Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Wed Apr 22 2020

Valid 00Z Thu Apr 23 2020 - 00Z Sun Apr 26 2020



...WA Cascades/Northern and Central Rockies...

Days 1-2...
A shortwave in confluent mid-upper level flow with an embedded 300
mb westerly jet streak has pushed into WA this afternoon. Ascent
from well defined upper divergence within a coupled jet region
shifts from the WA Cascades to the northern Rockies of ID/MT
through tonight before veering flow along drives the shortwave
southeast down the Rockies Thursday into Friday. Several inches of
snow are expected above the 5000 to 6000ft snow level in the
WA/northern OR Cascades and the ranges of north-central ID/western
MT and northwest WY, where Day 1 snow probabilities are moderate
to high for 6 or more inches. Snow showers continue on Thursday as
700 mb confluent flow pools moisture across ID and weak ascent
leads to additional snow.

The northwest flow allows the moisture and lift to progress
downstream across the rest of ranges of WY to  northern CO by
Thursday night. Day 2 snow probabilities for 6 or more inches are
moderate to high for the WY Rockies, particularly the Medicine Bow
Range, and northern CO

Residual light snow Thursday night into Friday occurs in the WY
Bighorns down to the front ranges in CO due to the persistence of
the moist confluent flow in the ranges of WY to northern CO, but
the approach of the 700 mb ridge from the Pacific northwest leads
to weakening lift across the region, so the mountain snow
generally tapers off by Friday night.

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

Jackson