Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Mon Jan 29 2024

Valid 00Z Tue Jan 30 2024 - 00Z Fri Feb 02 2024

...Upper Great Lakes to the Southern and Central Appalachians...
Days 1 and 2...
A shortwave trough moving off the top of an amplified ridge that
is currently centered over western Canada is forecast to amplify
as it dives south into the upper Midwest and northern Great Lakes
region overnight into early Tuesday.  Models show the system
continuing to amplify, with a closed mid-level center forming as
it carves out a deep trough into the Ohio Valley on Tuesday.  Snow
and ice accumulations associated with this system are expected to
remain light through Day 1, with its progressive nature helping to
limit amounts.  Accumulating snowfall of an inch or more is most
likely in the enhanced warm-moist advection centered ahead of the
low over northern Lower Michigan.  Rain changing to a wintry
mix/snow is expected further to the south along the advancing
front.  However, for most areas less than inch of snow and/or
minor icing is expected. 

Precipitation will spread into the central and southern
Appalachians Tuesday evening ahead of the advancing low/trough. 
Here too snow accumulations will be generally light, however
orographic effects will likely help support an inch or two along
some of the higher ridges from eastern West Virginia to the North
Carolina-Tennessee border on Day 2.  Probabilities for snow
accumulations of 4 inches or more remain less than 10 percent
through the period.  

...California into the Great Basin...
Days 2-3...

A longwave trough over the Pacific will amplify Wednesday into
Thursday noted by NAEFS height anomalies in the 700-500mb layer
reaching as low as -3 sigma. These lowered heights will pivot
onshore the OR/CA coast by the end of the forecast period,
producing enhanced ascent through height falls and PVA as spokes
of vorticity shed onshore. Accompanying these height falls, a
Pacific jet streak will surge zonally eastward towards CA, leaving
its favorable LFQ for ascent over the region to overlap the best
height falls to enhance the already robust ascent.

This impressive deep layer lift will spread eastward, and impinge
into an increasingly saturated column thanks to an atmospheric
river (AR) spreading onshore characterized by high probabilities
of IVT exceed 500 kg/ms on both the GEFS and ECENS, with even
modest probabilities for 250 kg/ms spilling into the Great Basin
by the end of the period. While this moisture will generally be
driven by accompanying WAA to raise snow levels, a surface low and
its associated cold front will traverse from CA into the Great
Basin on D3, with rapid snow level collapse occurring in the wake
of the cold front. This suggests that the heaviest snow will
remain above 6000 ft as the heaviest precipitation should occur
ahead of the cold front, but light to moderate precipitation
continuing after the FROPA will allow at least modest snowfall
accumulations into lower elevations, becoming more impactful to
the passes by Thursday night. The heaviest snow is likely to begin
in the Shasta/Trinity/Siskiyou region late D2, and before
expanding into the Sierra D3. WPC probabilities for more than 6
inches of snow D2 are as high as 70-80% in the Northern CA ranges,
generally above 5000 ft. By D3, high (>80%) probabilities continue
in the Shasta/Trinity/Siskiyou region, and expand along the length
o the Sierra, where locally 2-3 feet is possible. Lighter snowfall
of up to 6" is also possible D3 into the higher terrain of the
Great Basin.


The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.


Pereira/Weiss