Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EST Mon Dec 21 2020

Valid 12Z Mon Dec 21 2020 - 12Z Thu Dec 24 2020


Day 1...
...Central Appalachians...
A clipper system is expected to move across the Great Lakes on
Monday.  Increasing westerly flow behind the trailing front, along
with vigorous mid-to-upper level energy moving through the base of
the associated trough, are expected to support a period of
moderate to heavy snowfall across portions of the central
Appalachians Monday night.  Greatest threat for heavy
accumulations is expected along the Allegheny Mountains in West
Virginia, where WPC PWPF is showing higher probabilities for
accumulations of 4-inches or more.

Days 1-2...
...Pacific Northwest to the Northern and Central Rockies...
A lingering frontal boundary extending from the northern Rockies
and back along the Washington-Oregon border is forecast to lift to
the north on Monday ahead of an amplifying wave that is expected
to move onshore by late morning-early afternoon.  Strong low level
frontogenesis, along with favorable upper level forcing, will
support moderate to heavy precipitation, including high elevation
snow, across the northern Cascades and far northern Rockies.  As
the low moves inland, an amplifying upper trough will support a
strong trailing cold front that is expected to sweep across the
Northwest into the northern Rockies, with rapidly falling snow
levels Monday night into Tuesday. 

A strong area of low pressure move will into the northern Plains
on Tuesday, with a sharp trailing cold front sweeping southeast
from the northern into the central Rockies.  This will bring
widespread snow and strong winds from the Rockies into the Plains.
 Several inches of snow is possible across the higher reaches of
the northern and central Rockies.  This includes the central
Montana mountains, where northerly flow on the backside of the
departing system could enhance amounts.

Day 3...
...Upper Midwest/Great Lakes...
An upper trough and strong area of surface low pressure will
continue to move east -- tracking from the northern Plains early
Wednesday to the northern Great Lakes by Thursday morning.  The
00Z model runs showed a good deal of spread with respect to the
evolution of this system and potential winter weather impacts
across the northern Plains into the Great Lakes this period.  WPC
preferred the less amplified aloft solutions of the ECMWF, UKMET
and Canadian Global, with lighter QPF/snow totals across the
region.  However, do feel there is the potential for at least
locally heavy totals, especially across western portions of the
U.P. of Michigan and northern Wisconsin -- where strong northerly
flow on the backside of the system is likely to support some
lake-enhanced totals.

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

Pereira