FOUS11 KWBC 050844
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Sun Jan 5 2025

Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025

... Part 2/2 ...

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

A shortwave trough riding over a ridge axis centered off the
Pacific Northwest crosses the Northern Rockies today before being
absorbed into the trough over the Rockies that trails the low
developing over the Central Plains. Moisture streams in ahead of
this trough, making for enough precipitation to reach inland to
bring moderate snows to the Bitterroots and ranges around Glacier
NP and Yellowstone today with snow levels rising to around 4000ft.
Day 1 WPC snow probabilities for >4" are 50-80% over the greater
Idaho, western MT, and western WY ranges.

The next stronger shortwave trough rides over the ridge tonight,
diving across the OR Coast and becoming very positively- tilted
over the the Desert Southwest on Tuesday. High initial snow levels
around 7000ft over the OR Cascades only drop to around 6000ft under
the trough axis, so only the highest OR Cascades, well above pass
level will receive heavy snow. Snow levels will be closer to 5000ft
over the Great Basin and 4000ft over the northern Rockies. Day 2
snow probabilities for >4" are generally 30-60% over terrain in southern
ID, northeast NV, and northern UT.

As light to locally moderate precip shifts south with the trough
Monday night, snow levels in Utah will be around 5000ft. However,
on Tuesday there is a heavy snow potential on the eastern slopes of
the central/southern CO Rockies as strong high pressure (central
sfc pressure around 1040mb) shifts south down the Dakotas, aiding
an easterly low level flow to the moisture coming in from the
west. Day 3 snow probs for >4" are 20-50% for southern portions of
the Front Range and much of the Sangre de Cristos.


...Lake Effect Snow - Lake Ontario... Day 1...

Low pressure centered over James Bay weakens today as a rapidly
developing low off Nova Scotia becomes the dominant surface low in
a broad gyre over southeast Canada. As a result, the great pressure
gradient between James Bay and a 1044mb high over Manitoba will
weaken and the strong WNW flow over central Ontario will gradually
diminish. Sufficient WNW flow will persist over eastern Lake
Superior, northern Lake Huron, southern Georgian Bay, Lake Simcoe,
and Lake Ontario to provide maximum moisture fetch for LES single-
banding to persist through midday today between the Tug Hill and
Syracuse before drifting south and weakening this. Day 1 snow
probabilities for an additional >6" are 40-60% on an axis from
Syracuse west-northwest toward the shores of Lake Ontario.


Weiss/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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