Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Tue Nov 03 2020

Valid 12Z Tue Nov 03 2020 - 12Z Fri Nov 06 2020



...Pacific Northwest/Northern Rockies...
The initial surge of precip with the atmospheric river on Days 1-2
will be associated with warm conditions.  As a result, snow levels
are expected to remain high, confining significant snow
accumulations to the highest peaks of the northern Cascades
through late Wednesday. 
A mid level shortwave and associated cold front progresses across
the northwest on Thu and into the northern Rockies Thu night.
Post-frontal lift with mid level deformation/upper divergence
maxima support snow becoming locally heavy in the WA/OR Cascades
and then into the ranges of northern ID and northwest MT.  Snow
levels are expected to lower, expanding the threat for heavy
amounts across portions of the Cascades on Thursday and northern
Rockies Thu night into early Friday.

Much of the uncertainty revolves around how quickly cooling occurs
to support the change over from rain to snow. This favors higher
elevations with probabilities decreasing in valley areas due to
the limited duration of snow.  A second factor is in the QPF
amounts, as the ECMWF/ECMWF Mean/UKMET forecast more QPF than the
NAM due to a stronger 700 mb low on Thu.  More weighting was given
to the heavier solutions.

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

Petersen