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

Valid 00Z Sat Nov 14 2020 - 00Z Tue Nov 17 2020


...Western U.S....
Satellite imagery shows a well-defined, amplifying, upper trough
currently moving into the Pacific Northwest.  This system is
expected to move quickly across the western U.S., with models
showing a deep trough moving east of the Rockies by midday
Saturday.  The upper forcing associated with this system, along
with with a strong cold front pushing south and east, are expected
to help support a period of heavy snow across portions of the
northern to central Rockies Friday evening and overnight.  Areas
impacted are expected to include the western Wyoming and northern
Utah ranges, where WPC PWPF show high probabilities for localized
amounts of a foot or more Friday evening into Saturday. 
Meanwhile, progressive onshore flow, with embedded energy aloft
will continue to support periods of mountain snow across the
Northwest, with additional heavy amounts likely across the
Cascades Friday night into early Saturday.

Showers are expected to continue across the Northwest on Saturday,
before a warm front associated with a rapidly deepening low
approaching British Columbia brings a return of more organized
precipitation back to the region.  This will bring areas of heavy
snow back into portions of the Northwest and northern Rockies, but
with more limited impacts as warm air spreading across the region
pushes snow levels higher.  This system is expected to quickly
weaken as it moves onshore on Sunday, leaving behind a weak front
over the Northwest.  Fast zonal flow across the Northwest and
northern Rockies will continue to support shower activity along
and north of the boundary through Sunday.  By early Monday, the
front is expected to surge north as a strong upper ridge begins to
build ahead of an amplifying upper trough and deepening low
offshore.  With snow levels rising, any threat for heavy snow on
Monday will be largely confined to the higher peaks of the
northern Cascades. 

...Northern New England...
The upper trough associated with the system currently moving into
the West will continue to move quickly across the U.S. through the
weekend, with a negatively-tilted upper trough moving across the
Midwest into the Northeast on Sunday.  Cold air north of a
developing secondary low lifting across the Northeast, will
support a period of snow across northern New York and New England
late Sunday into early Monday, with some potential for impactful
accumulations across northern Maine.

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

Pereira