Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
217 PM EST Thu Nov 16 2023

Valid 00Z Fri Nov 17 2023 - 00Z Mon Nov 20 2023

...The West...
Days 2-3...

The upper low off the West Coast moves east and makes its way
ashore Saturday morning. The strongest IVT values will reside well
south of the Sierra Nevada, but modest PVA and diffluence aloft
working in tandem with SW upslope flow to generate periods of
heavy snow at elevations at/above 9,000ft. There is also an
anomalous moisture source present, as indicated by NAEFS showing
PWs >90th climatological percentile over the Sierra Nevada between
06-12Z Saturday. WPC probabilities show a high risk (70-90%) for
snowfall totals >4" for those higher elevations of the southern
and central Sierra Nevada.

By Saturday night, a new Pacific shortwave trough will track into
the Pacific Northwest. The trough will be taking on a negative
tilt as it approaches the OR coastline, fostering strong vertical
ascent at mid-upper levels of the atmosphere. Snow levels will
crash to as low as 4000ft in wake of a cold frontal passage over
the Olympics and Cascades Saturday night. Latest WPC probabilities
show the Washington Cascades featuring the highest chances
(60-80%) for snowfall totals >4". There are even low-moderate
chances (30-50%) for snowfall totals >8" in the highest peaks of
the Washington Cascades. The cold front continues to make its way
inland towards the Northern Rockies on Sunday with snow levels
crashing from the Bitterroots and Sawtooth of Idaho, to the Tetons
of Montana, the Wasatch and Uinta of Utah, and the Colorado
Rockies. WPC probabilities top out around moderate-high chances
(60-70%) for >4" in the tallest peaks of the Sawtooths,
Bitterroots, Tetons, and Colorado Rockies.

...Northern Maine...
Days 2-3...

A complex setup Friday night into Saturday is predicated upon the
timing of a cold frontal passage and track of a strengthening
storm system off the coast. Latest 12Z guidance has forced the
storm to track a little farther east compared to 24 hours ago with
a faster cold frontal passage as well. This would, in turn, keep
the bulk of QPF and resulting snowfall from transpiring over
northern Maine and more over the northwest Atlantic and Nova
Scotia. WPC probabilities still show a low risk (10-20% odds) for
>4" of snowfall over northern Maine, but the bulk of WSE members
show <3" of snow in places like Caribou. This situation will be
closely monitored as "last minute" trends west have occurred
before and placed the deformation axis over northern or eastern
Maine. At this time, snowfall in northern Maine would likely
result in Minor Impacts that included hazardous driving conditions
in some areas.

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

Mullinax