Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 PM EST Tue Nov 17 2020

Valid 00Z Wed Nov 18 2020 - 00Z Sat Nov 21 2020


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

Persistent weakly cyclonic upper level flow with an embedded
130+kt jet will drive moisture into the west through the week.
Embedded within this flow, several shortwaves will rotate onshore
accompanied by modest surface waves and associated warm/cold
front. Each of these features will drive ascent to produce
mountain snow across much of the western ranges through Thursday,
before a sharper trough digs southward into the Great Basin D3
shunting forcing and moisture away from the region.

The first impulse will lift onshore the OR/WA coast early
Wednesday accompanied by robust diffluence within the LFQ of the
upper jet streak. This will spread snowfall across many of the
ranges from the Olympics of WA, southward through the Sierra
Nevada, and as far east as the Tetons of WY. Snow levels will
gradually fall D1 from 4000-7000 ft early, to 2000-5000 ft late,
lowest in WA. WPC probabilities for 4 inches are high across most
of these ranges, with the highest snowfall likely in the Sierra
and Sawtooth where locally in excess of 12 inches is possible.

A second impulse quickly follows the first on Thursday with
additional height falls and PVA aided by modest jet diffluence in
a moist column. Snow levels of 2000-5000 ft will continue to
spread eastward, allowing periods of moderate to heavy snow to
impact many of the same ranges as on D1. Additional snow
accumulations could produce 2-day totals in excess of 2 ft in the
Cascades and Sawtooth.

Additional moderate to heavy snow will continue in the Cascades on
D3, but in general the forcing will begin to shunt southeastward
towards the Great Basin as a deeper trough digs across CA Friday.
The overlap of moisture and synoptic ascent will weaken by D3, and
WPC probabilities for 4 inches are moderate in the WA Cascades and
eastward into the Northern Rockies.


...Great Lakes...
Day 1...

Continued CAA behind Tuesday's cold front will leave lingering
multi-bands of lake effect snow, primarily SE of Lake Erie and
Lake Ontario tonight. The environment will gradually become less
conducive for LES as ridging builds northeast from the Ohio
Valley, causing low-level winds to back to the W/SW while the DGZ
dries and EL's lower. Lingering LES could produce a few inches of
snowfall in NW PA and parts of W NY, with a potential heavier band
focused in NW PA thanks to an effective fetch tapping moisture
from Lake Superior and Huron. WPC probabilities for 4 inches are
moderate in a narrow corridor of Erie and Crawford counties in PA,
but otherwise are less than 10%. Any accumulating snow should end
by early Wednesday morning.


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

Weiss