly (15-min) rainfall rates of
0.25"+, which would drive an elevated threat for mudslides and
debris flows in and around burn scars. While the bulk of the
rainfall should be largely beneficial for the region, the
heightened risk for significant flash flood impacts in association
with these burn scars has necessitated an upgrade to Slight risk
for a target region (generally encompassing the aforementioned burn scars).


...Texas Gulf Coast into Lower Mississippi Valley...

Moist southerly low-level flow from the western Gulf has returned
to much of the western Gulf Coast, spreading northeastward into
portions of the Lower Mississippi Valley. This return flow will
combine with a wave of low pressure and associated surface frontal
zone which will likely result in embedded moderate to heavy
showers by late morning to midday, gradually driven southeastward
towards the coastline with the progression of the front. While
hourly rates will generally be capped near 1", 24-hr totals could
add up to be locally as high as 3-4" (per 00z HREF 40-km
neighborhood probabilities for 3" exceedance of 20-30%). While
instability will be somewhat limited, MU CAPE up to 1000 J/kg
(maximized primarily in and between Corpus Christi to Houston)
could support a period of localized 1-2"/hr rainfall rates (which
adds a bit to the threat, especially within urbanized terrain).
While models are in relatively good agreement concerning the
magnitude of QPF (localized 3-4" totals), there is still a good bit
of uncertainty with regard to the placement of these higher totals
(with the ECMWF the most consistent in indicating these amounts in
and around Houston, while the latest HRRR-EXT run indicates totals
nearly this high as far southwest as Corpus Christi). Overall, the
CAM consensus is clustered farther southwest than much of the
coarser global guidance has been over the past several days, and
convection tends to verify farther southwest than expected (giving
credence to the HRRR solution, which resulted in an expansion of
the Marginal risk area farther southwest). A targeted upgrade to a
Slight risk remains possible with subsequent updates, which would
be most likely across portions of TX (where instability and the
potential for 1-2"/hr rainfall rates is expected to be maximized).

Churchill


Day 2
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES, LOS ANGELES BASIN, AND INLAND
EMPIRE OF SOUTHERN CALIFORNIA...

Shower activity in association with the deep closed low looks to
continue into Monday morning over portions of Southern California,
before the low pivots inland and brings an end to the potential for
sub-hourly (15-min) rainfall rates of 0.25"+. This will continue
the Marginal risk for excessive rainfall into Day 2 for both
coastal and more inland low elevation areas (with particular
concern for any lingering shower activity over aforementioned burn
scars). The threat is expected to last for only the first half of
Day 2 (prior to 00z Tuesday).

Churchill


Day 3
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill
$$
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FOUS30 KWBC 270750
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 AM EST Mon Jan 27 2025

Day 1
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES OF SOUTHERN CALIFORNIA...

Light to moderate shower activity in association with a deep
closed low looks to continue for a bit longer early this morning
over portions of Southern California, gradually diminishing by
midday as the low weakens and pivots inland and brings an end to
the (now very low) potential for sub-hourly (15-min) rainfall
rates of 0.25"+. Maintained the inherited Marginal risk for
excessive rainfall for this very low-end potential, primarily due
to elevated concerns for any lingering shower activity over the
recent burn scars. This activity should largely come to an end by
18z, and the Marginal risk will likely be discontinued with an
update later this morning or afternoon.

Churchill


Day 2
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill/Wegman


Day 3
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide the forcing for ascent for a developing low
over North Texas Wednesday and Wednesday night. Return flow of Gulf
moisture ahead of the low will advect north into Texas and
Oklahoma along with increasing instability. The typical diurnal
increase of the LLJ Wednesday night will greatly increase the
coverage and intensity of storms along the developing low's cold
front, while the warm front locally increases lift into Oklahoma.
Since the upper level low will be slow-moving (though increasing in
forward speed with time), this will allow the storms that form to
train over the same areas. Soils in the area have been drier than
normal, meaning most of the rain will be beneficial, however where
storms train over more flood-sensitive areas and urban areas,
localized flash flooding is possible. Ensemble guidance have
increased in the areal coverage of at least 1 inch of rain, though
the pattern is much more favorable to narrow streaks of much
heavier totals where the storms track. Thus, confidence has
increased enough to introduce a Marginal Risk area.

The more significant flash flooding risk will be largely north and
east of the Day 3 Marginal Risk area going into D4/Thursday.

Wegman
$$
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FOUS30 KWBC 281509
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1009 AM EST Tue Jan 28 2025

Day 1
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina


Day 2
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide forcing for ascent for a developing low over
the Texas and Oklahoma Panhandles Wednesday and Wednesday night.
Return flow of Gulf moisture, warmth, and instability ahead of the
low will advect north across Texas and into Oklahoma and Arkansas
Wednesday and especially Wednesday night. The typical diurnal
strengthening of the LLJ Wednesday night will greatly increase the
coverage and intensity of storms along the developing low's cold
front across Texas, while the warm front locally increases lift
into Oklahoma. The surface low will track north into Colorado,
contributing to a winter storm there. Meanwhile much of the
convection will be associated with the cold/occluded front as it
pushes east towards the Mississippi Valley. Since the upper level
low will be slow-moving (though increasing in forward speed with
time), this will allow the storms that form to train over the same
areas. Training convection will be of greatest concern as the
storms are first forming over Texas. Soils in the area have been
drier than normal, meaning most of the rain will be beneficial,
however where storms train over more flood-sensitive and urban
areas, localized flash flooding remains possible.

Wegman


Day 3
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID-SOUTH AND ARKLATEX REGION...

