FOUS11 KWBC 010929
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EST Wed Jan 1 2025

Valid 12Z Wed Jan 01 2025 - 12Z Sat Jan 04 2025

...The West...
Days 1-3...

A prolonged atmospheric river (AR) event continues to provide
moisture to the northwestern states through Friday night. A potent
trough crosses the Pacific Northwest coast on Friday which will
define the end of this AR, though an active winter pattern is
expected to continue over the Northwest through the weekend.

Rising snow levels in the core of the AR today into the OR/CA
border cause snow levels on the Klamath and southern Cascades to
rise above 7000ft.

...North-Central Rockies...
Days 1-2...
A shortwave impulse on the leading edge of the AR pushes ESE from
Washington state this morning and crosses Wyoming this evening.
Sufficient moisture, baroclinicity from existing stalled fronts,
and topographic lift will bring snow to the north- central Rockies
today into Thursday. Lee cyclogenesis aids develop over north-
central CO terrain that persists through Thursday morning. Day 1
snow probs for >8" is 40-80% for much of the central/southern ID
terrain, from the Tetons through the Wasatch, and northern CO
ranges that extend into southern WY.

Pacific Northwest into California...
Days 2-3...
Ridging behind the impulse causes further height rises as massive
AR moisture continues to stream in. A persistent frontal boundary
near the WA/OR border allows snow levels over the WA Cascades to
remain 4000ft or less through Thursday. Day 2 snow probs for >6"
are 20-40% over the WA Cascades. Meanwhile, sub-freezing wet-bulb
temps in the Columbia Basin and possibly Gorge Thursday afternoon
could result in some minor ice accumulations through Thursday
night.

A potent trough south of a mid-level low that tracks into western
WA on Friday provides a focus for lift with the robust moisture in
the AR for the Cascades through the northern Sierra Nevada while
height falls lower the snow level before the enhanced precip
diminishes Friday night. Snow levels of 7000-8000ft from central OR
through northern CA late Thursday night drop below 6000ft Friday
afternoon under the trough with snow levels of 4000-5000ft
persisting over WA. Day 3 snow probs for >8" are generally 20-40%
from the northern Sierra through southern WA terrain, while values
are 30-70% in the northern WA Cascades and in terrain of far
northern ID into MT.


...Great Lakes and Northeast...
Days 1-3...

Developing low over NY will track north over Maine through
tonight the drift farther north over the Canadian Maritimes into
Friday night causing snow over Northeast terrain today with lake
enhanced snow this evening becoming lake effect snow over the
Great Lakes that persist in earnest through Friday night. Snow this
morning is focused on the White Mtns above 2000ft snow levels with
heavy rates in the higher terrain and far interior Maine this
afternoon. Heavy lake enhanced snow develops off Lake Erie this
morning, shifting up off Lake Ontario and over the Adirondacks and
the Green Mtns this evening. Day 1 snow probs for >6" are highest
east of Erie, Ontario and the northern Adirondacks. Rates should
exceed 1"/hr for a few hours in each of these areas raising the
threat. Single banded LES persists from Lake Ontario through Friday
night with the WNW flow maximizing upstream moisture fetch with
Lake Superior, northern Lake Huron and the Georgian Bay (all of
which are nearly open water) in the stream lines for the Tug Hill.
Day 2 snow probs are 90% for >8" for the Tug Hill and around 60%
for Day 3.

Meanwhile, general multi- band LES is expected off Superior and MI
all three days on WNW flow. The heaviest snow is through Thursday
in the U.P. where preferred snow belts in WNW flow have >50% probs
for >6".



...Midwest through Central Appalachians...
Days 1-3...

The shortwave impulse crossing ESE over WY this evening and
promoting lee cyclogenesis tonight over CO will track over the
Central Plains late tonight into Thursday. Surface-850mb
frontogenesis over northeast Nebraska will help to draw an
inverted trough from KS which will track to KY by Thursday evening.
low pressure along the front and track east towards the Missouri
This wave is compact, but the presence of strong 850mb FGEN and air
cold enough for snow will generate narrow bands of moderate to
locally heavy snow from northeastern Nebraska through Iowa and
possibly IL/IN/OH (though the wave weakens). Day 1.5 snow probs
for >2" are 30-70% over northeast Neb and much of IA with values
generally 10% or less over northern IL/IN into OH.
A northern stream impulse interacts with this wave on Friday just
before it reaches the central/WV Appalachians which will enhance
lift along with topography and Great Lakes moisture to produce
enhanced upslope snow from the PA Laurels through the Allegheny
Highlands of WV. Day 3 snow probs for >6" are 50-80% from around Mt
Davis down the Allegheny Front through the Allegheny Highlands.


Jackson



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