FOUS11 KWBC 280750
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Mon Oct 28 2024

Valid 12Z Mon Oct 28 2024 - 12Z Thu Oct 31 2024


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

An amplified upper trough moving across the western U.S. is
forecast to produce widespread mountain snow from the Pacific
Northwest and the Sierra Nevada to the northern and central
Rockies. Areas of heavy accumulations are expected, especially for
portions of the central Rockies on Tuesday.

A well-defined shortwave moving through the base of the upper
trough is expected to lift northeast from the Southwest through the
central Rockies on Tuesday. In addition to strong mid-to-upper
level forcing, upslope flow on the northwest side of an associated
low level wave moving across Wyoming, will likely support periods
of heavy snowfall along the favored terrain. Ranges that will be
impacted include the Wind River, southern Absaroka, and the Big
Horns. WPC probabilities are 50 percent or greater for
accumulations of 8 inches or more across parts of these areas.
Locally heavy snows are also likely in the Black Hills, with
amounts of 6 inches or more likely, according to WPC probabilities.

As the system lifts out of the Southwest, other areas impacted
will include the southwestern and west-central Colorado ranges,
including the San Juan and Elk mountains, where heavy heavy snows
are forecast to begin tonight, with storm totals, according to WPC probabilities, likely reaching over a foot before snow diminishes
on Wednesday. Further west, widespread mountain snows are expected
along the Utah mountains as well, from the southern mountains to
the Uintas, with WPC probabilities indicating that locally heavy
amounts of 8 inches or more likely.

In the Northwest, onshore flow along the backside of the trough,
will continue to support unsettled weather across the region
through today. Snow levels will remain low enough to support light accumulations along the higher Cascades passes. Snow is expected to
diminish on Tuesday as a shortwave ridge builds ahead of the next
approaching system. Precipitation will then return by late
Wednesday with the arrival of a frontal band associated with a deep
upper low dropping south from the Gulf of Alaska. This will bring
the return of mountain snow to the Olympics and the Cascades and
the potential for impacts across the higher Cascades passes.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 290730
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Tue Oct 29 2024

Valid 12Z Tue Oct 29 2024 - 12Z Fri Nov 01 2024


...Central Rockies onto High Plains....
Day 1...

Widespread mountain snow, with locally heavy amounts, is expected
as a well-defined shortwave moving through the base of a broader-scale
trough lifts northeast from the Southwest, crossing the central
Rockies into the High Plains over the next 12 hours. Areas likely
impacted will extend from the southern Utah and southwestern
Colorado mountains to the Wyoming ranges and the Black Hills.
Strong upward ascent afforded in part by a developing upper jet
couplet and low-to-mid level frontogenesis, will generate moderate
to heavy snow that will translate northeastward across the region
today. HREF guidance indicates that locally intense bands,
producing rates of 1-2 in/hr, can be expected to develop initially
over the Utah and western Colorado mountains this morning before
lifting into Wyoming, where more widespread heavy snow is likely
to develop by the afternoon and continue into the evening. WPC
guidance shows locally high probabilities for accumulations of 8
inches or more along the central Utah and west-central Colorado
ranges. More widespread coverage is shown across the central
Wyoming ranges, including the Wind River and eastern Absaroka
ranges, the Owl Creek Mountains, and Big Horns. WPC guidance
suggests widespread amounts of 8 inches or more across these areas,
with totals topping 18 inches across some of the higher terrain.
Rain changing to snow will produce some lighter accumulations,
generally around 1-2 inches across the northeastern Wyoming Plains.
However, heavier accumulations of 8 inches or more can be expected
across the higher elevations of the Black Hills.

...Northwest to the Northern Rockies...
Days 2/3...

Much of the Northwest will be dry today as a shortwave ridge
builds and moves east across the region ahead of the next
approaching system. This next system will be a deep low that is
expected to rapidly develop over the Gulf of Alaska today. This
system is forecast to drop south, with its associated frontal band
expected to move into the Pacific Northwest on Wednesday. As the
low reaches the Northwest on Thursday, it is forecast to turn to
the east and move toward the northern Rockies as it gradually
weakens. The heaviest precipitation is expected to focus across
western Oregon and northwestern California, generated by strong
forcing and moisture advection. Significant snow accumulations are
likely along the Oregon passes as snow levels dip below 4500 ft. By
Wednesday night, snow will begin to develop and spread east from
the northeastern Oregon into the central Idaho ranges, where some
locally heavy amounts of 6 inches or more can be expected by Friday
morning.

The probability of significant ice across the CONUS is less than
10 percent.


Pereira


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FOUS11 KWBC 300830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Oct 30 2024

Valid 12Z Wed Oct 30 2024 - 12Z Sat Nov 02 2024

...Central Rockies...
Day 1...

Snow showers will continue across portions of the central Rockies,
as an upper trough moves across the region today. While widespread
heavy amounts are not expected, WPC probabilities indicate that
additional accumulations of 4 inches or more are possible across
the higher elevations of the west-central to north-central Colorado
mountains. Snows are expected to diminish as the upper trough moves
east into the central Plains this evening.

...Northwest to the Northern Rockies...
Days 1-3...

Precipitation will return to the Northwest and northern California
as a well-defined frontal band associated with a deep low dropping
out of the Gulf of Alaska moves onshore later today. Precipitation
rates are expected to increase later this morning and continue into
the afternoon as the front moves across the region. Rates will
diminish by the evening and overnight, but onshore flow will
continue to support unsettled conditions. Snow levels are expected
to quickly drop behind the front, supporting accumulating snow
across many of the Cascade passes.

The upper low is expected to support another period of organized
heavier precipitation as it continues to drop south on Thursday.
The heaviest precipitation is expected to center from southwestern
Washington through western Oregon into northwestern California.
Accumulations of a foot or more are likely for portions of the
Oregon Cascades, where snow levels are forecast to increase but
remain near or below 5000 ft through much of the period.

Further to the west, this series of systems will bring snow into
the eastern Oregon and northern Nevada mountains beginning this
evening, and the central Idaho ranges overnight, with periods of
snow continuing through Friday. Storm total accumulations of a foot
or more are likely across portions of the Blue Mountains in
northeastern Oregon and the central Idaho ranges, including parts
of the Salmon River Mountains and the Sawtooth Range.

As the upper low remains centered over Washington through much of
Friday, a well-defined shortwave digging to its west will move into
northern California late Friday into early Saturday, bringing
heavier precipitation across northwestern California into the
Shasta-Cascade region and the northern Sierra, where high
elevation heavy snow accumulations can be expected.

...Northeastern Minnesota to Upper Michigan...
Day 2...

The previously noted upper trough crossing the central Rockies
today is forecast to lift northeast through the central Plains and
into the upper Midwest by early Thursday. Guidance shows the upper
trough briefly assuming a negative tilt, with a closed 500 mb
center developing as moves into the upper Midwest Thursday morning.
As the surface, low pressure is expected to quickly deepen and
track northeast across the upper Mississippi Valley into the Great
Lakes. Beginning Thursday morning, models are showing rain
changing to snow within the strong low-to-mid level frontogenesis
band on the northwest side of the low, with some hi-res guidance
indicating snowfall rates increasing upward of 2 in/hr from east-
central Minnesota to the North Shore and northwestern Wisconsin.
While warm ground temperatures and solar insolation may help to
keep amounts in check, these rates, should they develop, will
likely to be sufficient to produce at least a few inches in some
locations. The overall trend in the overnight models was toward a
colder and wetter solution. So while probabilities for anything
above light accumulations have been low, it is expected these will
increase should these trends persist.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 301932
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Wed Oct 30 2024

Valid 00Z Thu Oct 31 2024 - 00Z Sun Nov 03 2024

...Northeastern Minnesota to Upper Michigan...
Days 1-2...

A vigorous upper level trough entering the Central Plains tonight
will lead to a strengthening area of low pressure that tracks into
the Upper Mississippi Valley by Thursday morning. This setup is
driven heavily by strong synoptic-scale ascent over eastern MN,
northern WI, and upper MI Thursday morning thanks to robust
250-500mb upper level divergence. As the storm strengthens Thursday
morning, dynamic cooling within the column coupled with intense
vertical velocities just below of, and including some lower
sections, of a saturated DGZ will support periods of heavy snow
from east-central MN to the MN Arrowhead, northern WI, and the
western most communities of MI's Upper Peninsula. Latest WPC
Snowband Probability Tracker does suggest the potential for 1-2"/hr
snowfall rates from just north of the Twin Cities metro area to
more prominently around Duluth, its surrounding communities, and on
north and east to include the coastal portions of the Arrowhead and
northwest WI. This event is a fairly fast moving setup with snow
starting mid-morning in parts of eastern MN, then concluding close
to sunset. The start and end times across northwest WI and the MI
Upper Peninsula would be closer to early afternoon for a start time
and ending around midnight local time.

Snowfall totals will likely be heavily driven by which areas reside
beneath the deformation axis, which is still not fully agreed upon
by guidance across the board. That said, most guidance does show an
opportunity for heavy snow in areas that would be experiencing
there first measurable snowfall of the season. Given that is would
be the first snow, the potential impacts are more amplified for
residents in these areas. The latest WSSI does show a large Minor
Impact area that stretches from the northern Twin Cities suburbs to
as far north as the MN Arrowhead and as far east as the western
most section of MI's Upper Peninsula. It is northwest WI to the
east of Duluth that show localized areas of Moderate to even Major
Impacts. These impacts are driven largely due to a combination of
the algorithm's Snow Amount and Snow Load components, signifying
that this is likely to be a heavy, wet snow in areas that could
result in detrimental impacts to trees branches and possibly power
lines. The good news is the storm will be racing east Thursday
night and any lingering snowfall is effectively finished around
sunrise Friday morning.

...Northwest to the Northern Rockies...
Days 1-3...

A potent storm system is arriving in the Pacific Northwest today
that is ushering in the next round of unsettled weather as far east
as the Northern Rockies and northern Great Basin. Initially, a cold front
will track through the Pacific Northwest today that begins the process
of lowering snow levels below 4,000ft by this evening throughout much
of the Pacific Northwest. NAEFS shows an upper level trough, sporting
500mb and 700mb heights below the 10th climatological percentile,
barreling into the Pacific Northwest and southern British Columbia
early Thursday morning. This upper level feature is also directing
a 300-400 kg/m/s IVT (above the 90th climatological percentile
over northern CA) into portions of the Northwest. There is not much
in the way of a sub-freezing air-mass either in front of, or in
wake of the upper low. This should keep most heavy snow to
elevations above 4,000ft in the Cascade Range, Siskiyou, and
Shasta. This same stream of moisture will reach the northern
Rockies late tonight and into Thursday where heavy snowfall

5,000ft is expected in the Blue, Sawtooth, Boise, and Teton

Mountains.

