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

Valid 00Z Wed Dec 29 2021 - 00Z Sat Jan 01 2022

...Upper Midwest to the Northeast...
Day 1...
A west-east oriented baroclinic zone will be the focus for mixed
precipitation from Wisconsin to upstate New York through Tuesday
night, with only light accumulations expected where snowfall rates
are able to overcome any melting at the surface.  Slightly heavier
snow is likely for northeastern Minnesota and across portions of
the Upper Peninsula of Michigan through Wednesday morning as a
surface low lifts northward across the region.  The greatest
totals are likely near the lake shore of the Arrowhead of MN where
some lake enhancement is expected, with locally 6+ inches of
accumulation possible.  Most of the snow should be over by 15Z on
Tuesday.

...Western U.S...
Days 1-3...
The parade of shortwaves tracking southward into the base of the
broader scale trough over the West Coast region is expected to
continue through the end of the week, with an amplifying
positively-tilted trough developing across California for the
Wednesday into Thursday time period.  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 on the order of 10-20+ inches are likely, with
locally heavier amounts where terrain enhancement happens. 
Another 2-3 feet of snow is likely for the central Sierra over the
next three days, adding to the already impressive snow pack in
place.  In addition to the heavy snow over 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, with 1-2 feet of accumulation
possible for those areas.

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 compared to the previous storm, thus bringing the
potential for heavy snows back into the Washington and Oregon
Cascades, as well as the Blue Mountains of Oregon and the
Bitterroot Mountains of Idaho beginning Thursday and continuing
into Friday.  By the end of the forecast period on Friday, heavy
snow is likely for the higher terrain of the Wasatch Mountains and
the central Rockies of Colorado.

Fracasso/Hamrick