Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Fri Dec 18 2020

Valid 12Z Fri Dec 18 2020 - 12Z Mon Dec 21 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
An upper level shortwave currently centered west of 140W is
expected move onshore late Friday -- supporting another round of
wet weather across the region beginning by late Friday and
continuing into early Saturday.  With snow levels relatively high
-- heavy snow accumulations will likely be limited to the higher
peaks of the northern Cascades.  This system will move quickly
east of the region on Saturday as a stronger and wetter system
approaches British Columbia.  Heavy precipitation is likely across
western Washington late Saturday into early Sunday as a long fetch
of deep, moisture-laden, westerly flow settles south into the
region.  Additional rises in snow levels Saturday night into
Sunday morning will confine heavy snow accumulations to the high
peaks of the northern Cascades.  On Sunday, focus for heavier
precipitation will shift farther south and east along with a
frontal boundary settling along the Washington-Oregon border and
across northern Idaho.  This is expected to increase the potential
for locally heavy snow accumulations across portions of the
northern Rockies, especially along and near the Bitterroot Range
on Sunday into early Monday.

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

Pereira