The upper low lingers along the Pacific Northwest coast through
Friday morning, then finally advances inland through the Northwest
Friday night. A cold front will advance south through northern
California and help to cause lowering snow levels throughout the
northern and central Sierra Nevada on Saturday. Periods of snow are
possible in the higher ridge lines of the northern Great Basin on
Saturday and as far east as the Tetons and Wasatch Saturday
afternoon.

WPC 72-hr PWPF shows high chances (>70%) of snowfall totals >18"
in the Oregon Cascades above 5,000ft, with some of the tallest
peaks above 6,000ft having similar high probabilities for snowfall
totals >30". Farther east, the Blue and Sawtooth Mountains at
elevations above 6,000ft have moderate-to-high chances (50-70%) for
snowfall amounts >18". Even portions of the Washington Cascades
are forecast to see some heavy snow with moderate-to-high chances

70%) for >12" of snowfall at elevations above 4,000ft. Farther

south, the Sierra Nevada and Siskiyou sport moderate-to-high odds
(50-70%) for snowfall >8" on Saturday with additional snowfall
likely to occur Saturday night. In terms of impacts, the
Probabilistic WSSI shows Moderate chances (40-60%) for Moderate
Impacts along the Oregon Cascades and the southern peaks of the
Washington Cascades. These areas can expect the more treacherous
travel conditions, particularly where heavy snow falls around pass
level. Elsewhere, the other mountain ranges listed sport generally low-to-moderate chances (30-50%) for Minor Impacts.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax





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FOUS11 KWBC 010623
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 AM EDT Fri Nov 1 2024

Valid 12Z Fri Nov 01 2024 - 12Z Mon Nov 04 2024

...Mountainous West...
Days 1-3...

Active period in the Western CONUS over the next few days
downstream of a very strong upper ridge (~99th percentile) between
Alaska and Hawaii. This will help guide several northern stream
systems into the West Coast and into the Intermountain West this
weekend into early next week. With the source region in the mid-
latitudes, moisture anomalies with each system will be modest and
thus the brunt of the snowfall will be driven by orographic upslope
into the Cascades initially then into the Rockies thereafter. Snow
levels will be on the higher side but still low enough at times to
bring some snow to the higher mountain passes.

D1 system will have the largest QPF amounts, primarily into the
WA/OR Cascades, as a cold front comes ashore. D1 probabilities of
at least 8 inches of snow are >50% above ~5000-6000ft. Into the
weekend, the lead system will move through the Great Basin with
light to moderate snow across central Idaho/western Montana and
into the Tetons southward through the Uintas and Wasatch. WPC
probabilities for at least 8 inches of snow D2 are low (<40%) in
these areas but a bit higher into the OR Cascades with the next
system quickly approaching the coast. By D3, that system will slip
through the Great Basin and into the central Rockies, with more
moderate totals possible into northern CO (Medicine Bow). Light
snow will work its way into the Front Range by the end of the
period as low pressure deepens over southeastern CO. WPC
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so and for at least 4 inches are >50% above 8000ft.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso





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FOUS11 KWBC 020637
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EDT Sat Nov 2 2024

Valid 12Z Sat Nov 02 2024 - 12Z Tue Nov 05 2024

...Mountainous West...
Days 1-3...

An active pattern remains for much of the West this period,
downstream of a strong upper ridge between Alaska and Hawaii. This
will take a series of systems on a NW to SE path from the PacNW
through the Great Basin and into the Rockies this weekend into
early next week. For day 1, ongoing system in the NW will continue
to bring snow to the Cascades eastward to the northern Rockies and
southward through the Bitterroots, Tetons, Wind River, into the
Wasatch. WPC probabilities for at least 8 inches of snow are >50%
above 5000ft or so in the NW to about 9000ft over the Uintas.

On day 2, the trough will dig into the Four Corners region and
support light to modest snow for the CO Rockies, especially above
10,000ft where WPC probabilities for at least 8 inches of snow are

50%. Light snow is probable into the I-25 corridor but amounts are

quite uncertain. Accumulating snow is favored farther south into
the Palmer Divide. By day 3, that upper trough will continue to
move slowly through NM with a focus for snowfall over the Sangre de
Cristos into the Raton Mesa. There, WPC probabilities for at least
8 inches of snow are moderate (40-70%). Back to the PacNW, the
next system will move ashore with another surge in moisture to the
area and significant snow for the WA Cascades. Snow levels will
rise to 4500ft or so with a decent moisture plume (300-400 kg-m/s
IVT) then fall behind the front. Higher mountains passes will
likely be affected with significant accumulations, but lowering
snow levels will likely affect some lower passes as well. WPC
probabilities for at least 8 inches of snow are >50% above
3000-3500ft or so.


The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 030705
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
205 AM EST Sun Nov 3 2024

Valid 12Z Sun Nov 03 2024 - 12Z Wed Nov 06 2024

...Mountainous West...
Days 1-3...

Upper troughing over the Great Basin this morning will continue to
dig into the Four Corners region into tomorrow, promoting broad
lift over the Wasatch and especially the CO Rockies. WPC D1
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so.

Into D2, upper low will be moving out of NM with lingering snow
for the San Juans, Raton Mesa, and Jemez Mountains where WPC
probabilities for an additional 8 inches of snow are moderate
(40-70%). Concurrently, a shortwave out of the northeastern Pacific
will push into Washington with a brief surge in moisture aimed at
the WA Cascades before weakening as the cold front moves inland.
Snow levels will rise from ~4000 to 6000ft as the core of the WAA
surges in ahead of the front, then will fall back below 4000ft
post-FROPA. Moisture should reach into the northern ID mountains
and into NW MT as well, where WPC probabilities for at least 8
inches of snow are >50% above 4000-5000ft or so.

By D3, the system in the NW will continue toward the Rockies with
upstream vorticity at the nose of a 140kt jet streaming in across
the PacNW, helping to amplify the pattern over the eastern Pacific
into western North America. This, in turn, will carve out a broad
upper trough that will dig into the Four Corners region through the
end of the period (and then beyond). Broad lift combined with more
localized upslope will maximize snow over the higher terrain as
snow levels fall to the valley floors in most areas of the Northern
Tier. WPC probabilities for at least 8 inches of snow are highest

70%) over the Little Belt Mountains in MT as well as the Big Snow

Mountains due to the favorable northerly flow. Lighter snow is
forecast for the central Idaho ranges, NW MT around Glacier NP,
southward through western WY and into the CO Rockies.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 040704
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
204 AM EST Mon Nov 4 2024

Valid 12Z Mon Nov 04 2024 - 12Z Thu Nov 07 2024

...CO/NM border...
Day 1...

Upper low over NM this morning will lift northeastward across the
TX Panhandle this evening. Snow on the NW side of the low within a
region of modest PVA and enhanced upslope into the Raton Mesa will
continue for the first part of D1 before tapering off later this
evening and overnight. Lighter snow will extend eastward into the
far western OK Panhandle as rain changes to snow. WPC probabilities
for at least 8 inches of snow are >50% above 9000-10,000ft over
portions of central/northern NM and across the CO line.


...Northwest and Northern/Central Rockies...
Days 1-2...

Next system coming into the PacNW will be the last for at least a
little bit in what has been quite a busy stretch. Warm front will
move ashore today bringing in a surge of moisture and rising snow
levels up to around 4000ft to the north and 6000ft to the south
over the WA Cascades, but only rising to around 5000ft farther east
toward the Divide. Strong and favorable upslope flow will maximize
over the WA Cascades where a few feet of snow will be possible at
the higher peaks. By late in the evening and overnight into
Tuesday, snow levels will fall to around 3000ft bringing
accumulating snow to many of the lower passes across the Cascades.
WPC probabilities for at least 8 inches of snow are >50% above
3500ft or so. Upper jet will continue inland and promote moderate
snow across the northern ID ranges into northwestern Montana. Into
D2, the mid-level vort will deepen and split, with the northern
portion closing off along the MT/Canadian border before pivoting
back southward Tuesday evening over central MT. This will maintain
modest snow over the central MT terrain (i.e., Little Belt and Big
Snowy Mountains) on upslope northerly flow. There, WPC
probabilities for at least 8 inches of snow D2 are >50% above
4000ft or so. The southern portion of the mid-level trough will dig
into the Great Basin through early Wednesday, spreading snow
across UT/CO where several inches are likely, especially into the
CO Rockies. By the end of D2, an upper low will start to close off
over the Four Corners region, setting up the next phase of the
system.


...Four Corners...
Day 3...

Aforementioned upper low is forecast to be situated over eastern
UT early Wednesday morning and will likely sink southward through
the day to a position over eastern AZ by the end of D3. This will
keep a relatively consistent SW to S flow over northern NM into
southern CO, which is quite favorable for upslope enhancement over
the San Juans and into the Sangre de Cristos among other ranges.
Multiple waves of PVA will rotate around the upper low and across
the region, favoring moderate to heavy snow at times with a high
likelihood (>60% chance) of moderate impacts per the
probabilistic WSSI. WPC probabilities for at least 8 inches of
snow are >50% above 6000-8000ft and probabilities for at least 4
inches >50% down to around 5000-7000ft. Colder air filtering into
northeastern CO will also promote light to perhaps modest snow over
the I-25 corridor with heavier totals likely into the Palmer
Divide. Heavy snow is forecast to continue past the end of D3 as
the system slowly moves through the region. Please see the Winter
Weather Outlook and the Probabilistic Winter Storm Severity Index
(WSSI-P) for more information.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 050721
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Tue Nov 5 2024

Valid 12Z Tue Nov 05 2024 - 12Z Fri Nov 08 2024


...Northern/Central Rockies...
Days 1-2...

