Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Wed Jan 27 2021

Valid 00Z Thu Jan 28 2021 - 00Z Sun Jan 31 2021

...Eastern Kentucky to northeastern North Carolina...
Day 1...
Quick hitting snow storm tonight. A mid-level trough currently
over the Mid-South states will quickly shift east over NC late
tonight. As the system crosses the south-central Appalachians
tonight, a surface low will rapidly develop off the Carolina coast
as the system moves out to sea. In the low levels a strong high
centered over the Upper Midwest will allow rather cold air to
spill south, resulting in increasing low-level frontogenesis (yet
still ahead of the mid-level trough axis) across the south-central
Appalachians and east across NC which will locally enhance
precipitation. Additional orographic enhancements will produce
locally heavy snow with moderate Day 1 snow probabilities for 8"
in far SW VA near the border with NC (much of this would fall in
6hrs). East-west oriented bands of snow the develop over the
northern NC (and far SW VA) Piedmont late tonight, possibly
persisting well east into the coastal plains of eastern NC where
there are moderate probabilities for 4 or more inches, much of
which would also fall in about 6hrs.


...Western U.S....
Days 1/2...
An ongoing major winter storm will produce multiple additional
feet of heavy snow and very strong winds across the Sierra Nevada
and mountains of Northern California through Friday morning.
Blizzard conditions are expected, especially across portions of
the Sierra Nevada.

Heavy amounts are forecast the Klamath Mountains and the Shasta
Cascade region. Models continue to show the band of intense
precipitation, fueled by highly anomalous moisture, and upper jet
intensifying to 110-130 kt that extends onshore across central CA
to the NV border through tonight that has embedded persistent
upper divergence maxima. This results in several feet of snow for
the length of the Sierra Nevada Mountains through the Day 1 snow
probabilities.

As the upper trough approaches the coast on Thursday, the upper
jet begins to weaken, so the strong upper divergence maxima
gradually wane with time. An additional 3 to 4 feet are possible
in the central to southern CA Sierra Nevada range. Since the upper
jet persists across eastern OR into ID and northwest WY,
continuing moisture transport and lift along the jet axis supports
widespread heavy amounts of a foot or more are also likely from
the eastern Oregon mountains and along the central to eastern
Idaho ranges into western Wyoming as noted in the Day 1 snow
probabilities. Back in CA, as the storm shift southeast, locally
heavy storm total accumulations on the order of a foot or more are
also likely along the Southern California Transverse and northern
Peninsular ranges as indicated by Day 2 snow probabilities. 

Heavy snow in CA diminishes on Friday as the upper trough ejects
inland. However, the upper jet crossing southern CA into AZ
reintroduces moisture transport into the Mogollon Rim of AZ and
also areas from Mt. Charleston NV across the southern Wasatch of
UT, so several inches of snow are possible Fri in these areas (low
to moderate probabilities for 8 or more inches here in the Day 2.5
snow probabilities). The snow coverage/intensity wanes as the
upper trough departs the southwest and crosses the southern
Rockies with Pacific moisture directed well south into Mexico.


...Eastern Great Lakes...
Days 1/2...
Northwest flow with trajectories are expected across Lake Huron
and then Lake Erie, with moistening of the air mass crossing the
lakes favoring snow showers in the lee of Lake Erie in northwest
PA to southwest NY Thursday into Friday. Further northeast, a
Georgian Bay connection to Lake Ontario supports snow showers in
the lee of Lake Ontario along the southern shore and areas
downstream, such as the Syracuse area.  Several inches are
possible south of Lake Ontario until drying aloft begins later
Friday, reducing snow shower coverage and intensity.


...Midwest...
Day 3...
The main trough from The West ejects east across the southern
Rockies Friday night before crossing the south-central Plains
Saturday. Gulf moisture wraps around the system and cold
antecedent conditions should allow for snow to wrap around the
north side of the low with the threat expanding across the Midwest
more Saturday night, so further information on this can be
expected tonight.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Jackson