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/La...

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