Vigorous upper trough moving through the NW will start its
transition to separate systems... the northern entity becoming an
upper low that will wobble through eastern Montana later today and
overnight. Broad divergence will favor light snow over much of
Montana but the eastward then southward movement of the forming
upper low will turn the flow northerly over central portions of the
state, which will enhance upslope flow into the Little Belt, Big
Snowy, and Big Horn Mountains. Snow will fall at a modest to heavy
rates per the 00Z HREF which shows >1"/hr probs of 50-90%. Lighter
snow is expected farther south through WY into CO as the southern
portion of the trough begins to split off and sink toward the Four
Corners, with some higher totals in the high terrain. Snowfall
should gradually taper off in most areas by Wednesday morning
(north) or overnight (south). WPC PWPF shows high chances (>70%)
for snowfall totals >8" over many of the MT ranges, especially

6,000ft, through Wednesday afternoon. Some of the tallest peaks of

these ranges could surpass 24" locally.

...Four Corners...
Day 2-3...

The southern part of the upper level feature responsible for the
heavy snow in the northern Rockies will sink southward through
eastern UT on Wednesday into AZ by early Thursday. After that, it
should slowly turn eastward into NM, but the guidance remains
inconsistent in the evolution/track. A cold front will bring in
colder air to the southern Rockies onto the High Plains as strong
upper level divergence aloft will support broad ascent over the
region. Moisture levels will be modest, but IVT on southwest to
southerly flow is forecast to be near the 90th percentile which
should act in concert with upslope enhancement to promote moderate
to heavy snow over the southern Rockies. Trend has been a bit less
and farther south with the QPF, resulting in a bit less snow for
northern areas in CO and continued uncertainty in snow amounts over
NM. Nevertheless, significant snow is still expected in at least
portions of southern CO into northern NM with this dynamic system.

The upslope component should favor both the SW/S-facing slopes of
the San Juans and Sangre de Cristos as well as farther north into
the Front Range via the cold front. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. By Friday morning, the upper low is
forecast to drift into NM with the brunt of the snowfall over NM.

WPC PWPF for the event through 12Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and into
parts of the far western High Plains. Highest totals are likely
over the Sangre De Cristo where 2ft+ is possible (>50% chance).
Around the Denver metro area, WPC PWPF for >4" of snow are around
30% but increase to the south (i.e., Palmer Divide) and then
increase again near/south of Colorado Springs and Pueblo to reach

70%.

The probability of significant ice across the CONUS is less than
10 percent.


Fracasso


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FOUS11 KWBC 060807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Wed Nov 6 2024

Valid 12Z Wed Nov 06 2024 - 12Z Sat Nov 09 2024


...Central/Southern Rockies and Adjacent High Plains...
Days 1-3...

Upper low at the base of a larger western CONUS trough lies over
southeastern UT this morning, and will continue sinking southward
today into AZ. It will slowly detach from the northern stream and
cut off as it wobbles southward then eastward into southern NM by
early Friday. Then, feeling the push of the subtropical jet, it
will lift northeastward across the northern TX Panhandle and far
western OK Panhandle to the CO/KS border by early Saturday. The
slow movement of the upper low combined with a fairly steady
stretch of moisture as well as upslope enhancement will lead to
significant snow accumulations in parts of the NM mountains and
across the CO line along the Sangre de Cristo range. Impacts may be
major to extreme in some locations.

For D1, frontal boundary over southeastern CO on the edge of the
height falls will see modest to locally heavy snow continue this
morning in an area of surface convergence and beneath bouts of PVA.
Through the day, as the upper low tucks farther southwest of CO,
heights should rise and the snow should lessen. Nevertheless,
snowfall rates of >1"/hr should yield an additional several inches
near/south of DEN and eastward over the High Plains. There, WPC
probabilities for at least 8 inches of snow after 12Z are moderate
(40-70%). Farther south over NM, closer to the stream of moisture
and vort maxes rotating around the upper low center, snow will
increase in intensity over much of the NM terrain where snowfall
just on D1 will likely exceed 8 inches (probabilities >70%) in the
terrain (e.g., Sangre de Cristos/Raton Mesa/Jemez Mnts).

By D2, slow movement of the upper low will allow for continued
snowfall to its east via persistent IVT values near/above the 90th
percentile. This could result in 1-2ft of snow over the NM terrain
(WPC probabilities for at least 12 inches are >70% over much of
northeastern NM) along with areas of blowing/drifting snow eastward
into the western OK Panhandle. Snow will continue over northern NM
into southern CO along the Sangre de Cristos and onto the High
Plains as the upper low sloshes toward the TX/OK Panhandles.
Moisture source will add in then largely switch to the western
Gulf, wrapping around the upper low and capitalizing on upslope
from the northeast through the column. Snow will finally start to
taper off by the end of the period (early Saturday).

All told, 3-day WPC probabilities for at least 24 inches of snow
are >50% above 6000-7000ft over the Sangre de Cristos and Raton
Mesa, with a large footprint of at least 6 inches over much of
eastern CO southward into central NM.


The probability of significant ice across the CONUS is less than
10 percent.


Fracasso



$$
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FOUS11 KWBC 090736
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 AM EST Sat Nov 9 2024

Valid 12Z Sat Nov 09 2024 - 12Z Tue Nov 12 2024


...Colorado Rockies...
Day 1...

The historic winter storm that has brought multiple feet of snow to
many parts of the Southern Rockies and central High Plains will
wane quickly Saturday morning as the low becomes vertically stacked
and occludes, shutting off the primary moisture feed. Despite this
occurring, the primary upper low will remain intense, and the
strong deformation axis responsible for the heavy snow banding
overnight will pivot across north-central CO into southeast WY
before slowly eroding through the day. The thermal profile to the
northeast is less conducive for snow, but some dynamic cooling due
to strong ascent could still result in some modest snowfall through
early Saturday. However, the greatest potential for any additional
significant accumulations will be across the higher terrain of
north-central CO, including the Park Range where WPC probabilities
for an additional 4+ inches of snow are 50-70%.


...Northwest...
Days 2-3...

A potent upper trough digging from the Gulf of Alaska will dig
towards Washington State, but likely come ashore over British
Columbia Monday night. This will have a two-pronged effect of
causing broad height falls across the Pacific and Interior
Northwest (height anomalies -1 sigma according to NAEFS) while also
causing some pinched broadly cyclonic flow into the Northwest
CONUS. Together, this will drive a corridor of moderate IVT for
which CW3E probabilities from the ECENS and GEFS suggest will
exceed 500 kg/m/s as an AR surges onshore, accompanied by a surface
cold front advecting eastward. This will result in a stripe of
heavy precipitation ahead of the cold front, with a secondary wave
of precipitation following in its wake embedded within the
confluent mid-level flow.

Snow levels ahead of the front will be generally 6000-7000 ft, so
the initial precipitation should fall as primarily rain except in
the highest terrain of the Cascades. However, as snow levels crash
behind the front and precipitation persists, accumulating snow will
occur as low as 3500-4000 ft by the end of the period. This will
result in significant snowfall in the WA/OR Cascades and the
Olympics, with increasing pass-level impacts possible.
Additionally, by Tuesday morning /end of D3/ precipitation falling
as snow is expected to spread as far east as the Northern Rockies
and areas around Yellowstone NP.

WPC probabilities for more than 4 inches of snow on D2 are confined
to the far northern WA Cascades and the higher terrain around Mt.
Rainier where they are above 70%. By D3, snowfall expands along the
Cascades and as far south as the Sierra, while additionally
pushing inland to the Northern Rockies. WPC probabilities for more
than 6 inches D3 are above 70% along the Cascades and near Mt.
Shasta, where 1-2 feet of snow is likely in the highest terrain.
Elsewhere, WPC probabilities are as high as 30-50%, highest near
the Salmon River and Sawtooth Ranges of Idaho. With snow levels
falling, impactful pass-level snow is also likely, especially at
some of the higher passes like Washington Pass and Stevens Pass in
the WA Cascades.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss

...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 100738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Sun Nov 10 2024

Valid 12Z Sun Nov 10 2024 - 12Z Wed Nov 13 2024

...Northwest...
Days 2-3...

Dual amplified shortwaves will bring an active period of weather to
the Pacific and Interior Northwest through mid-week. The first of
these will dig out of the Gulf of Alaska and approach the British
Columbia coast Sunday night, before sharpening into a short-
wavelength but amplified open wave advecting onshore WA/OR Monday
evening. This feature will then move quickly eastward into the
Northern Rockies while weakening through Tuesday. Although this
trough will be of modest amplitude and progressive, height falls,
divergence and dual jet streaks (one poleward arcing downstream of
the trough axis, another more zonally oriented upstream) will lead
to widespread deep layer ascent from the Pacific Northwest eastward
through the Northern Rockies. At the same time, this trough will
drive a cold front eastward, surging onshore the WA/OR coasts
Monday morning before racing into the Northern High Plains by the
end of the forecast period.

This synoptic lift will act upon an increasingly moist column noted
by rapidly rising 1000-500mb RH as warm/moist advection occurs
within the aforementioned jet streaks, driving IVT to 250-500
kg/m/s. This will support widespread moderate to heavy
precipitation, and as snow levels collapse from around 6000 ft
early to as low as 3500 ft D2, snow will become widespread in the
terrain of the Olympics, Cascades, Shasta/Trinity region of CA, the
Sierra, and then into the Northern Rockies. WPC probabilities for
at least 6 inches of snow are 70+% along the spine of the Cascades
of WA and OR, with amounts reaching multiple feet likely in the
highest volcanoes. There is also a 30-50% chance of more than 6
inches in the Shasta/Trinity region, the northern Sierra, the Blue
Mountains of OR, and the far Northern Rockies.

After a brief period of shortwave ridging across the Pacific
Northwest, a more pronounced impulse digs into the region during D3,
nearly reaching the coast by the end of the period. This will be
accompanied by more impressive moist advection with ECENS IVT
probabilities reaching above 90% for 500 kg/m/s, with several
members exceeding 750 kg/m/s. The accompanying WAA will surge snow
levels back to around 5000-6000 ft along the immediate coast, but
rise only to around 4000 ft across the Cascades. This will support
more widespread heavy snow, with generally below climo SLRs leading
to high snow load and greater impacts, especially across the WA
Cascades. WPC probabilities are above 9)% in the WA Cascades,
primarily above 5000 ft, with 1-2 feet likely in the higher
elevations. Some pass level impacts are also expected, especially
at Washington Pass, with lesser but still potential impacts at
Stevens Pass.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss


$$
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FOUS11 KWBC 110728
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 AM EST Mon Nov 11 2024

Valid 12Z Mon Nov 11 2024 - 12Z Thu Nov 14 2024

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

Two distinct and anomalous shortwaves will amplify and move
progressive across the Western CONUS through Thursday, bringing an
extended period of unsettled weather to the region, with several
days of moderate to heavy snow expected in the higher terrain.

