Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EST Tue Dec 22 2020

Valid 12Z Tue Dec 22 2020 - 12Z Fri Dec 25 2020


Day 1...
...Northern and Central Rockies...
Low pressure will slide southeast of the northern Rockies into the
Dakotas today as a strong trailing cold front plunges southeast
across the Great Basin and central Rockies.  Given a relative lack
of moisture and the overall progressive nature of the system,
widespread heavy snow accumulations are not expected.  However,
orographic effects will likely help support at least a few inches
across several high elevation locations.  This includes the
northern Idaho, western and central Montana, and western Wyoming
ranges. 

Days 1-2...
...Northern Plains and Upper Midwest...
An upper trough and its associated surface low will move east
across the Dakotas late Tuesday and then Minnesota during the
overnight hours.  As the system begins to tap some deeper
moisture, models show a developing comma head with light to
moderate precipitation developing across the Dakotas and Upper
Midwest on Wednesday into early Thursday.  Increasing low-to-mid
level frontogenesis, along with favorable upper forcing, is
expected to help organized precipitation west of the low.  As was
the case the previous night, the 00Z suite of guidance continues
to differ on the amplitude of the upper system, with impacts on
where the deformation will develop.  WPC continued to prefer the
ECMWF, UKMET and Canadian Global camp of solutions, with concerns
that the GFS and NAM may be too amplified aloft and in turn
spreading heavier precipitation too far west across the northern
Plains.  Given the model spread, forecast confidence on the
overall placement of heavier amounts is limited.  However, there
is some broader consensus that at least localized heavy amounts
are likely across portions of the western U.P. of Michigan and
northern Wisconsin, where a period of northerly flow on the
backside of the system as it moves across the Upper Great Lakes,
will likely support lake-enhanced totals Wednesday night. 
Regardless of precipitation, strong, gusty winds are a certain
impact as this powerful system moves across the region.

Day 3...
...Mid and Upper Ohio Valley/Central Appalachians/Lower Great
Lakes...
As low pressure over the Upper Great Lakes lifts into a Canada, a
sharp trailing cold front will continue to sweep east, moving
across the Ohio and Tennessee valleys on Thursday.  The sharp drop
in temperatures will result in rain changing to snow, with mainly
minor accumulations expected across much of the lower Ohio and
Tennessee valleys Wednesday night into early Thursday.  However,
the potential for heavy snow is expected to increase further east
by late Thursday.  A well-defined shortwave moving through the
base of the broader scale trough will support a surface wave
developing along the front as it moves into the southern and
central Appalachians on late Thursday.  As an upper low begins to
close off over the Ohio valley, this wave is expected to lift
north from the central Appalachians and through the upper Ohio
valley and Lower Great Lakes region, raising the potential for
heavy snows on the backside of the front late Thursday into the
overnight. 

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

Pereira