Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Tue Dec 18 2018

Valid 12Z Tue Dec 18 2018 - 12Z Fri Dec 21 2018

...Northwest...
Days 1-3...

A notable atmospheric river event will continue bring widespread
heavy snow to the mountains of the Pacific Northwest into tonight.
Snow elevations will lower from 6000 to 7000ft to 3000 to 4000ft
across WA/OR today as the upper trough axis comes ashore. Moisture
will spill over into the northern Rockies of ID/MT/WY, with
significant snowfall also expected across these areas as jet level
diffluence combines with 700mb upslope enhancement through
tonight. Snowfall will not be as deep in the northern Rockies as
across the Washington Cascades, but sufficient moisture and snow
levels of 4000 to 5000ft suggest accumulations of 1-2 feet are
possible in the high terrain of the ranges of Idaho north of the
Snake River Valley and western MT with notable amounts in
northwest Wyoming. WPC probabilities for 18 inches are moderately
high on Day 1 for the WA Cascades and along a portion of the ID/MT
border.

Ridging dominates the Pac NW Wednesday into Wednesday night with
the next major low taking aim at southern Vancouver Island late
Thursday with the associated cold front reaching the Pac NW coast
Thursday night. Another round of heavy snow for high elevations is
expected Thursday night with WPC probabilities for eight inches
moderate to high for the Cascades to northern Rockies.


The probability for significant icing (0.25 inches or more) is
less than 10 percent.


Jackson