The first of these will approach the Pacific Coast Monday morning,
with downstream SW flow driving robust IVT approaching 500 kg/m/s
onshore into CA/OR/WA. This IVT will help surge PWs to above climo
directly ahead of a surface cold front which will be positioned
beneath the trough axis. As this trough/front advect onshore Monday
aftn, the overlapping ascent into the moistening column will result
in heavy precipitation spreading from central CA through the WA
Cascades, and then pushing east, while weakening due to lessening
IVT, reaching the Northern Rockies and eastern Great Basin by
Tuesday morning. Snow levels ahead of this front will be generally
around 6000 ft during the period of heaviest precipitation, but
will fall quickly to as low as 3500 ft behind the front. At the
same time, while the primary forcing mechanisms will shift east,
continued onshore moist advection in pinched mid-level flow will
drive persistent moisture onshore, resulting in snow accumulations
lowering across the Cascades/Olympics. WPC probabilities D1 for
more than 6 inches of snow are above 70% along the spine of the
Cascades in OR and WA, with up to 2 feet possible in the highest
terrain. Lower probabilities exist into the Shasta/Trinity region,
the northern Sierra, and as far east as the Blue Mountains of OR.

During D2 /Tuesday and Tuesday night/ the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, while shortwave ridging briefly builds in
across the Northwest. This suggests that much of D2 will feature
waning snowfall with lesser accumulations. However, before D2 ends,
the subsequent trough will approach the Pacific Northwest once
again. This trough is progged to be deeper, noted by 700-500mb
height anomalies reaching -2 sigma according to NAEFS, with the
subsequent downstream warm/moist advection in the mid and upper
levels driving IVT to as high as 750 kg/m/s, and ECENS
probabilities suggest even a 30% chance of IVT exceeding that
value. This WAA will occur within a narrow channel ahead of yet
another cold front, driving a corridor of heavy precipitation in
conjunction with snow levels rising once again to 6000-7000 ft.
This will limit snowfall before D2 ends except in the highest
terrain of the Olympics and Cascades.

The trough axis responsible for pushing the front eastward and
driving the pronounced deep layer ascent across the Northwest will
struggle to move east as repeated lobes of vorticity rotate down
from the Gulf of Alaska, resulting in little movement of the trough
axis and persistent, although weaker, moist advection onshore. This
suggests that snow levels will only slowly fall, progged to drop
to 4500-5000 ft by the end of the forecast period, with primarily
light to moderate precipitation rates from northern CA through the
Northern Rockies. This will still result in heavy snow
accumulations at the highest elevations, reflected by WPC
probabilities for more than 6 inches exceeding 90% in the northern
WA Cascades, and then expanding east into Selkirk Range, the Blue
Mountains, and the Sawtooth/Salmon River area. The higher passes
above 5000 ft will also likely experience significant impacts from
snow accumulations, but in general most of the passes should be
below the heavier snow accumulation levels.


The probability of significant ice across the CONUS is less than
10 percent.


Weiss


$$
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FOUS11 KWBC 120820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Tue Nov 12 2024

Valid 12Z Tue Nov 12 2024 - 12Z Fri Nov 15 2024

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

An active period of winter weather continues across the Western
CONUS this period as dual shortwave trough cross the region.,

The first of these will be moving from the eastern Great Basin this
morning into the Central Plains by Wednesday evening, with minimal
amplitude gain. The combination of the progressive nature of this
feature with modest PW surge (at least until the Plains when
thermals support only rain) will somewhat limit snowfall potential
D1. However, sharp height falls downstream of the short-wavelength
trough combined with low-level convergence along the accompanying
cold front and post-frontal upslope flow into terrain features will
still support rounds of moderate to heavy snow. The greatest
potential for significant accumulations will be across the Wasatch
and Colorado Rockies, including the Park Range, where WPC
probabilities for more than 6 inches are above 70%, and locally up
to 10 inches is possible, primarily above 5500 ft.

Brief shortwave ridging blossoms across the Northwest behind this
first trough, but is quickly replaced by a more impressive impulse
digging along the Pacific Coast as it drops from the Gulf of
Alaska. Downstream divergence and accompanying WAA/moist advection
will begin to spread precipitation back into the Pacific Northwest
late tonight. Unlike its predecessor, this secondary trough will be
very slow to move east as it gets repeatedly reinforced just off
the coast by renewed lobes of vorticity swinging cyclonically
around it. This will have the two-pronged effect of driving waves
of ascent and persistent moisture onshore, while also keeping the
primary trough axis positioned just west of the region until Friday
when a more pronounced vort max swings through the base and pushes
the trough onshore CA late in the forecast period.

Within the most impressive WAA, a narrow channel of IVT exceeding
750 kg/m/s is likely, focused into the WA/OR and northern CA coast
late D2 into D2 before weakening with inland extent. This channel
/AR/ will also provide the highest increase in snow levels,
reaching as high as 7000 ft. However, dual cold front progged to
push onshore, one Wednesday morning and another Thursday morning,
driven east by periodic shortwaves, will cause gradual lowering of
snow levels to as low as 3000 ft by Friday morning across the
Cascades, and 4500 ft into the Interior Northwest and northern CA.
The lower snow levels will be accompanied by weaker forcing and
more transient moisture however, so heavy pass-level snow is still
not anticipated at this time.

Despite that, heavy snow accumulations are likely, especially on D2
when WPC probabilities for more than 6 inches spike above 90% in
the Cascades, Shasta/Trinity region, the Okanogan Highlands,
Selkirk Mountains, and Blue Mountains. Some of these areas could
receive as much as 2 feet of snow D2, and 6-12 inches is possible
at some of the elevated passes including Washington Pass. By D3,
the snow intensity wanes across most of the NW, but an increase in
WPC probabilities for 6+ inches reaching 30-50% in the Sierra as
the trough finally pushes onshore into CA.

The probability of significant ice across the CONUS is less than
10 percent.


Weiss

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FOUS11 KWBC 130751
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Wed Nov 13 2024

Valid 12Z Wed Nov 13 2024 - 12Z Sat Nov 16 2024

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

The weather pattern over the next few days will be driven largely
by a longwave upper level trough with a pair of upper level
disturbances tracking into the West Coast. The first storm system
is already impacting the Pacific Northwest with an IVT up to 750
kg/m/s (topping the 97.5 climatological percentile) escorting
anomalous moisture as far south as northern California and as far
east as the Lewis Range in Montana. Elevations above 5,000ft,
whether it be in the Cascades, Olympics, northern Sierra Nevada, or
inland through the Blue, Sawtooth, Bitterroots, and Lewis Range,
will be favored for witnessing accumulating snowfall that could
result in treacherous travel conditions. The WSSI does depict some
localized areas of Minor to Moderate Impacts in the >5,000ft
elevations of the Olympics, Cascades, Blue, and Shasta Mountains
today.

Onshore flow into the Pacific Northwest will keep high elevation
snow in the forecast through Thursday in the Northwest. By
Thursday night, the next Pacific storm system will track farther
south into California with 500mb heights that below the 2.5
climatological percentile on NAEFS. Heights will be low enough
Thursday night and into Friday morning that snow levels in the
Sierra Nevada will be as low as 4,000ft (5,000ft in the San Gabriel
and San Bernadino of southern California). The heaviest snowfall
will be observed above 6,000ft in the central Sierra Nevada. WPC
PWPF shows moderate-to-high chances (50-70%) for snowfall totals

4" in the central Sierra Nevada between Thursday evening and

Friday morning. By Friday afternoon, the upper trough will track
into the heart of the Intermountain West with strong upper level
ascent over the Northern Rockies. WPC PWPF shows moderate chances
(40-60%) for snowfall totals >4" in the >7,000ft ranges of the
Absaroka.

...Northern New England...
Day 3...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
plume of low level moisture back towards northern New England on
Friday. The air-mass over northern New England (particularly
Maine) will be sufficiently cold and dry enough for wet-bulb
temperatures to remain below freezing within the boundary layer.
Strong easterly mid-level flow with air-mass origins over the
Atlantic will lead to warm air-advection that causes a >0C nose
between 850-750mb Friday morning. This is a setup that will likely
result in periods of sleet/freezing rain over northern Maine and
possibly as far west as northern New Hampshire. WPC PWPF does show
low chances (10-30%) for ice accumulations >0.1" in parts of the
North Woods and Central Highlands of northern Maine. One notable
location with those potential ice accumulations includes Baxter
State Park. These icy conditions could result in Minor Impacts
according to the Probabilistic WSSI with low-moderate chances
(30-50%) through Friday afternoon. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions.

Mullinax


$$
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FOUS11 KWBC 140802
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Thu Nov 14 2024

Valid 12Z Thu Nov 14 2024 - 12Z Sun Nov 17 2024

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

A pair of Pacific storm systems will produce heavy mountain snow
from the West Coast to the Northern Rockies over the next few days.
Today, broad scale upper level ascent out ahead of a longwave
trough off the West Coast will also direct Pacific moisture at the
Northwest and northern California. Periods of snow will occur for
many of the Northwest U.S. mountain ranges which include (but are
not limited to) the Cascades, Siskiyou, northern Sierra Nevada,
Blue, Sawtooth, Bitterroots, Idaho Panhandle, and northern Montana
Mountains). As a potent upper level trough approaches California
Thursday night (500mb heights below the 2.5 climatological
percentile via NAEFS), snow levels will fall to as low as 4,000ft
in the Sierra Nevada and as low as 6,000ft in the San Gabriel and
San Bernadino of southern California. The heaviest snowfall will
be mainly confined to elevations at/above 6,000ft in the Sierra
Nevada. By Friday morning, the upper level trough will advance
through the Great Basin and into the Rockies by Friday afternoon.
Periods of snow will move through northern Nevada and into the
northern Rockies with the Tetons, Wind River, and Absaroka ranges
seeing the heaviest snowfall Friday evening. Snow will also pick up
in intensity along the Wasatch, Uinta, and Big Horn ranges Friday
night and into Saturday morning. WPC PWPF shows moderate-high
chances (50-70%) for >4" of snowfall along the Sierra Nevada above
6,000ft, the northern Great Basin above 7,000ft, and the Absaroka,
Tetons, and Wind River ranges above 8,000ft.

