Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 PM EST Fri Nov 17 2023

Valid 00Z Sat Nov 18 2023 - 00Z Tue Nov 21 2023

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

Snowfall for Day 1 (Sat 00Z - Sun 00Z) will occur in the Sierra
Nevada as the upper low off the coast opens up and moves ashore
later tonight. NAEFS shows IVT values of 200-200 kg/m/s, which
will supply both a plume of Pacific moisture and sufficient
upslope flow to cause topographically-enhanced snowfall rates.
Latest WPC probabilities show a moderate-to-high risk (50-80%) for
>6" of snowfall through Saturday afternoon. Meanwhile, as the
upper trough in California heads for the Four Corners region, a
more potent and negatively tilted upper trough arrives in the
Pacific Northwest Saturday night. Excellent upper level diffluence
ahead of the upper trough combined with crashing snow levels in
wake of a cold frontal passage will prompt heavy snow to ensue
over the Cascades, the Shasta, and the Sierra Nevada . Snow levels
will likely fall to as low as 3,000ft in the Cascades with heavy
accumulations occurring >4,000ft. WPC probabilities show the
highest risk for >6" of snowfall in the Oregon Cascades where
chances are high as 80%.

By Sunday morning, a steady stream of 700-500mb moisture will work
its way east into the heart of the Intermountain West. Falling
heights aloft and a cold frontal passage will cause snow levels to
drop to as low as 6,000ft in the valleys, but the heavier snowfall
amounts will likely be observed at elevations >8,000ft. WPC
probabilities sport moderate risks (50-70%) for snowfalls >6" in
the Wasatch, Uinta, the San Juan, the Colorado Front Range, and
Colorado's Sangre De Cristo. In terms of impacts, the WSSI shows
Moderate to Major impacts in the Cascades, Shasta, and Sierra
Nevada with Snow Load being the primary impact being identified.
Farther inland, the peaks of the aforementioned Intermountain West
ranges can expect Moderate impacts Sunday and into Sunday night.

...Northern Maine and the Northeast...
Days 1-2...

As a cold front tracks through Maine and a coastal low quickly
forms off the East Coast, precipitation is likely to fall in the
form of snow over northern New England early Saturday morning.
Given the storm system's and cold front's quick progression, snow
will only get to fall at a good clip for a couple hours at most.
WPC probabilities for >4" of snowfall are topping out at 10% just
north of Caribou. Meanwhile, a potent shortwave trough diving
south and east from Ontario will provide strong vertical ascent
aloft combined with adequate moisture content to generate periods
of snow across the northern Appalachians. Latest WPC probabilities
contain a low risk (10-20%) probabilities for snowfall >4" in
parts of the Adirondacks and Green Mountains, but the footprint
for >2" are moderate-to-high in some cases (50-80%) in these same
mountain ranges. Some spotty areas of Minor Impacts are possible
in the northern Appalachians on Sunday with snow covered roads and
reduced visibilities the most notable hazards for motorists.


For Days 1-3, the probability of significant icing (at least
0.25") is less than 10 percent.

Mullinax