Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 PM EST Fri Nov 20 2020

Valid 00Z Sat Nov 21 2020 - 00Z Tue Nov 24 2020


...Pacific Northwest/Northern Rockies...
Day 3...

A sharp shortwave and accompanying potent 130+kt Pacific jet
streak will drop onshore Washington Sunday producing ascent and
heavy snow in the highest terrain of the WA and OR cascades as
well as the Olympics. The strongest LFQ ascent occurs late D3 so
additional snow is likely beyond this period, and heavy snow may
be ongoing Sunday night, especially in the WA Cascades. WPC
probabilities are high for 6 inches in the highest terrain of the
WA Cascades D3, with locally more than 12 inches possible.


...San Juan Mountains...
Day 3...

Mid-level shortwave will eject eastward into the Great Basin while
sharpening to produce height falls into the Four Corners region.
Atop this feature, a zonally oriented jet streak will push
eastward as well, extending into the Plains and strengthening to
drive in-situ RRQ diffluence to aid in ascent. These features
together will spawn weak cyclogenesis in the CO high plains, aided
lift which will be at times intense through 700mb southerly WAA
upsloping into the San Juans. Periods of intense omega with
increasing mid-level RH will produce rounds of snow, which will
likely be heavy at times. WPC probabilities indicate a high chance
for 6 inches, with locally more than 12 inches possible in the
highest terrain.


...Northeast...
Day 3...

An area of low pressure will develop along a cold front across the
Ohio Valley Sunday in response to a shortwave and RRQ jet dynamics
interacting with the low-level baroclinic zone. This low is likely
to lift rapidly northeast into New England by the end of D3
/Sunday night/ with robust ascent aided by WAA spreading
precipitation from Pennsylvania to Maine. Cold high pressure in
advance of this system will slowly retreat to the northeast, with
overrunning precipitation expected to help lock in the cold air,
at least initially, across the region. As the WAA intensifies, a
warm nose is likely, and a brief period of snow lifting northeast
will quickly transition to sleet and freezing rain from SW to NE.
The WAA should eventually overwhelm the column causing p-type to
change to rain except in the terrain where WPC probabilities for
0.1" of freezing rain are low to moderate in the Adirondacks.
Elsewhere, a light glaze of freezing rain is possible. Further
north, some heavier snow is likely where the WAA provides ascent
but the overall column is colder to prevent a warm nose exceeding
0C. WPC probabilities indicate a low chance for 4" of snow in the
White Mountains, with some heavier accumulations possible over far
northern Maine.


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

Weiss