FOUS11 KWBC 310905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Tue Dec 31 2024

Valid 12Z Tue Dec 31 2024 - 12Z Fri Jan 03 2025

...Pacific Northwest to Northern Rockies...
Days 1-3...

Lull today over the Northwest ends this evening as a plume of
Pacific moisture streams in ahead of a trough emanating from a
Gulf of Alaska low. A vort lobe from this trough ejects ESE over WA
tonight and Wyoming Wednesday night. The front with this wave
stalls near the OR/WA border tonight before slowly lifting north
through Thursday as the intensity of moisture advection increases.
Snow levels of 2000 to 3000 ft linger over the Cascades into
Wednesday before rising steadily south of the front with levels in
southern OR reaching 7000ft while they remain 2000ft in the North
WA Cascades (though rates generally remain light north of the
front). Day 1.5 snow probs for >8" are 50-90% in the OR Cascades
and 30-50% in the southern WA Cascades. This moisture shifts inland
with Day 2 snow probs for >8" 30-60% over the Sawtooths and Tetons
south along the WY/ID border. A powerful low which will be
directing the atmospheric river approaches the OR Coast Thursday
night. Snow levels of 6000-8000ft persist over OR with lower values
in WA allowing some Day 3 snow probs for >8" around 30% in the
North WA Cascades. Sufficient cold air pools in the Columbia Basin
by Thursday night to allow a threat of ice accretion in the lower
Basin into the Columbia Gorge Thursday night.


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

The surface low currently crossing St. Louis is presenting
deformation banding over IA and northern IL, but
uncharacteristically for a late December night, there is
insufficient cold air for significant snowfall. However, cold
conveyor flow from the east and further development should allow
for some decent snow bands to develop later this morning over
southern MI and far northern IN. Tonight expect LES bands to form
off Lakes Superior and Michigan that persist into Thursday. Day 2.5
snow probs are high near Whitefish Bay.
A coastal low develops at the triple- point over the central Mid-
Atlantic this evening and quickly becoming the dominant low. This
low tracks up the New England coast through Wednesday with dynamic
cooling driven snow over terrain of northern New England late
tonight into Thursday before lake enhanced snow develops in to lake
effect snow bands off the still ice free Lakes Erie and Ontario.
Day 2.5 snow probs for >8" are 50-80% for the Tug Hill and northern
Adirondacks as well as the length of the Greens in NT and the
Presidential Range in NH.
Upslope flow into the central Appalachians turns into snow late
tonight that continues through Wednesday. Day 1.5 snow probs for
>6" are 40-80% from the PA Laurels through the Potomac and
Allegheny Highlands west from the Allegheny Front. The progression
of the low slows over Atlantic Canada Wednesday night through
Thursday night which allows LES to persist. Day 2 snow probs are
50-80% from Erie through southern Buffalo and around 80% on the Tug
Hill. The focus shifts a bit north for Day 3 with snow probs for
an additional >8" over the Tug Hill around 60%.


Jackson



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