Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 PM EST Tue Jan 26 2021

Valid 00Z Wed Jan 27 2021 - 00Z 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.

An occluded front, associated with a deep low dropping southeast
along the Washington and Oregon coasts, is expected to move
inland, bringing heavy precipitation into portions of Northern
California this evening.  Snow levels are expected to be low at
the onset -- dropping below 1000 ft in some locations.  Therefore,
in addition to heavy amounts across areas that will likely include
the Klamath Mountains and the Shasta Cascade region, significant
accumulations are possible closer to the coast and across the
northern Sacramento Valley -- where several inches are possible
Tuesday night.  Models continue to show a band of intense
precipitation, fueled by highly anomalous moisture, developing
along the associated cold front as it drops south Tuesday night
into Wednesday -- supporting very intense snowfall rates along the
Sierra.  Despite gradual weakening of the parent low along the
Washington and Oregon coasts on Wednesday, energy digging south
through the eastern Pacific will continue to amplify the trough
and support a prolonged period of deep southwesterly flow across
central California into the Sierra.  Deep southwesterly flow is
expected to persist across California as a new low develops and
drops south from the British Columbia to the Washington and Oregon
coasts on Thursday.  The threat for heavy snow is forecast to
diminish on Friday as the upper trough finally begins to move
inland.  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.  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.

...Northeast...
Day 1...
Light to moderate precipitation associated with a low level front
lifting out ahead of a wave moving through the Great Lakes, will
continue to move east across the Northeast Tuesday evening.  While
mostly light snow accumulations are expected, some areas of the
Adirondacks, Green, Berkshire and White mountains could see
additional snow accumulations of 4-inches or more before
precipitation ends.

...Southern and Central Appalachians...
Day 2...
A shortwave trough, associated with the remnants of an upper low
currently moving out of the Southwest, is expected to move from
the Ohio and Tennessee valleys into the Appalachians Wednesday
night.  This system is expected to support widespread light snows
across the region, with some potential for locally heavier amounts
falling across the southern and central Appalachians Wednesday
night.

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

Pereira