FOUS11 KWBC 052020
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Sun Jan 5 2025

Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025

...Central Plains through the Mid-Atlantic... Days 1-2...

...Major winter storm continues from Central Plains and Ohio
Valley today through the Mid-Atlantic...

The major winter storm which has been advertised for several days
now is ongoing to start the forecast period. This system is being
forced by a closed 500mb low which will be positioned over Kansas
City, MO to start the period. This closed low will move
progressively east, reaching Indianapolis, IN by 12Z Monday, and
then northern VA at the start of D2 /00Z Tuesday/. During the
latter half of D1, however, this feature is progged to open into a
wave, while still potent, but with subtly less downstream ascent.
At the same time, however, a potent jet streak rotating around the
base of the trough will amplify towards 150 kts, helping to offset
some of the loss of deep layer lift resulting from the opening of
the trough. At the surface, the accompanying low pressure will
skirt east across the lower OH VLY/TN VLY, with secondary
development likely occurring across the Carolinas and then pressing
east off the Mid-Atlantic Coast by Tuesday morning.

Together, this will result in a large swath of heavy
precipitation, with all p-types likely. Downstream of the primary
surface low, strong isentropic ascent along the 290K-295K surface
will draw impressive moisture northward into the system (NAEFS PW
above the 97th climatological percentile) supporting an expansion
of widespread precipitation. At the same time, the accompanying
theta-e ridge rotating cyclonically into the system will lift into
a TROWAL, especially early in the period, before the upper low
opens and the low occludes to the secondary development. Despite
that, some enhanced elevated instability beneath this TROWAL will
support heavy precipitation rates, leading to widespread
significant precipitation amounts of snow, sleet, and freezing
rain. While confidence in heavy precipitation causing impacts is
high, the exactly placement of the transition zone and associated
ice/snow amounts remains uncertain.

There continues to be two primary camps with the track of this
low. THe GFS/NAM/high-res members remain a bit north of the
CMC/ECMWF. While latitudinal spread has decreased, it is still
significant in terms of where the transition zone will occur.
However, regardless of the exact track, a stripe of very heavy snow
(rates 1-2"/hr) is likely within the WAA, and the conceptual model
for a laterally translating band from west to east fits this
system. This suggests a narrow but intense band of snow, supported
by both NBM and DESI probabilities, as well as the WPC snowband
tool which indicates a high potential for 1-2"/hr snow rates from
northern MO eastward to MD and DE. Since this band will be
translating along its long/axis (more eastward motion than north)
some locally very high snow totals are possible as reflected by
10-30% chance for 12+ inches of snow from northern WV eastward
through southern DE, but in general WPC probabilities support a
high (>70%) risk for 6+ inches along this same axis with a sharp
N/S gradient on both sides.

Some of this snow will be additionally enhanced by the post-system
comma head/deformation which will pivot across the Mid-Atlantic
Monday night. The guidance continues to trend a bit stronger with
this feature, and while latitudinal placement of the most
significant forcing remains quite uncertain within this axis,
renewed moderate to heavy snow, with much fluffier SLRs than
earlier, will likely regenerate across the region adding up to a
few more inches before exiting by Tuesday morning, but WPC
probabilities after 00Z Tuesday for additional snow accumulations
exceeding 2" are just around 10%, highest around the Chesapeake Bay.

South of the heavy snow area, this system will also pose a
significant to damaging ice threat within the warm nose/p-type
transition zone. Intense WAA over-topping the cold surface layer
accompanied by persistent E/NE dry-bulb advection is a classic
setup for damaging freezing rain. Although there is some
uncertainty into how efficiently the ice can accrete at times due
to what should be intense precipitation rates, the threat for
damaging ice remains from central KY into the southern/central
Appalachians and Foothills where WPC probabilities for 0.25"+ are
50-90% across northern KY and until WV/VA. The greatest risk for
damaging ice however continues to focus across eastern KY where a
10-30% chance exists for 0.5" of ice. This could result in downed
limbs and power lines leading to widespread long-lasting power
outages and impossible travel.

Finally, between the heavy snow and heavy icing, the forecast
profiles indicate a narrow stripe where the p-type may remain sleet
for a considerable temporal duration. This is due both to a subtly
weaker surge of the warm nose as well as a deeper sub-warm- nose
cold layer which will allow refreezing of droplets. At the same
time, cold/dry wet-bulb advection on E/NE winds will maintain this
cold layer effectively to prevent a more rapid p-type transition.
Heavy accumulations of sleet are difficult to achieve, but in this
setup it is possible that up to 1" of sleet will accumulate causing
tremendous driving impacts. The greatest risk for the significant
sleet accumulations D1 extend west to east from northern KY through
southern VA.

