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

Valid 12Z Wed Jan 27 2021 - 12Z Sat Jan 30 2021


...Western U.S....
Days 1-3...
A major winter storm is expected to produce multiple feet of heavy
snow and very strong winds across the Sierra and mountains of
Northern California through Friday morning.  Blizzard conditions
are expected, especially across portions of the Sierra. Snowfall
rates of 4 inches per hour were reported tonight near Mt. Shasta,
CA.

Heavy amounts are forecast the Klamath Mountains and the Shasta
Cascade region. Models continue to show a 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 in the central CA Sierra
Nevada Mountains on Day 1 today.

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. In addition to the very heavy totals that
are likely across the Northern California mountains and the
Sierra, 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 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.

The threat for heavy snow in CA is forecast to diminish on Friday
as the upper trough finally begins to move 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. The snow
coverage/intensity wanes as the upper trough departs the southwest
and crosses the southern Rockies.

...Great Lakes...
Day 1...
Light snow are expected in the lee of Lake MI Day 1 as winds back
from northeast to north, favoring snow ending on the IL side of
the lake and developing on the IN side.  Long duration northerly
trajectories should favor moistening the air mass to support
accumulating snow showers.

Days 2-3...
The models indicate northwest flow with trajectories crossing Lake
Huron and then Lake Erie, with moistening of the airmass crossing
the lakes favoring snow showers in the lee of Lake Erie in
northwest PA to southwest NY. 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 early on Day 3 reduces snow shower
coverage and intensity.


...Oh Valley to Southern and Central Appalachians...
Day 1...
A low-mid level trough is expected to move from the Mid MS Valley
to Ohio and Tennessee valleys into the Appalachians Wednesday
night.  This system is expected to support widespread light snows
across the region, Due to orographic enhancements, locally heavy
amounts are expected across the southern and central Appalachians
Wednesday night, focused near the NC/TN border and southwest VA. 
As the low pressure develops off the NC coast, cold air arrives on
the back edge of the precip in southern VA to northern North
Carolina, with rain changing to snow. Light accumulations are
possible here.

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

Petersen