FOUS11 KWBC 310743
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Mon Mar 31 2025

Valid 12Z Mon Mar 31 2025 - 12Z Thu Apr 03 2025

...The West... Days 1-2...

An active stretch of wintry weather has begun from the West Coast
to the Front Range of the Rockies through Tuesday due to the
prolonged influence of a deep longwave trough over the western
U.S. set to move inland over the next few days. Extreme impacts
forecast throughout much of the central Sierra Nevada.

Focus shifts to an exceptional closed upper low in the northeast
Pacific today that slowly makes its way east towards the Pacific
Northwest by tonight. Heavy high elevation mountain snow (above
5000ft in the central Sierra and above 4000ft in both the Cascades
and northern CA ranges) will be underway, but as the upper low
inches closer to the Northwest U.S., falling heights and colder
temperatures aloft will force snow levels to lower additionally on
Day 1 to around 3000ft. The primary IVT axis will be aimed at
southern CA through Day 2, but broad onshore flow and an axis of
greater convergence just to the south of the upper low will provide
ample moisture and upslope flow for the Pacific Mts to experience
very heavy snowfall. The northern extent of the IVT will still sport
values >90th climatological percentile through the Sierra Nevada
and into the Great Basin this afternoon. From northern California
to the northern Rockies, the divergent left- exit region of a 200mb
jet streak whose wind speeds (>130 kts) are above the 99th
climatological percentile will be placed directly overhead.

The best moisture advection and upslope enhancement will be seen
from California's Siskiyou and Salmon mountains on south and west
along the spine of the Sierra Nevada. Through tonight, the flood
of Pacific moisture aloft will advance well inland through much of
the Intermountain West. Just about every notable mountain range in
the central and northern Rockies is likely to see measurable snow
into Tuesday. Meanwhile, cyclonic flow around the upper low will
still support ongoing upslope snowfall in the central and northern
California ranges through Tuesday night. Then by early Tuesday the
parent upper low is expected to dive southeast across southwest OR
and northern California, proving a final surge of Pacific moisture
within an area of cold air aloft. The bulk of the Pacific moisture
advection will be finished Tuesday night and into Wednesday, but
the core of the longwave trough will be over the Rockies with NAEFS
showing 500mb temps that are below the 10th climatological
percentile from the Sierra Nevada to the Front Range of the
Colorado Rockies. With the help of daytime heating, expect numerous
snow showers to envelope most of the western U.S. mountain ranges
given the steep lapse rates.

Through 12Z Wednesday, WPC probabilities show the northern ranges
of California (Siskiyou, Salmon, Shasta) and the length of the
central and northern Sierra Nevada have high chances (>70%) for
snowfall totals >24". Most of these ranges above 6,000ft are
currently forecast to see localized amounts between 3-5 feet
through 12Z Wednesday, with WSSI depicting Extreme impacts due to
both snowfall amounts and blowing snow. Travel delays and road
closures are likely at many of the major Sierra passes. Heavy snow
also reaches into the Oregon Cascades with high probabilities for
at least 12". Farther inland, many Intermountain West ranges
(Sawtooth, Blue, Uinta, Bear River, Wind River, Absaroka, Big Horn,
and CO/WY central Rockies) are all likely to see snowfall totals
of 6-12" with locally higher amounts in these ranges as well.
Expect hazardous travel conditions for many road ways that remain
open in these mountain ranges.

...Northern Plains and Upper Midwest... Days 2-3...

By Tuesday morning, the upper trough responsible for the barrage of
mountain snow in the West will position the 250mb jet streak's
divergent left-exit region over the Great Plains. Modest 500mb PVA
and strengthening WAA at low levels will give rise to lee
cyclogenesis east of the Rockies. Throughout the day Tuesday a
strengthening LLJ, (NAEFS shows 850mb winds reaching the 97.5
climatological percentile 18Z Tuesday from north Texas to eastern
Kansas), will deliver both increased moisture and increasing WAA
aloft. The upper trough to the west will also direct an IVT of
300-500 kg/m/s into the Upper Midwest, which is topping the 90th
climatological percentile Tuesday afternoon. There appears to be
sufficiently cold enough air present that precipitation will
transition over from rain to snow from the Black Hills on east
across South Dakota and as far east as central Minnesota Tuesday
afternoon that is largely due to strong 300K isentropic ascent and
850-700mb WAA resulting in FGEN at those mandatory height levels.
Given the early-April sun angle, snowfall will struggle to
accumulate during the day on Tuesday unless rates can reach >1"/hr.
These rates appear most likely after the 21z timeframe once WAA
strengthens and could kick start treacherous travel conditions
into the overnight period.

There are still some ongoing difference with respect to details,
including how far north the rain/snow line reaches in central MN on
Wednesday, as well as banding potential in the
northern/northwestern precipitation shield within the deformation
zone from the Dakotas through northern MN. However, most guidance
highlights a similar area of heavy snowfall extending from
northeast SD and southeast ND across much of northern MN.

