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

Valid 12Z Wed Apr 22 2020 - 12Z Sat Apr 25 2020



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

Days 1-3...
The models show confluent mid-upper level flow with an embedded
300 mb jet streak that moves onshore today.  Ascent occurs with
well defined upper divergence within a coupled jet region crossing
the WA Cascades and then inland into northern ID.  Several inches
of snow are expected at higher elevations of the Cascades and then
into the ranges of northern ID/northwest MT, where snow showers
continue on day 2 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
ranges of WY, and then northern CO by Fri morning.
Day 2 snow probabilities for 6 or more inches are high in the
Mission/Swan Ranges of Northwest MT, the Bitterroots along the
ID/MT border, and the Tetons and Medicine Bow Mtns in WY.

Residual light snow 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 forecast snow amounts are light.  Consequently, Day
3 only has a low probability of 6 inches of snow in WY/CO.

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

Petersen