This system has prompted the issuance of key messages which are
linked at the bottom of the discussion.

...Great Lakes and Northeast... Days 2-3...

Behind the large system moving off the Mid-Atlantic coast Tuesday
morning, a push of arctic air will dig into the Great Lakes and
Northeast, fueled initially by a potent shortwave dropping nearly
due south from Ontario Tuesday morning. 850mb temps are progged to
crash to -15C to -20C, steepening lapse rates considerably, and
resulting in a favorable setup for both lake effect snow (LES) and
upslope snow into the upwind terrain of the Northeast.

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it.
This will mark the leading edge of the coldest air, and cause
impressive NW flow into a moistening column. The temperatures
during this time will be very cold, so the DGZ will be quite low,
resulting in effective upslope into the snow growth region. Light
and fluffy snow will efficient accumulations, so despite a short
duration, WPC probabilities for 4+ inches of snow are moderate
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan
early D2, the greater coverage and intensity of LES is likely D3
behind this front. GLERL lake-surface temperatures are still
sampled to be +5C to +8C, so the very cold air moving overhead will
support lake-induced instability to drive LES. At this time the
LES appears less intense as the last round, but WPC probabilities
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.

...Western/Southwestern U.S.... Days 1-3...

Pacific shortwave will enter Oregon later this evening with
another surge in moisture to the OR Cascades into the northern
Great Basin as it weakens and slips southeastward into Nevada
overnight. High initial snow levels around 6000-7000ft over the OR
Cascades only drop to around 5000-6000ft under the trough axis,
restricting significant snow to the highest OR Cascades well above
pass level. Snow levels will be closer to 5000ft over the Great
Basin and 4000ft over the northern Rockies, where several inches of
snow are likely D1 over northern NV, southeastern ID, western
WY/southwestern MT and then into the CO Rockies. WPC probabilities
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate
through the Southwest and sharpen, eventually closing off over the
lower CO River Valley late Tue. With high pressure nosing down out
of the northern Plains, upslope snow will expand across the CO
Rockies into the Front Range via an easterly low level flow. The
upper low will likely move into northwestern Mexico by the end of
the period, helping to drag the snowfall southward as well through
the Mogollon Rim, Sangre de Cristos, Sacramento Mountains, etc. For
the D2-3 period, WPC probabilities for at least 4 inches of snow
are 20-50% over the Front Range into the CO Rockies, San Juans, and
higher elevations in AZ/NM above 8000ft or so.

Weiss/Fracasso/Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 060759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Mon Jan 6 2025

Valid 12Z Mon Jan 06 2025 - 12Z Thu Jan 09 2025

...Ohio Valley through the Mid-Atlantic... Day 1...

...Major winter storm continues from the Ohio Valley through the Mid-Atlantic...

The major ongoing winter storm which has already dumped over a foot
of snow from central KS to northern MO is forecast to rapidly slide
eastward today while producing additional areas of heavy snow and
freezing rain between the Ohio Valley and Mid-Atlantic. This
system is being forced by a closed 500mb low which will be
positioned over the IL-IN border to start the period. This closed
low will weaken into a potent open wave today while pushing due
east toward the Mid-Atlantic by this evening. The weakening is
being aided by a separate stronger upper low diving southward
towards New England and helping shear out the storm system as it
exits the East Coast. At the same time, however, a potent jet
streak rotating around the base of the trough will amplify towards
150 kts, helping to offset some of the loss of deep layer lift
resulting from the opening of the trough. At the surface, the
accompanying low pressure will skirt east across the lower OH
VLY/TN VLY, with secondary development likely occurring across the
Carolinas and then pressing east off the Mid- Atlantic Coast by
Tuesday morning.

Together, this will result in a large swath of heavy
precipitation, with all p-types likely (at least at the beginning
of the forecast period). Downstream of the primary surface low,
strong isentropic ascent along the 290K-295K surface will draw
impressive moisture northward into the system (NAEFS PW above the
90th climatological percentile) supporting the expansiveness of
widespread precipitation.

A stripe of very heavy snow (rates 1-2"/hr) is likely within the
WAA during the first half of D1, and the conceptual model for a
laterally translating band from west to east fits this system. This
suggests a narrow but intense band of snow, supported by both NBM
and DESI probabilities, as well as the WPC snowband tool which
indicates a high potential for 1-2"/hr snow rates from northern VA
and MD eastward to DE following the band weakening across southern
OH right before 12z. Since this band will be translating along its
long/axis (more eastward motion than north) some locally very high
snow totals are possible as reflected by 40-50% chance for an
additional 8+ inches of snow from northern VA eastward through
southern DE, but in general WPC probabilities support a high (>70%)
risk for 4+ inches along this same axis with a sharp N/S gradient
on both sides. These similar probabilities are also found in
western MD and WV where an upslope component following the passage
of the low pressure system.

