Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
315 PM EST Fri Dec 02 2022

Valid 00Z Sat Dec 03 2022 - 00Z Tue Dec 06 2022

...Pacific Northwest, California, and Northern Rockies...

Days 1-3...
The primary winter weather maker in the short term is a cut-off
upper low off the West Coast that will direct a steady stream of
Pacific moisture at the West Coast. This persistent onshore fetch
will also overrun colder than normal temperatures in the
Northwest, resulting in some light snow and potentially light
wintry mix accumulations tonight and tomorrow along parts of the
I-5 corridor in western Washington and western Oregon. Aside from
the Olympics, which sports a 30-40% chance for >12" of snowfall on
Day 1, it will be the mountain ranges of California that receive
the heaviest snowfall throughout the upcoming weekend. Initially,
the best swath of 850-700mb moisture will be directed at the
central and southern Sierras where for Day 1, WPC probabilities
show a high (>70% chance) for snowfall >12". As the upper low
drifts south on Saturday, improved diffluent flow at 250mb and an
improved 850-700mb moisture fetch will become introduced into
northern California. WPC probabilities for >8" of snow will be as
high as 70% in the central/northern Sierras on Day 2, while the
tallest peaks of the Shasta and Trinity Ranges have 40-60% odds of
receiving >8" of snow. In terms of impacts, the PWSSI shows
anywhere from 40-60% probabilities for "Major" impacts up and down
the Sierra Nevada from Days 1-2. This implies snowfall amounts are
likely to cause considerable disruptions to daily life, including
dangerous or impossible driving conditions.

This steady stream of Pacific moisture and divergent flow aloft
pushes inland late Day 2 into Day 3 with heavy snow expected from
the Boises and Sawtooth of Idaho on east into the Tetons and
Absarokas. Favored upslope areas and the tallest ridge lines are
the most likely areas to see snowfall totals >6", which according
to latest WPC probabilities range from 30-50% most often. Some
heavy snow is also possible as far north as the northern Rockies
of western Montana and northern Idaho, but WPC probabilities for
>6" of snowfall on Day 3 are <20% on average, but could increase
should the moisture fetch become oriented more to the north in
future forecast cycles.

...U.P. of Michigan...

Day 1...
A vigorous shortwave trough traversing the Great Lakes Friday
night and into Saturday morning will help to strengthen a storm
system as it tracks northeast into the heart of southeast Canada.
Upper level divergence ahead of the trough with a corridor of
500-700mb moisture aloft will foster a supportive environment for
precipitation. Meanwhile, a cold front racing east across the
Upper Great Lakes will cause temperatures to plummet below
freezing. Precipitation will then fall in the form of snow across
the U.P. of Michigan. The combination of NW flow and 850mb temps
as low as -15C should support a few hours worth of lake effect
snow bands with the Porcupine Mountains likely seeing the best
chances for heavy snow. Latest WPC probabilities show 40-50%
probabilities for snowfall totals >4" with some WSE plumes showing
as much of 6" northeast of Ironwood. The upper trough is quite
progressive and drier RH values will rush in, which should keep
any lake effect bands overnight and into Saturday morning brief.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Mullinax