Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 PM EST Sun Nov 22 2020

Valid 00Z Mon Nov 23 2020 - 00Z Thu Nov 26 2020


...Great Lakes/Northeast Day 1...

Low pressure tracking up the St Lawrence River tonight and into
Quebec should maintain a low-mid level warm advection pattern
across northern New York and northern New England.  The primary
snow band is expected in Canada, with mixed precip occurring in
the NY Adirondacks to the VT Green mountains, NH White Mountains,
and ranges of western Maine.
Warm advection results in snow changing to sleet and freezing rain
before ending.
Multiple precip types limit amounts of any one type, so the threat
of heavy amounts is minimal.

The trailing cold front crosses the lower lakes, with a brief
period of cross lake flow within the cold advection pattern
allowing lake effect snow showers Monday morning in the lee of
Lakes Erie and Ontario.  The setup is temporary, with the models
showing pockets of dry air aloft advecting across the region Mon
afternoon.  This limits potential amounts.
Likewise a trough/secondary front in the upper lakes allows for
snow showers in the lee of Lake MI in northwest lower MI, bit the
approach of a low level ridge from the Upper MS Valley leads to
drying aloft and sinking motion, so the duration of snow should be
limited. Some light snow accumulations are likely.

...Upper MS Valley to the Great Lakes Days 2/3...
A low-to-mid level warm front associated with an approaching upper
trough will help generate widespread precipitation, with cold air
supporting snow across the upper Mississippi Valley into the upper
Great Lakes Monday night through Tuesday.
A surge of lift occurs with 850-700 mb theta-e advection across
the region, so snow may be moderate at the theta-e advection
peaks.  The warm advection leads to a precip type change over
further south in SD/IA/southern MN/southern WI, limiting amounts
there. The NAM and Canadian regional GEM hold on the snow longer
than the GFS and ECMWF, so the duration of snow is causing
uncertainty.

Accumulating snows appear likely from eastern Minnesota to the
U.P. of Michigan, with a few inches most likely across portions of
northern Wisconsin into the U.P. of MI. The event winds down as
the continuing warm advection with the front crosses MI to the
Canadian border on Wed.

...Central Rockies/high Plains Days 1-2...
Deep southwesterly flow ahead of an approaching upper trough
combines with upper divergence maxima in the left exit region of a
jet over NM to support snow across the mountains of western and
central Colorado tonight into Monday before diminishing as the
upper trough moves east of the region on Tuesday.  Areas impacted
will likely include the Elk and San Juan Mountains and Sawatch
Range.  The long duration leads to high amounts of snow, with one
to two feet of snow in the San Juan Mountains.

On Day 2/Mon night-Tue, as the low level front produces cooling on
the central high Plains, a band of snow may occur within the band
of low-mid level frontogenesis/deformation in eastern CO.  There
is potential for several inches, with the parent NAM higher than
the Nam Conus Nest, but the Canadian global/regional GEM/ECMWF/GFS
have increased amounts from last night. The event ends late Tue as
the 700 mb trough moves east of the area and ascent declines and
ends.

...Pacific Northwest/Northern Rockies Days 1-3...
An upper trough and a pair of weak frontal systems are expected to
produce periods of mainly light snow from the Olympics and
Cascades to the northern Rockies Sunday and Monday.  A better
defined frontal system and upper jet maximum crosses the Olympics
and Cascades Tue, bringing potential for several inches to the
ranges. As the next 700 mb trough approaches on Wed, another round
of precipitation and a threat for heavier snow accumulations into
the Olympics and northern Cascades on Wednesday. As
heights/temperatures aloft fall,  snow level decrease on
Wednesday.  This allows for an increase in snow coverage/duration,
with the 09z SREF Mean indicating maxima near an inch liquid
equivalent in both the WA/OR Cascades, indicating potential for up
to a foot of snow for the 24 hour period ending 00z Thu. 

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

Petersen