Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EST Sat Dec 19 2020

Valid 12Z Sat Dec 19 2020 - 12Z Tue Dec 22 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
Strong westerly flow is expected to direct a plume of anomalously
deep moisture into the Pacific Northwest beginning as early as
late Saturday afternoon -- supporting the development of heavy
precipitation that is expected to continue into the overnight
across portions of western Washington and Oregon.  This
precipitation will include snow for the Cascades.  However, rising
snow levels late Saturday into early Sunday are expected to
confine heavy accumulations mainly to the higher elevations of the
northern Cascades.  With ample moisture spreading east, locally
heavy accumulations are also expected east of the Cascades -- with
areas impacted expected to include the northeastern Oregon
mountains and the northern Rockies -- especially the northern
Idaho and the northwestern Montana ranges west of the Divide.  But
here also, rising snow levels are expected to limit heavy
accumulations mostly to the higher peaks.

With onshore flow forecast to ease a bit, expect to see some
decrease in precipitation rates by late Sunday.  However, a
lingering frontal boundary and coinciding axis of deep moisture
will continue to support additional locally heavy amounts,
including mountain snows for the Cascades and northern Rockies.

This front is expected to lift north ahead of an amplifying wave
approaching the Washington coast Monday morning.  This will
support an additional round of organized precipitation across the
northern Cascades and Rockies as the front lifts back through the
region on Monday.  Snow levels are expected to once again increase
ahead of the low, before dropping rapidly as the low and a strong
trailing cold front move across the Northwest into the northern
Rockies late Monday into early Tuesday.  Additional heavy snow
accumulations are likely across the northern Cascades.  Potential
for heavy snow for the northern Rockies east of the Divide is
expected to increase late Monday.  A boundary lifting north ahead
of the wave is expected to support increasing low level
convergence/upslope flow -- raising the threat for heavy
precipitation developing along the Lewis Range in northwestern
Montana by late Monday and continuing into early Tuesday.

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

Pereira