Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 AM EST Thu Nov 12 2020

Valid 12Z Thu Nov 12 2020 - 12Z Sun Nov 15 2020


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

An active pattern for the Pacific Northwest resumes today with
precipitation expected there for much of the next week. The
leading impulse on the next trough to affect the Pacific Northwest
reaches Vancouver Island this afternoon with the main reinforcing
trough axis reaching the WA/OR coast around midday Friday. This
main trough axis shifts southeast to the CO Rockies by Saturday
morning with the next shortwave trough approaching the Pacific
Northwest Saturday.

Cold air is in place across the northern tier of the CONUS west of
the Great Lakes under a broad trough over much of the CONUS. Snow
levels rise on the Olympics/Cascades to around 2500ft in warm air
advection ahead of the approaching wave before precip breaks out
late this afternoon/into the evening. While this system is
expected to weaken and move progressively downstream, the second
system is more potent and both systems have ample Pacific moisture
to work with. Areas impacted by heavy snow through Friday night
are likely to include the Olympics, WA/OR Cascades, the Wallowa
Mountains in northeastern Oregon, the central and northern Idaho
ranges, and the Tetons where Day 2 probabilities for 12 or more
inches of snow are high.

This reinforcing system will rapidly move southeast across the
West, with a well-defined cold front dropping south across the
Rockies and Great Basin on Saturday. However, upstream onshore
flow with embedded energy aloft, will support periods of high
elevation snow across the Northwest into the northern Rockies
through Saturday. Additional heavy snows are likely for portions
of the Cascades on Saturday, as well as the central Idaho ranges,
including the Sawtooth. Areas of heavy snow are also expected to
develop across western WY and briefly into central CO. Day 2.5
snow probabilities are generally moderate to high for 8 or more
inches across very similar areas to Day 2, just expanded farther
southeast across the rest of western WY, northern UT, and central
CO.

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

Jackson