Thunderstorms will likely be ongoing on the northern and western
sides of the Slight and Marginal Risks at the start of the period
Thursday morning. A vigorous upper level cutoff low will begin to
rejoin the jet stream by Friday morning. Before it does so however,
it will provide the forcing for numerous showers and thunderstorms
across much of the Mid-South and Arklatex region. A 50-60 kt LLJ
will pump plentiful Gulf moisture across the Arklatex region
Thursday morning. The trailing cold front will push east across
Oklahoma and Texas, providing ample forcing for the showers and
storms. The primary limiting factor for the storms will be
instability. The greatest instability (around 500 J/kg) will be
across southern and central Texas, while the greatest forcing will
be further north from the Arklatex north and east into the
Mississippi Valley. This disconnect may help to diminish the flash
flooding potential a bit. However, this will be partially offset
by upsloping into the Ozarks across Oklahoma and Arkansas.

It appears that many of the storms will be associated with a line
that moves east across the area. However, storms embedded within
the line may move somewhat parallel to the line towards the
northeast, this will allow for the potential for training. Since
there will be plenty of Gulf moisture (PWATs above 1.5 inches),
the showers and storms will have the potential to cause localized
flash flooding issues, especially in the terrain of the Ozarks
where upsloping may contribute to locally higher rainfall totals.
By Thursday night, the storms will push across the Mississippi
River. By then the eastward forward speed of the storms should
increase, thus diminishing the flooding potential with time.

Wegman

$$
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FOUS30 KWBC 290824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Wed Jan 29 2025

Day 1
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHEASTERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide ample forcing for ascent (along with the left
exit region of a ~120 kt jet streak) for a developing low over the
Texas and Oklahoma Panhandles into tonight. Return flow of Gulf
moisture, warmth, and instability ahead of the low will advect
north across Texas and into Oklahoma and Arkansas today (and
especially into tonight). The typical diurnal strengthening of the
LLJ overnight will greatly increase the coverage and intensity of
storms along the developing low's cold front across Texas
(particularly after 06z), while the warm front locally increases
lift into Oklahoma. The surface low will track north into Colorado, contributing to a winter storm there. Meanwhile much of the
convection will be associated with the cold/occluded front as it
pushes east towards the Mississippi Valley. Since the upper level
low will be slow-moving (though increasing in forward speed with
time), this will allow the storms that do form to have a high
likelihood of training over the same areas. Training convection
will be of greatest concern as the storms are first forming over
North TX and into southeast OK (where a targeted upgrade to Slight
risk was made, given 00z HREF 40-km neighborhood probabilities for
3" exceedance of 50-70%, and primarily over a relatively short
period from 06z-12z Thurs). Soils in the area have been drier than
normal, meaning most rainfall should (in theory) be beneficial,
however the rapid development of rainfall rates of 1-2"/hr over the
dry soils may locally exacerbate the flash flood threat (due to
hardened and compacted soil resulting in reduced infiltration and
increased runoff).

Churchill/Wegman


Day 2
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
ARK-LA-TEX, OZARKS, AND MID-SOUTH REGIONS...

Thunderstorms will likely be ongoing on the northern and western
sides of the Slight and Marginal Risks at the start of the period
Thursday morning. A vigorous upper level cutoff low will begin to
rejoin the polar jet by Friday morning. Before it does so,
however, it will provide the forcing for numerous showers and
thunderstorms across much of the Mid-South and Ark-La-Tex region.
A 50-60 kt LLJ will pump plentiful anomalously high Gulf moisture
across the Ark-La-Tex region Thursday morning, as the trailing
cold front provides ample forcing for showers and storms across
Oklahoma and Texas (and depending on how much rainfall occurs in
the 6-12 hours prior to the start of Day 2, some ongoing flash
flooding for prior training storms may be locally significant in
portions of North TX into southeast OK, where probabilities for
excessive rainfall are towards the higher-end of the Slight risk
spectrum, being 25%+). The main limiting factor will likely be
decreasing instability (to around 500 J/kg of MU CAPE into Thursday
morning), which looks to be maximized across southern and central
Texas. The greatest forcing looks to be displaced farther north
(from the Ark-La-Tex north and east into the Mississippi Valley).
This disconnect may help to diminish the flash flooding potential a
bit (though concerns are greatest where the best instability and
forcing may meet over aforementioned areas of North TX into southeast OK).

It appears that many of the storms will be associated with a line
that moves east across the area. However, storms embedded within
the line may move somewhat parallel to the line towards the
northeast, this will allow for the potential for training farther
upstream into portions of the Middle MS Valley and OH Valley.
Since there will be plenty of Gulf moisture (PWATs above 1.5
inches, above the max moving average using BNA as a proxy
sounding), the showers and storms will have the potential to cause
isolated to scattered flash flooding. By Thursday night the flash
flood threat should diminish as the last storms cross the MS River
with eastward forward speed of the storms increasing along with
continued decreasing instability, thus diminishing the flooding
potential with time.

Churchill/Wegman


Day 3
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST COASTAL RANGES AND FOOTHILLS...

A wetter pattern will take shape across the Pacific Northwest
region beginning Thursday night, as a series of storm system look
to impact the area (originating from a closed low from the North
Pacific interacting with another closed low/digging trough moving
south along the west coast of British Columbia). After an initial
round of light to moderate precipitation late on Day 2 in the
coastal ranges and foothills, rainfall will become moderate to
heavy at times throughout much of Day 3. While forecast maximum
IVT values are relatively weak overall (500-700 kg/ms), a longer
duration event could lead to a more 'moderate' atmospheric river
event. 24 hour rainfall totals are forecast to generally range from
1-3" (though may locally exceed 3" in favored upslope terrain).

Churchill

$$
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