Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Mon Nov 16 2020

Valid 00Z Tue Nov 17 2020 - 00Z Fri Nov 20 2020


...Western U.S....
Days 1-3...

A modest Pacific jet streak of 110-130kts will move onshore the
Pacific Northwest beginning Tuesday night, with LFQ diffluence and
increasing moist advection driving precipitation across the West.
Much of the precipitation through D3 across the west should remain
north of this jet, from Northern California to Washington, and
stretching as far east as Wyoming. Embedded within this fast flow,
at least two shortwaves will move onshore driving modest cold
fronts into the West to enhance ascent and precipitation. The
first of these is likely Tuesday evening, with a secondary
shortwave/front moving onshore Wednesday night. Each of these
fronts will cause an enhancement in precipitation, with snow
levels falling in succession. Although heavy snow is limited D1,
by D2 /Tuesday night through Wednesday/ heavy snow is likely in
the mountains from the Olympics eastward to the Northern Rockies,
and southward as far as the Sierra Nevada, including all the
in-between terrain. WPC probabilities are high for 4 inches across
the terrain, with isolated amounts to 12 inches in the Sierra,
Sawtooth, and Cascades on D2. By D3, the first shortwave/front
decays and shifts east to be followed closed by the second. This
will bring another round of heavy snow to these same areas, with
high WPC probabilities for 4 inches persisting, and isolated
totals again eclipsing 12 inches possible in the WA and OR
Cascades. 3-day snowfall may reach 30 inches in the Cascades.


...Great Lakes...
Days 1-2...

A shortwave embedded within broad cyclonic flow across the eastern
CONUS will drive a cold front and associated wave of low pressure
across the Great Lakes overnight into Tuesday morning, bringing
rounds of lake effect snow (LES) to the region. There is likely to
be two waves of LES over the next 1.5 days. The first will be
associated with the more robust synoptic forcing associated with
the passing shortwave and cold front. The combination of height
falls and low-level convergence in a moistening column will drive
both modest synoptic snow showers and LES from the U.P. of MI into
New England. During this period of ascent, the EL atop the lakes
will be deepest, but instability through CAA will be modest and
total snowfall through Tuesday morning is likely to be light.

However, as CAA intensifies behind the front, LES should become
more robust and widespread despite lowering ELs as synoptic ascent
wanes. More intense low-level forcing off the lakes due to
increasing CAPE should produce favorable ascent into the DGZ for
periods of moderate LES. While the NW flow is not ideal for heavy
snow rates due to the limited fetch, guidance is producing at
least one more steady band thanks to an effective fetch from Lake
Huron across Lake Erie, and the heaviest snow through day 1.5 is
likely in the favored NW snow belts downwind of Lake Erie. WPC
probabilities for 4" are moderate in NW PA and SW NY, but less
than 10% elsewhere. While not accumulating much, there is
potential for isolated snow squalls behind the front across PA/NY
Tuesday as well, although low-level theta-e lapse rates around 0
suggest limited intensity of any squalls, and it may be more
snow-showers moving across the Mid-Atlantic and into New England
Tuesday night into Wednesday.

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

Weiss