Meanwhile, the Pacific Northwest will see periods of snow gradually
diminish throughout the day today with a fairly tranquil day
weather-wise on tap for Friday. By Saturday, the next Pacific
trough approaches with a stronger IVT (>500 kg/m/s, or above the
90th climatological percentile via NAEFS) will direct another plume
of Pacific moisture at the region. Strong synoptic-scale support
will also be present as 200-500mb winds over western Washington top
the 90th climatological percentile. As the warm front arrives
Saturday night, snow levels will rise to as high as 5,500ft in some
cases, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Through
12Z Sunday, the northern Cascades and the Idaho Panhandle above
5,000ft sport high chances (>70%) for snowfall >6" with a low
chance (10-30%) for >12" in the tallest peaks of the northern
Washington Cascades.

...Northern Maine...
Days 1-2...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
stream of low-level moisture back west towards northern New England
late Thursday night into Friday. The air-mass over northern New
England (particularly Maine) will be sufficiently cold and dry
enough for wet-bulb temperatures within the boundary layer to
remain below freezing for at least a few hours. Strong easterly
mid-level flow with air-mass origins over the Atlantic will lead to
warm-air advection (WAA) that causes a >0C nose between 850-750mb
Friday morning. This setup will result in periods of sleet/freezing
rain over northern Maine and possibly into far northern New
Hampshire. WPC PWPF does depict some spotty areas with low chances
(10-30%) for ice accumulations >0.1" in portions of the North Woods
and Central Highlands of northern Maine. One notable location with
those potential ice accumulations includes Baxter State Park. These
icy conditions could result in Minor Impacts according to the
Probabilistic WSSI with low chances (10-30%) through Friday
afternoon. The primary impacts would be icy roads in some areas
that could lead to slick travel conditions.

Mullinax



$$
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FOUS11 KWBC 251905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Fri Oct 25 2024

Valid 00Z Sat Oct 26 2024 - 00Z Tue Oct 29 2024


...Pacific Northwest...
Days 2-3...

An impressive closed low spinning off the British Columbia coast
will shed a shortwave and accompanying vorticity maxima into the
Pacific Northwest Sunday. This feature will spread height falls and
PVA to drive ascent eastward, with a collocated upper jet streak
also pivoting onshore to enhance lift. The best moisture advection
will occur downstream of this lead trough, but snow levels D2 will
be quite high, 7000-8000 ft, so despite impressive IVT exceeding
500 kg/m/s leading to widespread precipitation, snow will be
confined to only the highest peaks of the Cascades and Olympics.
However, during D3, the parent upper low shifts southward and opens
into a still sharp upper trough, lifting onto the WA/OR coast
Monday. While residual moisture will gradually decay through D3,
sufficient ascent downstream of this trough combined with upslope
ascent will drive continued moderate precipitation, with snow
levels falling to 4500-5000 ft. This will allow snow to accumulate
into lower elevations by the end of the forecast period, but
most snow is still expected only above pass level.

WPC probabilities for more than 6 inches of snow are only above
30% in the highest peaks of the WA Cascades D2, where locally 2
feet of snow or more is probably near the top of Mt Rainier. By D3
snowfall becomes more widespread, with WPC probabilities for more
than inches reaching 30-50% across much of the spine of the
Cascades from WA to OR, and several feet of event-total snow is
likely on the higher mountain tops.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss



$$
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FOUS11 KWBC 260845
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Sat Oct 26 2024

Valid 12Z Sat Oct 26 2024 - 12Z Tue Oct 29 2024


...Pacific Northwest...
Days 2-3...

Upper level ridging will give way to a broad upper trough, carved out
by a series of shortwaves moving south of a low moving into British
Columbia. This will support the return of unsettled weather and
falling snow levels in the wake of a strong cold front moving from
the Northwest into the northern Rockies on Sunday. Strong mid-to-
upper level forcing and a shot of deeper moisture are expected to
generate some of the heaviest precipitation across western
Washington and Oregon Saturday night into Sunday morning. However,
with snow levels above 6000 ft in most locations, winter weather
impacts will be limited, initially. Snow levels are expected to
steadily drop through Sunday, dipping to around 4500-5000 ft along
the Washington and Oregon Cascades by Monday morning. Conditions
will remain unsettled as onshore flow persists. WPC probabilities
indicate that most significant snow accumulations will remain above
the major Washington and Oregon passes through Monday Morning.

Unsettled weather will continue across the Northwest through
Monday, with snow levels dipping further as additional shortwaves
move through the base of the broader scale trough as its axis
shift inland. WPC probabilities indicate that potentially
impactful accumulations will become more likely along the Oregon
Cascades, with probabilities for accumulations of 4 inches or more
climbing above 50 percent in some of the passes.

Snow will also be spreading south into the Sierra Nevada and east
into portions portions of the northern and central Rockies Monday
into Tuesday. The highest probabilities for significant
accumulations through Tuesday morning center over north-central
Idaho and southwestern Montana and northwestern Wyoming -- in and
around Yellowstone NP. WPC guidance shows at least low
probabilities for accumulations of 6 inches or more in the higher
elevations of these areas.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 271922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EDT Sun Oct 27 2024

Valid 00Z Mon Oct 28 2024 - 00Z Thu Oct 31 2024


...Pacific Northwest to the central Rockies...
Days 1-3...

A much more active pattern is set to develop this week as a large
trough begins to expand across the western CONUS.

A Pacific trough will encroach on the coast D1, with the primary
trough axis pivoting onshore by Tuesday morning. The most
impressive vorticity impulse embedded within this trough is progged
to surge into central CA, which will then help to amplify this
feature as it advects east on Tuesday into the Great Basin and then
Four Corners. Initially, this trough will remain progressive and
positively tilted, but should deepen, and some of the guidance
suggests a negative tilt or even closing off of the upper height
fields late in the forecast period. Although there is, as expected,
some uncertainty into the placement and speed of this evolution,
500mb heights across the West feature minimal spread in the IQR
according to DESI, so confidence is pretty high in the progression.

As this trough surges eastward, it will drive increasingly intense
ascent through height falls and downstream divergence, especially
during the period of best amplification D2, and combine with a
strengthening 250mb jet streak pivoting from CA into the central
Rockies to force impressive deep layer ascent, especially in the
Rockies. At the same time, moisture will become increasingly robust
due to onshore flow of Pacific moisture streaming ahead of the
trough axis, and pushing PWs to +1 to +2 sigma according to NAEFS
ensemble tables. This will result in widespread and expanding
precipitation, first in the WA/OR Cascades, but then becoming more
robust across the central Rockies beginning Tuesday aftn.

The challenge with this event will be how much snow will spread out
of the higher terrain and into the High Plains Tuesday and
Wednesday. While forcing will remain impressive, a cold front
digging southward will begin to cool the column, while also
providing a region of enhanced baroclinicity leading to stronger
frontogenesis. This should result in a surface wave developing near
UT/CO and then lifting E/NE into the Plains by Wednesday. The
combination of fgen, this modestly strengthening surface low, and
the southward push of the cold front should help cause snow levels
to crash from around 8000ft early to 4500-5000 ft late. This,
combined with periods of dynamic cooling, will result in
significant snow accumulations in many areas.

The other challenge with this system will occur D3 as the low
ejects into the Plains and leaves what could be a potent comma
head/deformation axis with precipitation to its NW. There is
considerable model spread in the placement/timing of this feature,
so confidence is lower, but some of the more aggressive guidance
indicates the band will be strong enough to cause dynamical
cooling to support of swath of snow into the Dakotas and maybe even
MN before the end of the forecast period. Despite favorable omega
into a subjectively deep DGZ, the weaker/less snowfall solutions
are preferred based on consensus and ensemble means, but it will be
worth monitoring the next few model runs to see if this stripe of
heavier snow could materialize.

At this time, while uncertainty still exists in both the elevations
at which significant snow can accumulate, and the eastern footprint
of the heavy snow axis, heavy snow is likely in a lot of the higher
elevations. On D1, WPC probabilities for more than 6 inches are as
high as 70-90% in the WA Cascades as a weak low pressure approaches
the coast beneath the primary mid-level trough. By D2, more
expansive precipitation begins in the central Rockies, and WPC
probabilities for more than 6 inches are above 50% in the San
Juans, Uintas, Big Horns, Wind Rivers, and around Yellowstone NP.
By D3 precip begins to wane, but periods of moderate to heavy snow
will persist across the higher terrain of CO and WY, and as far
east as the Black Hills of SD where WPC probabilities are 30-70%
for an additional 6+ inches. Storm total snow of 12-18" is likely
in some of the highest elevations by Wednesday night.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss



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FOUS11 KWBC 230703
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Wed Oct 23 2024

Valid 12Z Wed Oct 23 2024 - 12Z Sat Oct 26 2024

...Northern Rockies...
Day 1...

Mid-level low off the WA/OR coast opens into compact shortwave
and ejects east over the OR Coast this afternoon before weakening
as it reaches western WY late tonight. Lee-side low pressure
develops over northern WY overnight which aids upslope flow back on
the western WY terrain where snow reaches moderate rates overnight.
Snow levels drop from around 8000ft to 7000ft overnight during this
precip, particularly around greater Yellowstone. Day 1 PWPF are
10-40% for >4" over the northern Absarokas in southern MT and in
the Tetons.


The probability of significant ice across the CONUS is less than
10 percent.


Jackson


$$
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FOUS11 KWBC 250730
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Fri Oct 25 2024

Valid 12Z Fri Oct 25 2024 - 12Z Mon Oct 28 2024


...Pacific Northwest...
Day 3...

Upper level ridging will give way to a broad upper trough, carved
out by a series of shortwaves moving south of a low moving into
British Columbia. This will support the return of unsettled weather
and falling snow levels in the wake of a strong cold front moving
from the Northwest into the northern Rockies on Sunday. Strong mid-
to-upper level forcing and a shot of deeper moisture are expected
to generate some of the heaviest precipitation across western
Washington and Oregon Saturday night into Sunday morning. However,
with snow levels above 6000 ft in most locations, winter weather
impacts will be limited. Snow levels are expected to steadily drop
through Sunday, reaching to around 4500 ft in the northern Cascades
by Monday morning. Conditions will remain unsettled as onshore
flow persists. However, WPC probabilities indicate that most
significant snow accumulations will remain above pass level.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira

$$
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FOUS11 KWBC 210820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Mon Oct 21 2024

Valid 12Z Mon Oct 21 2024 - 12Z Thu Oct 24 2024

...Colorado Rockies...

