Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
232 PM EST Sun Feb 2 2025

Valid 00Z Mon Feb 03 2025 - 00Z Thu Feb 06 2025


...Western U.S. across the Northern Rockies...
Days 1-3...

...Atmospheric River (AR) and upper low maintain prolonged heavy
snow from Oregon/California border and Sierra Nevada to the
Northern Rockies through Wednesday...

Deep upper low (under the 1st climatological percentile tonight per
the GEFS) persists over Vancouver Island through Monday before
drifting southwest off the PacNW coast with troughing lingering
there through Thursday. Another upper low moving east to the north
of Hawaii will maintain onshore flow into the West over the next
several days, focusing the primary moisture axis from CA/OR border
through southern Idaho and western WY terrain before a brief break
later on Wednesday. Along the West Coast, this moisture axis will
shift south over CA beginning on Tuesday as the next upper low
approaching interacts with the larger upper system churning off the
PacNW. The prolonged nature of anomalous IVT (+4 to 8 standardized
anomaly per the 00z NAEFS) will likely lead to a corridor of very
high snowfall totals in excess of 2 feet, with the most anomalous
snowfall totals expected across southern OR east of the Cascades as
highlighted by ECMWF EFI values of 0.95-1.0. A tight baroclinic
zone north of this moisture axis will also maintain lower elevation
snow with enhanced totals in terrain through this time.

72hr probs for >2' ending 00Z Thursday are 50-90% over terrain
from the Shasta/Siskiyou, Fremont Mtns in southern OR, the
Sawtooths, and Yellowstone through Wind Rivers. Beneficial precip
for this area, but major impacts to any passes in this area with
snow levels around 4000ft in the heavier snow. Snow levels are
5000-8000ft in the core of the AR moisture axis over northern
CA/NV/UT. However, on Monday afternoon height falls as troughing
expands over the NW low brings snow farther south into CA. Heavy
snow reaches the northern Sierra Nevada by Monday evening with a
slow progression of heavy snow down the length of the Sierra with
hourly rates exceeding 2"/hr into Wednesday. Day 2 and Day 3 probs
for >1' are 60-95% for the length of the Sierra Nevada with totals
over 3' likely in the High Sierra (5000-6000ft).

Cold continues over western WA/OR with occasional snow in the
Seattle/Portland metro areas through Tuesday night. A few to
several inches at a time are possible given the banding potential
under/adjacent to the upper low just to the north. Total 72hr
probabilities for at least 2 inches are 20-40% for Seattle and
40-60% in Portland.

The moisture plume extending across the northern Great Basin and
into the Northern Rockies on Day 1 will gradually lift north on Day
2 and Day 3 as the tilting axis that brings snow to CA Monday
night also shifts snow across the northern Rockies through MT
into Wednesday. Day 1 snow probs for >8" are high (70-90%) over
the Sawtooths, Absarokas, and much of the greater Yellowstone
region of western WY, with 40-60% probs over all the southern and
southwestern MT ranges. Low probs (10-30%) for at least 8" extend
east into the adjacent northern High Plains of southeastern MT. By
Day 2 these heavy snow probabilities shift north into the
Bitterroot Range and all of the northern ID and western MT ranges.
For Day 2 and Day 3 the highest WPC probabilities for at least 8"
remains in the Sawtooths, but with 40-60% probabilities into
northern ID and western MT.


...Northern Plains, Great Lakes, Northeast...
Days 1-2...

Zonal flow sends two shortwave troughs east over the northern tier
through Monday night followed by a third over the Northern Plains
on Wednesday.

Shortwave trough axis over the Great Lakes speeds across New
England tonight with light to locally moderate snow spreading
across the Interior Northeast and New England before sunrise on
Monday. Local terrain enhancements over the Tug Hill/southern
Adirondacks/Berkshires/Greens/Whites and coastal Maine where Day 1
snow probs for >4" are 20-40%. Elsewhere, amounts are generally
expected to remain in the 1-3" range throughout New England.
Additionally, some light icing is possible across central/southern
PA, but with amounts expected to remain below 0.1".

The next shortwave trough entering the northern Plains this
afternoon will trigger east-west oriented snow bands forming in
the left exit region of a 130kt+ jet streak stretching across the
northern Plains, Upper Midwest and Great Lakes through Monday
afternoon. This will lead to a narrow axis of moderate snow along
the SD/ND border through central MN, northern WI, and the northern
MI Lower Peninsula, where Day 1 snow probs for >4" are 30-60%.
These bands slide mainly north of and weaken near northern New
England as precipitation crosses the region Monday night along a
cold front. However, upslope and some lake enhancement off Lake
Ontario lead to Day 1-2 probabilities for at least 4" of 30-60% for
the northern Adirondacks, northern VT/NH, and northern ME.

By Day 3 another short wave trough exits east of the Northern
Rockies and spreads another round of snow into the northern
Plains. For now probabilities of at least 4" are low (10-30%)
across northern MT and northern ND with the expectation that this
snowfall would expand eastward across the Upper Midwest Wednesday
night.


...Midwest to Mid-Atlantic...
Day 3...

By the end of Day 3 (Wednesday) a couple of subtle short waves
within a weak trough traversing the Midwest will help blossom a
precipitation shield to the north of a stalled stationary front
expected to drape from the southern Mid-Atlantic to the middle
Mississippi Valley. With surface high pressure reaching across the
Great Lakes and more prominent over the Northeast, while above
average 500mb height anomalies span the region, icing is
anticipated to the main winter weather hazard between the Midwest
and Mid-Atlantic through Wednesday night. WPC probabilities for at
least 0.1" are currently low (10-30%) from northern MO and
southeast IA across northern IL and IN. Additionally, low
probabilities show up across the Allegheny Mts of western MD and
WV, with these probabilities expected to increase and expand
throughout the Mid-Atlantic once the forecast period includes
Thursday.


Snell/Jackson