Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Tue Dec 28 2021

Valid 00Z Wed Dec 29 2021 - 00Z Sat Jan 1 2022

...Upper Midwest to the Northeast...
Day 1...
A west-east oriented baroclinic zone will be the focus for mixed
precipitation from the southern Great Lakes region to the northern
Allegheny Plateau.  Widespread heavy amounts are not expected,
however low-to-mid level frontogenetic forcing along with
favorable forcing along the right-entrance region of a powerful
upper jet may support a brief period of heavy precipitation
translating east from the Great Lakes in the late afternoon to
northern Pennsylvania and the Southern Tier of New York during the
evening/overnight.  HREF guidance suggest rates may briefly reach
1-2 in/hr, supporting a window of rapidly accumulating snow before
changing over to mixed precipitation.  For most areas only an inch
or two of snow is expected.

...Western U.S...
Days 1-3...
The series of shortwaves diving south into the base of the broader
scale trough over the West Coast region is expected to continue,
with an amplifying positively-tilted trough developing across
California Wednesday into Thursday.  This will support yet another
round of heavy snows spreading south along the Sierra and into the
Transverse and Peninsular ranges.  For the southern Sierra and the
Transverse and northern Peninsular ranges, accumulations of 1-2
feet, with locally heavier amounts are expected.  In addition to
the heavy amounts across California, guidance shows low-to-mid
level frontogenesis enhancing the threat for locally heavy amounts
farther east into the southern Nevada and southwestern Utah
mountains.

This trough is expected to swing east across southern California,
raising the threat for additional heavy snows across the elevated
terrain of the Southwest and the central to southern Rockies on
Thursday into early Friday.  Meanwhile, yet another shortwave is
expected to begin impacting the Pacific Northwest on Thursday. 
This system is forecast to take a more inland track across the
Northwest, bringing a threat for heavy snows back into the
Washington and Oregon Cascades, as well as the Blue Mountains
beginning Thursday and continuing into Friday.

Fracasso/Hamrick