Day 1...
An upper low centered over central CO early this morning will shift
east into KS by early afternoon. Comma head precip, moderate at
times, on the back side of the low will persist over the north-
central CO Rockies through this morning with snow levels around
9000ft. Probs for an additional >6" snow between 12Z and 18Z are
around 50 percent in portions of the Front Range.


...Glacier National Park...
Day 1...

An upper trough axis reaches the PacNW coast early this morning and
shift ENE through western MT this evening. An inland surge of
Pacific moisture ahead of this trough maintains high snow levels
over the Northwest today, but height falls are sufficient by this
evening to allow snow levels to drop on the Lewis Range through
Glacier NP to drop from around 7000 to 5000ft tonight. PWPF for >6"
is around 30 percent on the higher peaks of the park.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 220712
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EDT Tue Oct 22 2024

Valid 12Z Tue Oct 22 2024 - 12Z Fri Oct 25 2024

...Greater Yellowstone...
Day 2...

A rather positively tilted trough extending WSW from southern BC
closed off into a mid-level low today west of WA. This low then
opens into a compact trough and ejects east Wednesday with the H5
trough axis crossing greater Yellowstone late Wednesday night. Snow
levels in the moisture ahead of this wave are 8000-9000ft. The
only snow probs for >4" in the WWD forecast period are on Day 2 and
in the 20-40% range over the northern Absarokas in MT and for the
Tetons. There is about a 10% risk for 6" in the Red Lodge portion
toward Beartooth Pass.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 180830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Oct 18 2024

Valid 12Z Fri Oct 18 2024 - 12Z Mon Oct 21 2024

...Central and Southern Rockies...
Days 1-2...

Potent low closes off over northern AZ this morning where it stalls/occludes/fills through Saturday. This will result in a
prolonged period of intense synoptic ascent focused across the Four
Corners and southern/central Rockies, where height falls,
downstream divergence, and upper level diffluence in response to
modestly coupled jet streaks overlap. At the same time, a surface
cold front will shift east over the Four Corners today and over the
southern Rockies tonight. This will have a two-pronged effect of
enhancing ascent, as a wave of low pressure develops along it,
while impressive baroclinicity results in sharpening fgen to drive
locally more intense lift. Where this most intense ascent occurs,
especially in areas of upslope flow, precipitation rates will be
heavy.

Downstream of this upper low, strong moisture advection leads to
PWs +2 to +3 sigma, additionally supporting widespread heavy
precipitation. Snow levels ahead of the low are quite high,
varying between 9000 to 10000ft, but will fall to around 6500ft
underneath the upper low over AZ tonight. Day 1 PWPF for >8" is
over 50% in the higher southern UT ranges and the Kaibab Plateau
and around Humphreys Peak in northern AZ along with over 80% for

12" additional in the San Juans of CO.

The filling low causes precip rate to drop to light to moderate
with snow levels lingering around 9000ft over the San Juans. Day 2
PWPF for additional >8" is 30-50%.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 150737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Tue Oct 15 2024

Valid 12Z Tue Oct 15 2024 - 12Z Fri Oct 18 2024

...Northeast...
Day 1...

Following a brief tapering off of snowfall overnight, periods of
snow will pick back up over the northern Appalachians Tuesday and
linger into Tuesday night. This is due to an elongated upper level
trough pivoting around the western flank of a closed 500mb low
situated over the St. Lawrence Seaway that will traverse the
Northeast from west to east. In addition to the usual upslope flow
into the Adirondacks, Green, and White Mountains, the steep lapse
rates aloft will trigger potent snow showers that could cause brief
bursts of heavy snow at elevations >2,000ft. A few snow showers may
linger around northern New England Wednesday morning, but
accumulations would be minor and limited to the elevations >3,000ft
on Wednesday. WPC probabilities show low chances (10-30%) for
additional snowfall totals >4" along the higher elevations of the
Adirondacks, Green, and White Mountains through Wednesday morning.

...Pacific Northwest...
Days 2-3...

An upper level shortwave trough bringing a surge in 850-700mb
moisture and causing 500-700mb heights falls will cause snow levels
to gradually drop from 6,000ft Tuesday night to around 4,000ft
Wednesday evening over Washington's Olympic and Cascade Ranges. As
this initial 500mb vort max exits to the east Wednesday night, a
second trough will dive south through the region on Thursday with
snow levels bottoming out just below 4,000ft in some cases. Periods
of heavy snow are expected along the Oregon Cascades during the day
on Thursday. Snow should taped off some time Thursday night. WPC
PWPF showed high chances (50-70%) for snowfall totals >4" above
5,000ft in the Cascade Range through Thursday.

...Northern & Central Rockies...
Day 3...

Pacific moisture out ahead of the first upper level shortwave to
traverse the Pacific Northwest is forecast to produce periods of
snow in the higher elevations (>6,000ft in northern ID, >7,000ft in
the Bitterroots, Sawtooth, Absaroka, Tetons, Wind River, and Big
Horn Ranges) early in the day on Thursday, but it is with the
approach of the second trough where snowfall rates look to
increase. Stronger height falls Thursday evening will accompany an
air-mass that NAEFS shows at the 500mb and 700mb layers to be below
the 10th climatological percentile. Plus, NAEFS shows an anomalous
IVT that tops the 90th climatological percentile from the Great
Basin to Utah. The region will also be placed favorably beneath a
coupled jet structure that will help to maximize upper level
divergence from the Absaroka on south to the Wasatch Thursday night
and into Friday morning. Waves of low pressure will develop along
a strengthening frontal boundary that will track through the
Wasatch and into southern WY by Friday morning, prompting
precipitation that starts out as rain to make a quick changeover to
heavy snow.

WPC PWPF sports moderate-to-high chances (50-70%) for >6" of
snowfall in the Absaroka of southwest Montana with some moderate
chances (40-60%) for >12" in some of the Absaroka's tallest peaks.
Farther south, the Bighorns and Wind River Ranges sport low-
moderate chances (30-50%) of seeing >8" of snowfall through 12Z
Friday, while the Uinta how moderate-high chances (50-70%) for the
same snowfall totals through 12Z Friday. This is likely to be the
first winter storm of the season for these mountain ranges this
season and will result in impacts residents have not dealt with in
months (poor visibility, slick/snow covered roads). The WSSI-P does
sport moderate-high chances (50-70%) for Minor Impacts in the
Absaroka, Wind River, Big Horn, and Uinta ranges, as well as some
of the tallest peaks of the Wasatch through Friday morning.


The probability of significant ice across the CONUS is less than
10 percent.


Mullinax




$$
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FOUS11 KWBC 160752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Oct 16 2024

Valid 12Z Wed Oct 16 2024 - 12Z Sat Oct 19 2024

...Pacific Northwest...
Days 1-2...

An upper level shortwave trough ushering in a surge of 850-700mb
moisture and resulting in upper level height falls will cause snow
levels to gradually drop from 6,000ft early this morning to around
4,000ft by Wednesday evening over Washington's Olympic and Cascade
Ranges. As this initial 500mb vort max exits to the east Wednesday
night, a second trough will dive south through the region on
Thursday with snow levels bottoming out just below 4,000ft in some
cases. The heaviest snowfall is likely to occur along the
Washington Cascades Wednesday evening with moderate-to-heavy
snowfall possible along the tallest peaks of the Oregon Cascades
Thursday morning. WPC probabilities depict high chances (>70%) for

4" of snowfall in the Washington Cascades and peaks above 5,000ft.

The taller volcanic peaks of the Washington Cascades sport
moderate-high chances (50-70%) for snowfall >8" through Thursday.


...Northern & Central Rockies...
Days 2-3...

**First Significant Winter Storm of the Season to Impact the
Central Rockies**

The evolution of the first significant winter storm of the season
across portions of the Intermountain West begins as an upper level
trough enters the Pacific Northwest on Thursday. As Thursday
progresses, the upper trough will deepen as it tracks south through
the Great Basin Thursday night, then becoming a closed low along
the UT/AZ border by midday Friday. The low continues to slowly
track across northern Arizona Friday night but remain stalled over
the Four Corners region early Saturday morning.

NAEFS shows upper level evolution that is quite anomalous from a
variety of parameters. At 250mb, a jet streak over the northern
High Plains will be coupled with a jet streak at the base of the
amplifying upper trough, thus maximizing upper level divergence
over the Intermountain West on Thursday. As the upper trough cuts
off into an upper low by Friday, strong divergence beneath the left
exit region of the 250mb jet streak will be placed over Utah and
the Central Rockies. NAEFS shows 500-700mb heights and temperatures
that are routinely below the 10th climatological percentile,
particularly over the Great Basin and Utah late Thursday into
Friday. Perhaps most impressive are the IVTs which will eclipse
300 kg/m/s (above the 99th climatological percentile) in southern
UT Thursday night. Mean flow out of the SW will result in strong
upslope ascent into the Uinta and San Juans in particular.

In the Absaroka, Big Horns, and Wind River Ranges, WPC PWPF shows
high chances (>70%) for storm event snowfall totals >8" with at
least moderate chances (>50%) for snowfall >12" in the tallest
peaks. Farther south, the Uinta and San Juan sport high chances

70%) for snowfall totals surpassing 12". In fact, the Uinta sport

moderate chances (40-60%) for >18" of snowfall through Saturday
morning, with high chances (>70%) in the San Juan for >18" of
snow. The WSSI-P Moderate probabilities are keying in on the San
Juans as seeing the most impactful snow from this event, primarily
due to a combination of the algorithm's focus on snow totals,
snowfall rates, and snow load. Probabilities are showing moderate
chances (40-60%) for snowfall that would result in hazardous
driving conditions and potential closures and disruptions to
infrastructure in the San Juans, Wasatch, and Uinta. While Moderate
Impacts are deemed lower chance (10-30%) in the Absaroka, Wind
River, and Big Horn Ranges, potential impacts are elevated compared
to what the algorithm suggests given it is the first significant
winter storm of the season in these Ranges above 7,000ft. There are moderate-high chances (50-70%) for Minor Impacts in these Ranges,
as well as far south as parts of the Sangre De Cristo and far
northern mountains of Arizona.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



$$
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FOUS11 KWBC 140709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Mon Oct 14 2024

Valid 12Z Mon Oct 14 2024 - 12Z Thu Oct 17 2024

...Northeast...
Days 1-2...

