Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Wed Nov 04 2020

Valid 12Z Wed Nov 04 2020 - 12Z Sat Nov 07 2020


...Pacific Northwest/California/Northern Rockies...

Sustained upper level jet maxima and moisture fluxes and coupled
upper divergence/lower convergence maxima support periods of heavy
precipitation along the orographically favored regions of the
Northwest through midweek.  However, snow levels are expected to
remain high through Wednesday -- confining the threat for heavy
snowfall accumulations to the highest peaks of the northern
Cascades.  Then on Thursday night and Friday as an amplifying
shortwave begins to move onshore, snow levels will begin to lower
across the Northwest -- expanding the potential for heavy snow
accumulations across the WA Cascades and in the front range of MT.


Overall consensus of the models shows an upper trough continuing
to amplify over the northwestern U.S., with an upper low closing
off over Oregon/Northern California on Friday and drifting inland
into the Great Basin Friday night.  As the low passes, cold
advection aloft will lower snow levels and lead to rain changing
to snow.  This will bring the threat for high elevation heavy snow
accumulations into the OR/northern CA Cascades.  Meanwhile, strong
low-to-mid level frontogenesis extending from western Montana to
eastern Oregon, along with falling snow levels, will help set the
stage for locally heavy snows beginning to develop from the
northeastern Oregon mountains to the ranges of northwestern
Montana, particularly the front range/Glacier National Park.  The
primary uncertainty is duration of snow, and secondarily amounts
of QPF once precip becomes snow.  

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

Petersen