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

Valid 00Z Fri Nov 13 2020 - 00Z Mon Nov 16 2020


...Western U.S....
An amplifying shortwave positioned along the leading edge of a
strong Pacific jet is expected to approach the Northwest coast
Thursday night.  The combination of moist onshore flow and strong
upper forcing is expected to produce widespread precipitation,
including mountain snow, across western Washington and
northwestern Oregon beginning overnight, with heavy amounts across
the orographically favored terrain.  As the upper trough moves
onshore, precipitation is likely to spread farther south into
northern California and east across the northern Intermountain
West into the northern Rockies on Friday into early Saturday. 
Areas impacted by heavy snow through early Saturday are likely to
include the Olympics, Cascades, and the northeastern Oregon,
central and northern Idaho and northwestern Montana ranges.  WPC
PWPF indicates the potential for at least localized amounts of a
foot or more across these areas, with several feet likely across
the higher terrain of the northern Cascades. 

Snows will continue to push through the northern Rockies on
Saturday and south along the associated frontal band dropping
south through the central Rockies.  This is expected to result in
locally heavy accumulations across the western Wyoming and
northern Utah ranges.

Even as the system moves east into the Plains by late Saturday,
progressive westerly flow, with embedded energy aloft will
continue to support periods of mountain snow from the Northwest to
the northern Rockies. 

With periods of precipitation continuing, warm advection ahead of
a system moving inland is expected to result in rising snow levels
across the Northwest into the Rockies Saturday night into Sunday.

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

Pereira