Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
249 PM EST Sat Nov 14 2020

Valid 00Z Sun Nov 15 2020 - 00Z Wed Nov 18 2020


...Western U.S....
Progressive onshore flow will direct another plume of deep
moisture across the Pacific Northwest, with a strong warm front
helping to organize heavy prepitation as it moves across the
region Saturday evening and overnight.  Although snow levels will
begin to rise through the evening and overnight, cold air in place
at the onset is expected to support heavy snow across portions of
the Cascades, northeastern Oregon and central Idaho ranges.  As
the system pushes farther east, it is expected to bring additional
heavy snows to portions of the eastern Idaho and western Wyoming
ranges Saturday night into Sunday.  WPC PWPF indicates widespread
potential for 8-12 inches, with locally heavier snowfall
accumulations, across these areas during the Day 1 period (00Z Sun
- 00Z Mon).

Boundary lingering across the Northwest and northern Rockies, will
continue to support periods of snow into Monday before surging to
the north as an upper ridge begins to build ahead of an amplifying
trough moving across the eastern Pacific. 

...Northeast/Great Lakes...
A deep upper trough currently centered over the Plains is expected
to continue to move quickly across the U.S., with models showing a
negatively-tilted upper trough moving from the Great Lakes into
the Northeast Sunday night.  As the associated surface front moves
across the Northeast, a secondary low surface low developing on
the triple point is expected to help hold enough cold air in place
to maintain wintry precpitation across northern New Hampshire and
Maine, with impactful snow accumulations possible Sunday night. 

Deep cyclonic, westerly flow and cold air advection in the wake of
the system is expected to support developing lake effect snow
showers in the lee of lakes Erie and Ontario, with the potential
for locally heavy accumulations increasing Monday and Tuesday.

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