Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EST Mon Dec 10 2018

Valid 00Z Tue Dec 11 2018 - 00Z Fri Dec 14 2018

...Mountain West...
Days 1-3...

A strong shortwave trough comes ashore into the Pac NW Tuesday
night with an Atmospheric River in advance (with a second one
approaching late Day 3). High 1000-500mb RH will spread across
WA/OR and into the northern Rockies Tuesday night through
Wednesday night. Snow elevations as low as 2000 feet will quickly
rise with the warm atmospheric river. 850-700mb flow will become
aligned perpendicular to the Cascades, which combined with intense
moist advection as noted by PWAT anomalies approaching +2 standard
deviations above the mean, and jet level diffluence will likely
produce more than 2 feet of snow in the Washington Cascades, with
8-12 inches likely in the higher terrain of the Oregon Cascades
and Bitterroots of Idaho.

Shortwave ridging builds across the Pac NW Wednesday with
continued snowfall in the higher terrain making event totals of
more than 4 feet likely in the Cascades of Washington, with 1-2
feet probable across the high terrain of ID/OR. With snow levels
lowering to 2-3 kft under the building ridge, the valleys also
have the potential to see a few inches of accumulation as well.

Persistent subfreezing air pooled east of the Cascades in WA/OR
will continue to result in pockets of freezing rain until the
arrival of the AR Tuesday. Moderate probabilities for a tenth inch
of ice are present in the lee of the Cascades in WA Tuesday.

Jackson