Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Fri Nov 13 2020

Valid 12Z Fri Nov 13 2020 - 12Z Mon Nov 16 2020


...Western U.S....
An amplifying shortwave trough on the leading edge of a strong
northwesterly Pacific jet will cross the WA/OR coast this morning
before continuing to shift southeast, reaching the southern
Rockies by Saturday morning. The combination of leading moist
onshore flow and strong upper forcing will produce widespread
moderate to heavy precipitation through tonight across the
northwest CONUS, including heavy mountain snow on orographically
favored terrain above snow levels around 3500ft in WA to 5000ft in
northern CA ahead of a strong cold front. Precipitation will
spread south across northern California and east across the Great
Basin and the northern and central into early Saturday. Areas
impacted by heavy snow through tonight/Day 1 are the entire
Cascades, Blue Mtns of northeastern Oregon, central and northern
Idaho and northwestern Montana ranges, and western WY and northern
UT where snow probabilities are moderate to high for a foot or
more. Several feet are likely across the higher terrain of the
WA/OR Cascades today alone. 

The next shortwave trough will approach the Pacific northwest
coast Saturday night with a strong westerly jet shifting it east
across Pacific Northwest and northern Rockies through Sunday. Snow
levels increase in the warm/moist air that pushes into WA/OR
Saturday and continues into Sunday. Therefore Days 2/3 snow
probabilities are lower with a moderate risk for 8 or more inches
for the highest WA/OR Cascades to western WY (with the same ranges
as Day 1 affected in between).


...Upper Midwest...
Days 2/3...

The wave currently approaching the WA/OR coast spills onto the
central Plains Saturday. The mid-level trough takes on a negative
tilt over the Midwest Saturday night. While there is still
uncertainty for the strength and track of the low, as well as how
much comma head precip will be present over the Upper Midwest
Saturday night, there is risk for a swath of accumulating snow
around the northwest side of the low as well as for lake effect
snow in the cold air in the wake of the low - mainly on Sunday. As
of now Day 2.5 WPC snow probabilities are low for 4 or more inches
only in the western UP of MI which is where there may be a pivot
of TROWAL-caused snow followed by lake effect. This Midwest low
bears further attention in subsequent forecasts.


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

Jackson