Some of this snow will be additionally enhanced by the post-system
comma head/deformation which will pivot across the Mid-Atlantic
Monday night. The guidance continues to trend a bit stronger with
this feature, and while latitudinal placement of the most
significant forcing remains quite uncertain within this axis,
renewed moderate to heavy snow, with much fluffier SLRs than
earlier, will likely regenerate across the region adding up to a
few more inches before exiting by Tuesday morning.

South of the heavy snow area, this system will also pose a
continuing ice threat within the warm nose/p-type transition
zone. Damaging ice accretion of 0.5-0.75" has already been realized
along a stretch spanning from eastern KS to KY, but thankfully
precipitation will be coming to an end across this area as light
snow/freezing rain pushes eastward today on the backside of the
low. Intense WAA over- topping the cold surface layer will continue
this morning across parts of the southern Mid-Atlantic, however.
WPC probabilities for at least an additional 0.1" beginning 12z
today fall between 10-40% across southern VA.

This system has prompted the issuance of key messages which are
linked at the bottom of the discussion.

...Great Lakes and Northeast... Days 2-3...

Behind the large system moving off the Mid-Atlantic coast Tuesday
morning, a push of arctic air will dig into the Great Lakes and
Northeast, fueled initially by a potent shortwave dropping nearly
due south from Ontario Tuesday morning. 850mb temps are progged to
crash to -15C to -20C, steepening lapse rates considerably, and
resulting in a favorable setup for both lake effect snow (LES) and
upslope snow into the upwind terrain of the Northeast.

The greatest potential for upslope snow will occur D2 /Tuesday/ as
the potent shortwave dives south pulling a cold front with it.
This will mark the leading edge of the coldest air, and cause
impressive NW flow into a moistening column. The temperatures
during this time will be very cold, so the DGZ will be quite low,
resulting in effective upslope into the snow growth region. Light
and fluffy snow will efficient accumulations, so despite a short
duration, WPC probabilities for 4+ inches of snow are moderate
(50-70%), highest in the NW Adirondacks and N Greens.

Although some heavy LES may occur downstream of Lake Michigan
early D2, the greater coverage and intensity of LES is likely D3
behind this front. GLERL lake-surface temperatures are still
sampled to be +5C to +8C, so the very cold air moving overhead will
support lake-induced instability to drive LES. At this time the
LES appears less intense as the last round, but WPC probabilities
D3 feature a low risk (10-30%) for 4+ inches of snow in the favored
N to NW snow belts.

...Intermountain West, Rockies, and Central High Plains.... Days 1-3...

Pacific shortwave diving across the Intermountain West today will
surge moisture ahead of it and feature favorable divergence to
promote scattered high elevation upslope snow spreading southward
across the central Great Basin and Central Rockies today before
reaching the Southern Rockies late D2/early D3. Several inches of
snow are likely D1 over northern NV, southeastern ID, western
WY/southwestern MT and then into the CO Rockies. WPC probabilities
for at least 4" of snow are >50% in these mountainous regions.

Into D2, height falls (and trailing height falls) will rotate
through the Southwest and sharpen, eventually closing off over the
lower CO River Valley late Tue. With high pressure nosing down out
of the northern Plains, upslope snow will expand across the CO Front
Range via an easterly low level flow along with a relatively low
DGZ. The upper low will likely move into northwestern Mexico by the
end of the period, helping to drag the snowfall southward as well
through the Mogollon Rim, Sangre de Cristos, Sacramento Mountains,
etc. For the D2-3 period, WPC probabilities for at least 4 inches
of snow are 20-50% over the Front Range into the CO Rockies, San
Juans, and higher elevations in AZ/NM above 8000ft or so.

...Southern Plains... Day 3...

As the aforementioned upper low closes off over the Southwest late
D2 it continues to dig into northwestern Mexico D3 and become
anomalously deep (below the 0.5th climate percentile per the 12z
GEFS). Meanwhile, a separate shortwave rounding the far eastern
Pacific ridge is forecast to dive southward and interact with this
feature and spawn strong southwest flow containing rich moisture
and prominent upper divergence. Given the strong high situated to
the north over the Central Plains and cold the low-level airmass in
place, snow and mixed precipitation is expected over much of
western and north-central TX. Currently, WPC probabilities for at
least 2 inches are low (10-30%) through Thursday morning but these
may trend up if precipitation start time trends faster. This storm
will need to be monitored as 00z ensembles depict heavier snowfall
impacting parts of the Southern Plains on D4.

Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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