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

Valid 00Z Sat Dec 19 2020 - 00Z Tue Dec 22 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
...Rounds of Moderate to Heavy snow expected through the weekend...
An upper level shortwave is expected move onshore this evening,
supporting another round of mountain snow along the path of the
wave from the WA Olympics/Cascades to the ranges of northern Idaho
and northwest Montana.  This system will move quickly east of the
region across the northern Plains.
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 briefly shifts farther
south and east along with a frontal boundary settling along the
Washington-Oregon border.
The models are now focusing on the development of a wave of low
pressure in the northeast Pacific that moves towards WA that
drives the warm front back north again across WA
Sun night into Mon. This is expected to increase the potential for
locally heavy snow accumulations across the ranges of northern WA
into northern ID and northwest MT.
Here model differences relate to the track of low pressure, with
the ECMWF and GFS clustering well, and the NAM/UKMET tracking the
low precipitation/resultant snow in concert with the low.  More
weighting was given to the 00-12z ECMWF/12z GFS/12z Canadian
global QPF/temperature profiles. Strong warm advection in the
preferred solutions keeps snow at higher elevations, stabilizing
as the cold front approaches Monday afternoon or night. 

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

Petersen