An anomalous upper level trough over the Northeast will lead to a
strengthening cyclone over northern New England this morning. The
850mb low positioned over Upstate New York will track north and
east through the Green and White mountains by this afternoon. To
the north and west of the 850mb low, a surge in both moisture and
NW winds will result in a stronger upslope component into the
Adirondacks, Green, and White mountains throughout the day. In
addition to the sufficient upper level ascent, an ari-mass that
is quite cold by mid-October's climatology will support sub-
freezing boundary layer temps for in the northern Appalachians. Locally
heavy snowfall rates are expected with 0.5-1"/hr rates possible in
the tallest peaks of the Adirondacks this morning and afternoon
according to the 00Z HREF on WPC's Snowband Probability Tracker. As
the storm tracks into Quebec this evening, snow will taper off and
be limited to just snow showers through Tuesday morning. Another
round of snow is likely to occur Tuesday late morning and through
the afternoon as a trough axis pivoting on the back side of the
storm system in eastern Quebec brings about an additional surge of
low- mid level moisture. In addition, steep lapse rates could
trigger potent snow showers, especially in the higher terrain of
northern New England above 2,000ft. Snow showers look to linger in
the northern Appalachians until Wednesday morning when the trough
axis moves east and off the New England coast.

Any locally heavy snowfall totals (>4") would likely be confined
to elevations above 2,000ft in the Adirondacks, Green, and White
mountains. WPC probabilistic guidance does depict some low-
moderate chances (20-40%) for >6" of snowfall in the tallest

3,000ft peaks of the Adirondacks. Below 2,000ft, snowfall amounts

will struggle to top 2" in the 1,000-2,000ft elevation layer, with
mainly a coating to 1" in the elevations below 1,000ft. Given this
snowfall is the first of the season for parts of the region,
motorists will be subjected to poor visibility and some snow
covered roads for the first time in months. The WSSI-P does show
low-moderate chances (20-40%) for Minor Impacts in parts of the
Adirondacks through Monday evening.

...Northwest...
Days 2-3...

A 500mb shortwave trough bringing with it a surge in 850-700mb
moisture and associated height falls will cause snow levels to
gradually drop to as low as 4,000ft by Wednesday. The air-mass is
not particularly cold for the time of year, so this setup favors
snowfall accumulation in the Olympics and Cascades to elevations

5,000ft. WPC probabilities do suggest low-moderate chances

(20-40%) for >4" of snowfall on Wednesday along the windward side
of the Cascade Range at elevations >4,000ft. As 500mb heights and
snow levels continue to fall, some of the higher level Washington
passes may start to see accumulating snow by Wednesday night. The
potential for accumulating snowfall reaches the Northern Rockies
(the Bitterroots, Absaroka, and Tetons most notably) by early
Thursday morning. WPC probabilistic guidance suggests the Lewis
Range of western MT could see locally higher totals surpassing 4"
at elevations >7,000ft, while the other mountain ranges mentioned
are most likely to see their best accumulating snowfall above
8,000ft on Thursday.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax


$$
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FOUS11 KWBC 150811
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EST Fri Nov 15 2024

Valid 12Z Fri Nov 15 2024 - 12Z Mon Nov 18 2024

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

This morning, an upper level trough will advance through the Great
Basin and into the Rockies by this evening. Periods of snow
along the Sierra Nevada and northern Great Basin this morning will
move into the northern Rockies tonight with the Tetons, Wind
River, and Absaroka ranges seeing the heaviest snowfall. Snow will
also pick up in intensity along the Wasatch, Uinta, Big Horn,
Little Belt, and Big Snowy ranges tonight and into Saturday
morning. WPC PWPF shows moderate chances (40-60%) for >6" of
snowfall along the Sierra Nevada above 9,000ft. High probabilities

70%) for snowfall >8" are forecast along the ridgelines of the

northern Great Basin above 6,000ft and the Absaroka, Tetons, and
Little Belt, and Big Snowy ranges above 7,000ft.

Elsewhere, the Pacific Northwest will witness a fairly tranquil
day weather-wise today. This changes by Saturday as the next
Pacific trough approaches with a stronger IVT (>500 kg/m/s, or
above the 90th climatological percentile via NAEFS) will direct
another plume of Pacific moisture at the region. Strong synoptic-
scale support will also be present as 200-500mb winds over western
Washington top the 90th climatological percentile. As the warm
front arrives Saturday night, snow levels will rise to as high as
5,500ft, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Heavy
snow will continue to push inland through the interior Northwest
Sunday night while persistent upslope flow leads to a prolonged
stretch of accumulating snowfall above 3,000ft in the Cascades.

Through this weekend, the northern Cascades and the Idaho
Panhandle above 5,000ft sport high chances (>70%) for snowfall >12"
with low-moderate chances (30-50%) for >18" in the tallest peaks.
In the Oregon Cascades and Blue Mountains, WPC PWPF shows
moderate-high chances for >12" of snowfall. WSSI-P depicts
moderate-high chances (50-70%) for Major Impacts in the WA/OR
Cascades above 4,000ft, which includes some notable passes such as
Snoqualmie Pass.

...Northern Maine...
Day 1...

Some light ice accumulations are expected across portions of
northern Maine this morning. Warm air at mid levels overrunning
sub-freezing low level temperatures will support periods of
freezing rain/drizzle, especially from the St. John Valley on south
through the Southern Aroostook. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions. WPC
PWPF shows low-moderate chances for ice accumulations >0.01"
through this afternoon.


Mullinax


$$
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FOUS11 KWBC 160820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Nov 16 2024

Valid 12Z Sat Nov 16 2024 - 12Z Tue Nov 19 2024

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

As one storm system originally over the Intermountain West last
night departs (leaving some light-moderate mountain snow over parts
of the central Rockies in its wake this morning), attention shifts
to the next Pacific storm system that is set to bring an onslaught
of heavy snow to the Pacific Northwest and Northern Rockies this
weekend and into early next week. Precipitation will arrive
Saturday morning as a warm front approaches from the West. Snow
levels start out around 3,000ft Saturday morning, but strong warm
air advection associated with the warm front will cause snow levels
to rise above 7,000ft in the OR Cascades, while they are more
focused around 5,000ft in the northern WA Cascades Saturday night.
This same moisture plume, which is associated with a >500 kg/m/s
IVT that also happens to be above the 90th climatological
percentile according to NAEFS, will advect rich low-mid level
moisture well inland into the northern Rockies Saturday night. By
Sunday, while moisture advection into the Cascades will not be as
strong, a cold front passing through on Sunday will cause snow
levels to plummet to as low as 2,000ft in the WA Cascades and
2,500ft in the Oregon Cascades by Sunday evening.

These falling snow levels will also occur inland across the
Northern Rockies Sunday night and into Monday. NAEFS by 00Z Monday
shows 500mb heights that are below the 10th climatological
percentile throughout the northwestern U.S.. By Monday afternoon,
700mb temperatures are unusually cold across the Pacific NW with
temps as cold as the 2.5 climatological percentile. This unusually
cold air-mass is in place at the same time as steady onshore flow
perpendicular to the Cascades and northern Rockies continues to
support moderate-heavy snow from the Olympics, Cascades, and
northern Rockies to as far south as the Salmon Mountains of
northern California. Heavy snow not only looks to continue through
Monday night in the Cascades and Olympics, but a much stronger
Pacific storm system arrives on Tuesday with more heavy snow and
much stronger winds.

Through 12Z Tuesday, WPC PWPF shows which chances (>70%) for
snowfall >18" in the Oregon Cascades, the northern Bitterroots of
the Idaho Panhandle, and the tallest peaks of the Lewis Range. The
northern Washington Cascades show high chances (>70%) for over 2
feet of snowfall with some of the tallest peaks above 7,000ft
topping 3 feet. Impacts will be the worst in these aforementioned
areas with Moderate to Major Impacts shown on the WSSI, which
includes some of the notable passes in the OR/WA Cascades.
Elsewhere, WPC PWPF shows high chances (>70%) for >12" of snowfall
in the Blue, Boise, and Sawtooth Mountains. Similar high chance
probabilities exist in the Tetons for >8" of snowfall. The WSSI
shows Minor to localized Moderate Impacts in these mountain ranges.

...Eastern MT & Western ND...
Day 1...

Over the past 24 hours, guidance shows the upper level trough
tracking through southern Montana tonight to be more potent and
compact than originally shown. The region remains beneath the
divergent left-exit region of a 250mb jet streak located over the
Four Corners region. As the 700mb low tracks through eastern
Montana this morning, a ribbon of anomalous 700mb moisture aloft
will wrap around the northern and western flank of the low while
strong vertical velocities aloft and dynamic cooling allow for
heavy snow to ensue over northeast Montana. Latest CAMs show
excellent upward vertical velocities within a highly saturated DGZ
around 600-550mb. The 00Z HREF Snowband Probability Tracker (SPT)
does show a swath of snowfall rates that could hover around 2"/hr,
if not higher in some cases. Residents in these areas can expect
rapidly accumulating snowfall this morning with US Routes 2 and 85
to the north and west of Williston likely to feature treacherous
travel conditions. Slick roads and significantly reduced
visibilities are expected where the heaviest snowfall occurs. By
this afternoon, the storm system will be over eastern North Dakota
and tracking to the northeast. Periods of snow will continue across
western North Dakota before finally concluding Saturday evening.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax

$$
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FOUS11 KWBC 170812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Nov 17 2024

Valid 12Z Sun Nov 17 2024 - 12Z Wed Nov 20 2024

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

A developing active pattern is in its early stages as the first in
a series of Pacific storm systems revolves beneath the base of a
longwave trough that will become quasi-stationary in the Gulf of
Alaska to start the period. Then, as a strong ridge builds over the
Aleutians mid-week, the longwave trough will dive south into the
northeast Pacific and direct a steady barrage of rich Pacific
moisture at the region into the second half of the week. Tonight,
the warm front associated with the initial surge in Pacific
moisture is working across the interior Northwest. Meanwhile, a
cold front will push across western WA/OR Sunday morning that will
cause snow levels to plummet to as low as 2,000ft in the WA
Cascades and 2,500ft in the OR Cascades by this afternoon. By
Sunday evening, some locations on the windward side of the Cascades
could see snow levels as low as 1,000ft. It is also worth noting
that there are likely to be some instances of freezing rain today
with low-moderate probabilities for >0.01" of freezing rain in
parts of eastern OR and the northern ID Panhandle.