The ECMWF EFI continues to depict an strong signal (values of
0.8-0.9) for a potentially disruptive winter storm from the far
eastern Dakotas on east across northern MN. This also aligns fairly
well where WSO values >50% exist on Days 2-3. Both snow amounts,
along with snow load and blowing snow, are likely to be hazards
that the Northern Plains and Upper Midwest contend with through
Wednesday, with freezing rain most likely on the eastern edge of
the WAA across northern Wisconsin and northern Michigan. Current
WPC snowfall probabilities (through 00Z Thursday) for >8" are
moderate- to-high chance (50-70%) across northeast SD, southeast
ND, and northern Minnesota. High probabilities (70-90%) for >8"
exist across the MN Arrowhead. There are also low probabilities
(10-30%) for snowfall totals >12" in this region, highest and up to
50% in the MN Arrowhead. Residents and those traveling to/from
these regions should keep close eyes on the forecast as additional
changes in the types of impacts, the severity of those impacts, and
where/when these impacts occur are likely to fluctuate for another
day or so.

*Key Messages have been issued for this winter storm, and the link
to view them is posted below.

Snell/Mullinax

...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 050819
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Thu Feb 5 2026

Valid 12Z Thu Feb 05 2026 - 12Z Sun Feb 08 2026

...The Carolinas... Day 1...

A deep shortwave trough will track across the northern Gulf and
then off the Florida coast through this aftn. This will help
strengthen secondary low pressure off the coast of the Carolinas,
with some lingering moisture extending back onshore, at least
through the morning. However, forecast soundings indicate that
pronounced dry air above the surface will inhibit much in the way
of precipitation. However, what does fall, will occur in a sub-
freezing column, leading to a continuation of very light freezing
rain or freezing drizzle for a few hours after 12Z across the
eastern Carolinas. This is reflected by WPC probabilities that
indicate a 10-30% chance (locally 50% chance) of more than 0.01" of
ice, highest from Cape Fear through the Outer banks.

...Great Lakes, Central Appalachians, Northeast, Mid-Atlantic... Days 1-3...

A fast moving shortwave diving out of Manitoba will race southeast
today, reaching the Mid-Atlantic states by Friday morning. This
lead impulse will be followed almost immediately by a second, more
strung out, shortwave, with this secondary impulse responsible for
driving a strong arctic cold front southward beneath it. This
secondary impulse will help develop a surface wave of low pressure
across Upstate NY Friday evening, with this low scooting rapidly
eastward to off the New England coast by the end of D2. The lead
cold front will trail off this low, with a secondary reinforcing
surge of arctic air occurring Saturday behind a secondary front.

Together, these features will result in an extended period of
moderate ascent, but in an environment with normal to below normal
PWs on the sharpening NW flow. Although the moisture will be
modest, the forcing will be sufficient to wring out snowfall across
the area, although most accumulation will be very light. There may
be a few exceptions:

1) Upslope snow into the Central Appalachians. This is expected
behind the first cold front beginning Friday aftn and then
intensifying Friday night before waning Saturday evening.
Intensifying N/NW flow will become quite strong and direct into the
terrain, with the cooling column providing a deepening DGZ into
which this ascent will maximize. Although atmospheric moisture is
modest, this flow will direct at least some Great Lakes enhanced
moisture to improve the potential for heavy upslope snowfall.
Initially, Froude numbers Friday and Friday night suggest critical
or blocked flow indicating the heaviest snowfall will be along or
just upwind of the Appalachians crests, but as flow becomes
unblocked on Saturday, more snow may spill over towards the east.
Still, the heaviest accumulations are likely in the higher terrain
and upwind areas, where WPC probabilities for the entire event
suggest a high risk (>70% chance) for at least 8" of snow, with
locally 12+" possible (30% chance).

The increasing W/NW flow behind the arctic front will also support
some enhanced lake effect snow (LES), especially in the favored
more N/NW snow belts south of Lake Ontario and Lake Superior. LES
is not expected to be extremely heavy or prolonged, but WPC
probabilities indicate a 70-90% chance for at least 4" south of
Lake Ontario, and a 30-50% chance across the eastern U.P. of Michigan.

Finally, as a surface low strengthens well offshore and east of
the Mid-Atlantic states, increasing NW flow with a potential
inverted trough will focus some moisture and ascent across eastern
New England from near Portland, ME southward to Cape Cod, MA. The
combination of onshore flow and ocean effect snow combined with the
potential focused inverted trough may enhance snowfall along the
coast. The intensity of this snowfall is still quite uncertain as
most of it is early D3, but these inverted troughs can sometimes
pivot favorably for locally much heavier snowfall. Confidence is
modest at this time range due to wide variations in the models, but
current WPC probabilities indicate a 50-70% chance for at least 4"
of accumulation from Cape Ann southward through Cape Cod,
including the Boston metro area.

Finally, although not expected to be widespread, scattered snow
squalls are possible along the arctic front as it digs southward
Friday night into Saturday across the Northeast. Any convective
snow showers or squalls that develop will contain brief heavy snow
rates and strong winds, leading to hazardous travel.

Extremely cold temperatures this weekend have also prompted the
issuance of Key Messages related to this event. Those are linked below.

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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