FOUS11 KWBC 170923
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EST Wed Dec 17 2025

Valid 12Z Wed Dec 17 2025 - 12Z Sat Dec 20 2025

...Cascades and Northern Rockies... Days 1-3...

Active weather pattern for the Pacific Northwest and Northern
Rockies continues into next week. As of 09Z, a mid level low is
apparent in satellite imagery over the northern BC coast with a
trough axis extending south to the OR Coast. Pacific moisture had
surged inland ahead of this trough and is reaching the northern
Rockies now per regional NEXRAD returns. Snow levels on the
Cascades are quickly crashing to around 2000ft in WA and 3000ft in
OR with moderate precip rates persisting behind this cold front
today bringing impactful snow below pass level. The lower snow
levels spread across the northern Rockies through midday. Precip
rates briefly drop to light tonight. Day 1 WPC snow probs for >8"
are 60-90% for the WA Cascades as well as the higher western WY
ranges, and more like 40-80% for the northern OR Cascades, much of
western MT ranges through central ID and the Uinta in UT and
northern CO ranges.

A focused and powerful atmospheric river (AR) surges into the
northern OR coast Thursday before shifting south down the coast
through far northern CA Thursday night and Friday. Snow levels in
the core of this AR will rise to over 9000ft in OR and 5000 to
6000ft in WA through Thursday with a powerful cold front shifting
south over WA Thursday night. Moisture and higher snow levels push
over the northern Rockies ahead of this cold front Thursday night
with levels generally 5000 to 7000ft over MD/ID/WY with the cold
front pushing south over this area Friday/Friday night. Day 2 WPC
snow probs for >8" are 60-90% over the WA Cascades again and 50-80%
over the western MT/central ID ranges, and 40-80% over the western
WY ranges. Day 3 snow probs depict the southern shift with 40-80%
chances for >8" on the WA and OR Cascades along with the
Bitterroots with 60-90% for the Sawtooth and western WY
ranges. For three day totals: A few feet are likely over much of
the northern Rockies and several feet over the higher Cascades.

...Northern Plains, Upper Midwest, Great Lakes... Days 1-3...

Powerful and deep low pressure develops in the lee of the southern
Canadian Rockies today before tracking over northern North Dakota
tonight and northern Minnesota Thursday morning then across Lake
Superior Thursday afternoon and Ontario/Quebec Thursday
night/Friday.

The potent shortwave emerging from the Pacific Northwest/British
Columbia this afternoon will be the driver of this developing
system as it tracks rapidly eastward embedded within pinched zonal
flow. This shortwave will amplify as it dives over the Northern
High Plains, closing off to manifest as 700-500mb height anomalies
falling below the 1st percentile according to NAEFS. As this low
tracks east it will interact with a powerful 160kt Pacific jet
streak diving across the Northern Rockies to create the intense
surface low pressure.

Downstream of this surface low, impressive theta-e advection from
the Gulf, with a modest TROWAL developing as moisture curls back
to the NW of the system and lifts isentropically. Beneath this
modest TROWAL, which will be well positioned into the left exit of
the strong jet streak, a band of heavy snow is possible as far west
as north-central MT, but likely for northeast ND through far
northern MN. The progressive nature of this system will limit the
potential for heavy snow, but intense ascent into a deep DGZ will
result in at least a short period of heavy snow within a possible
fgen-forced band. The greatest risk for >6" snow is in the Day 1.5
range along the northern MN border where probs are around 60% with
greater than 30% probs for >2" over central ND through north-
central MN and the western U.P. Although the total snowfall from
this event will be modest, increasingly strong winds, which will
likely eclipse 60 mph in gusts, will create dangerous travel due
to blowing snow and potential blizzard impacts.

Additionally, the leading edge of comma head could trigger snow
squalls from eastern MT through the Dakotas into MN tonight through Thursday.

...Great Lakes and Interior Northeast... Days 2/3...

The mid level low tracks across Michigan Thursday night with
reinforcing troughs swinging the trough axis around it to a
negative tilt as it lifts over the Northeast Friday. In the wake
of this low, strong CAA crosses the Great Lakes to produce periods
of lake effect snow (LES) especially across the eastern U.P.,
northwest L.P., and east of Lakes Erie and Ontario. Inversion
depths are forecast to be generally modest (850mb) and plentiful
moisture will be somewhat transient. Still, efficient forcing into
the DGZ will likely produce at least short periods of heavy LES,
reflected by Day 2.5 WPC probabilities for >4" that are 30-50%
east of Erie/Ontario. The wrap around flow and CAA cross the
Interior Northeast late Friday with Friday night snows bringing
30-60% Day 3 probs for >4" to the Adirondacks, Greens and Whites.

The probability of significant icing is less than 10 percent.

Jackson

...Winter Storm Key Messages are in effect and linked below...

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

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