The falling snow levels will occur well inland across the Northern
Rockies Sunday night and into Monday. NAEFS shows 500-700mb temps
and heights that are below the 10th percentile throughout the
Northwest on Monday. This unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Following a brief lull Monday night, a rapidly
strengthening storm system (both the GFS and ECMWF have the low sub
950mb by Tuesday afternoon!) will direct a stronger atmospheric
river at the Pacific Northwest with not only exceptional moisture
advection (>750 kg/m/s IVT on NAEFS would be above the 97.5
climatological percentile Tuesday afternoon), but intense winds
along the mountainous terrain. Robust warm air-advection late
Tuesday and into Tuesday night will lead to snow levels rising to
above 3,000ft in the WA Cascades and Olympics, around 4,000ft in
the OR Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern would be for the potential for a
heavier/wet snow type to rapidly accumulate, while powerful wind
gusts potential lead to tree damage and power outages Tuesday
night and into Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along not just the Cascades above 3,000ft, but the
Olympics, the Salmon/Shasta/Siskiyou of northern California, and
well inland through the Blue, Sawtooth, Boise, Bitterroot, Tetons,
and Lewis Mountain Ranges. The latest WSSI shows Moderate to Major
Impacts in the Cascades and Bitterroots with impacts including not
just dangerous travel conditions (including notable Cascade
passes), but the weight of the snow and strong winds by Tuesday
night could result in tree damage and power outages in some areas.

...Northern Plains...
Day 3...

Confidence is increasing in a powerful late November cyclone to
track from the Southern Plains on Monday and race north into the
Midwest Monday night. Both NAEFS and ECMWF SATs support a highly
anomalous area of low pressure Monday night, but the ECMWF is
farther west and closer to the Missouri River than the NAEFS, which
is located farther east over central Iowa. By Tuesday morning, the
NAEFS (a GFS/CMC ensemble based tool) shows the storm about to
cross the Mississippi River in southeast MN while the ECMWF is over
southwest MN. The primary reason for this is the ECMWF shows a
slightly slower and stronger 500mb solution than the GFS, albeit
they are not overly different in the storm evolution.

Where they differ is that the ECMWF (RGEM as well) phases sooner
with an upper level disturbance over the northern High Plains and
is pulled farther west than the GFS. By 21Z Tuesday, both the ECMWF
and RGEM show a closed 500mb low near Bismarck, while the GFS is
more elongated and does not close off fully until a few hours later
near the ND/Canada border. These details are critical in
determining where the deformation zone (or heaviest snow bands) set
up; over the Dakotas or more over south-central Canada. For the
moment, WPC PWPF shows moderate-high probabilities (50-70%) for

4" of snowfall over north-central ND. It is worth noting that the

WPC PWPF does show low chances (10-30%) for snowfall >8", and near
the North Dakota/Canada border, a small 10% chance area for >12" of
snowfall. Residents in the Dakotas should monitor the forecast
closely over the next 24-48 hours.

While exact amounts are unclear, the exceptionally strong storm
system over Minnesota coupled with high pressure building in over
the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is still expected to fall across much of
central and western North Dakota and wind gusts of 30-40 mph would
result in whiteout conditions for areas that, in some cases, would
experience their first round of accumulating snowfall this season.
The Probabilistic WSSI (WSSI-P) shows moderate-high chances
(50-70%) for Minor Impacts across much of central North Dakota with
the highest probabilities focused in north-central North Dakota.
The WSSI-P shows Snow Amount and Blowing Snow to be the primary
impacts in these areas. In summary, while exact amounts remain low
confidence at this range, the combination of some accumulating snow
and hazardous visibility reductions due to blowing snow are still
likely to cause some treacherous travel conditions in these areas
late Tuesday and into Wednesday morning.


The probability of significant ice across the CONUS is less than 10
percent.

Mullinax
$$
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FOUS11 KWBC 220831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Nov 22 2024

Valid 12Z Fri Nov 22 2024 - 12Z Mon Nov 25 2024

...Pacific Coast through the Interior Northwest and Great Basin... Days 1-3...

Persistent closed low centered off the Pacific Northwest coast will
remain energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification of
the gyre, noted by an expansive region of 500-700mb height
anomalies reaching below -1 sigma, and approaching -3 sigma in the
core of the trough. Beneath this trough, deepening surface low
pressure will approach the WA coast today, but likely get pulled
back to the west on Saturday before landfall in response to
secondary shortwave energy digging to its south. This will result
in persistent onshore and divergent mid-level flow, accompanying
upper level jet energy to spread moisture onshore as a continuation
of the ongoing atmospheric river (AR) across northern CA through
early Saturday. While IVT values exceeding 500 kg/m/s wane and
pivot southward by the end of D1, this will still result in
plentiful moisture and widespread moderate to at times heavy
precipitation pushing onshore the Pacific Coast into the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades on D1 where they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D1-D2. WPC probabilities
D1 are high for more than 6 inches of snow in the northern WA
Cascades, the Salmon River Range, and portions of the Northern
Rockies near Glacier NP. On D2, snowfall is more robust and
expansive, with a greater than 90% chance of 6 inches again in the
Northern Rockies and Salmon River/Sawtooth Ranges. More
impressively, the Sierra will be favorably located to be impinged
upon by moisture by the end of D1, leading to pronounced upslope
snow. WPC probabilities are above 80% for 12+ inches through
Saturday, with 2 feet possible above 8000 ft.

By D2 onward, snow levels crash again as a cold front pushes
onshore, but this is accompanied by reduced moisture advection as
the column begins to dry out. Some moderate snow will persist
through onshore flow, especially in the Sierra where an additional
6-12 inches is possible, but otherwise WPC probabilities for more
than 6 inches are between 30-50% and focused across the Wasatch,
Uintas, and ranges of NW WY.

...Appalachians and Northeast... Days 1-2...

Impressive upper low over the Northeast will feature a complex
evolution through the weekend resulting in a couple surface lows and
varying areas of heavy snow.

The Central Appalachians, especially from the Laurel Highlands
southward across WV and as far south as the Great Smokey Mtns of
TN/NC, will benefit from prolonged upslope flow on the western
periphery of the upper low resulting in waves of heavy snowfall
through Saturday. WPC's snowband probability tracker depicts an
extended period of 1-2"/hr snowfall rates during the D1 period
across the Allegheny Mtns. WPC probabilities in this area are high

70%) on D1 for more than 8 inches of snow. Event total snowfall

could peak around 2 feet in central WV.

The ongoing potent and compact surface low churning near Long
Island is forecast to loop westward and eventually toward the south
today over eastern PA as it remains embedded within the large
500mb gyre overhead. As this occurs, an impressive plume of low-
level moisture will continue to wrap northwest into the system,
driving rich theta-e advection into a TROWAL to support some
elevated instability and enhance available moisture for heavy
precipitation early on D1. Increasing deformation is noted on
current radar trends on the W/SW side of the system as it becomes
exceptionally wrapped. This setup will support intense mesoscale
ascent and bands of heavy precipitation rotating W/SW as the low
retrogrades. Although the column will be marginally supportive of
accumulating snow outside of higher elevations (above about 1500
ft), the intense lift should dynamically cool the column and allow
efficient snowfall accumulation at times even in lower terrain.
SLRs for this event will likely be below climo due to near freezing
temps and warm soils/compaction, but still, rates of 1-2"/hr will
promote heavy accumulations, especially in the Poconos and
Catskills and surrounding areas early this morning. Most of the
ongoing snowfall is expected to occur before or right around the
start of the D1 period (12z Fri), so WPC probabilities don't
reflect the ongoing event. Nonetheless, the greatest snowfall
amounts are expected in northeast PA and neighboring areas of New
York and northern NJ, with 2-day snowfall potentially reaching
above 12" in a few highly elevated spots. This will likely result
in dangerous travel and impacts to infrastructure including
scattered power outages.

In the lower elevations of the Mid-Atlantic, some snow bands could
pivot as far SE as the I-95 corridor, so while accumulations in
the major urban areas from Washington, D.C. to New York City are
expected to be minimal, if any, these cities could see their first
"falling" snow of the season Friday morning/aftn.

Along the Adirondacks, Green, and White Mountains, the
upper low will draw moist easterly flow northward and into these
ranges Friday night through Saturday while another surface low
develops east of Maine and pivots northwest. Snow levels should be
high enough that only the highest terrain will receive significant
snow, but WPC probabilities have increased and now feature a 30-50%
chance on D2 of more than 4 inches, primarily in the White
Mountains of NH and the peaks of northern ME.

...Northern Rockies through the Northern Plains... Days 1-2...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the far northern High Plains, interacting with increasing baroclinicity as a front wavers near the international border. The
overlap of synoptic lift with the enhanced mesoscale ascent
through fgen will result in weak cyclogenesis, leading to an
expanding area of precipitation falling as snow. The column will be
plenty cold for wintry precipitation thanks to high pressure from
Canada extending southward, and as the WAA along the warm front
lifts north, it will result in an exceptionally deep DGZ to support
fluffy aggregate dendrites which should accumulate efficiently. The
biggest question remains whether or not this favorable environment
and most impactful snowfall will span south over the U.S.-Canada border.

The heaviest snowfall is likely in the terrain near the Northern
Rockies where upslope flow will enhance ascent, and WPC
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP on D2. However, heavy snowfall is also possible farther
east across northern MT along the international border D1-D2 where
the most robust synoptic ascent will materialize. A leading surge
of modest upper diffluence and WAA may lead to a burst of snow
today across northwest and northern MT. WPC probabilities for at
least 2" on D1 are high throughout northern MT. For the main
activity related to the approaching shortwave on D2, the guidance
has continued to trend a bit farther north from central MT eastward
to ND, with current WPC probabilities suggest around a 30% chance
for more than 4 inches right along the international border as far
east as north- central MT on D2, with higher probabilities for
heavy snowfall located in southern Alberta and Saskatchewan.

The probability of significant ice across the CONUS is less than
10 percent.

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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