Heavy Snow and Winter Weather in Idaho, December 30 to 31, 2020

Area Forecast Discussion
National Weather Service Missoula MT
242 AM MST Wed Dec 30 2020

.DISCUSSION...Regional radar and surface observations reveal an
area of light snow moving across eastern Washington into north
central Idaho and northwest Montana near the ID/MT border this
morning. Isentropic lift in the warm sector of the approaching
shortwave trough will continue to produce light snowfall across
north central Idaho and northwest Montana through the early
afternoon. A southerly pressure gradient will cause a transition
to mixed precipitation along the Highway 12 corridor by mid-
morning as air temperatures warm above freezing, but ground
temperatures remain below freezing. A similar set up will occur
across Mineral, Sanders, and Lincoln Counties in northwest Montana
during the late afternoon and early evening time frame. Snowfall
rates will increase Wednesday afternoon and evening along the
Idaho and Montana border for elevations above 2,500 feet as the
upper level trough rounds into eastern Washington.

An secondary shortwave trough embedded within northwesterly flow
will lead to an increase in snow shower activity above 3,000 feet
Thursday. Snowfall expectations have trended higher across the
northern Bitterroot and Clearwater Mountains Thursday, with
several inches of additional snow being possible.

Confidence continues to increase for a period of active weather
this weekend through early next week. A longwave trough extending
southeastward from the Gulf of Alaska through the Pacific
Northwest will direct a train of Pacific storm systems into the
western U.S.

The first upper level disturbance will move across the Northern
Rockies Friday night into Saturday morning. This quick hitting
system will only bring minor snowfall accumulations across
mountain ranges in northwest Montana and along the Idaho and
Montana border.

A potent Pacific storm system will arrive into the region
Saturday night into Sunday. A warm front will transport a plume
of Pacific moisture across the Northern Rockies. Anomalously high
atmospheric moisture and a strong southwesterly low level jet will
favor a period of heavy mountain snowfall for elevations above
3,500 feet in the Purcell, Cabinet, and northern Clearwater and
Bitterroot mountain ranges. Minor valley snowfall is possible
Sunday night into Monday morning as a cold front sweeps across
western Montana.

Ensemble cluster model analysis suggests a deep shortwave trough
will nudge closer to the Pacific Northwest by next Wednesday and
Thursday. Modest confidence exists for periods of moderate to
heavy mountain snowfall to continue, with valley temperatures
remaining mild.

&&

.AVIATION...Light snow will continue to spread across north
central Idaho and northwest Montana this morning. Confidence for
accumulating snowfall remains high for areas along the Idaho and
Montana border and across north central Idaho above 2,000 feet.
Only minor snowfall accumulations (generally one inch or less) are
expected for areas along US Highway 93 and the I-90 corridor south
and east of Missoula. Pockets of freezing rain are possible for
valley locations in north central Idaho and northwest Montana this
afternoon as a southerly pressure gradient leads to a transition
to rain below 2,500 feet and ground temperatures remain below
freezing for some. A moist and unstable northwesterly flow will
maintain widespread snow shower activity for elevations above
3,000 feet across north central Idaho Wednesday night into
Thursday morning.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...     Winter Weather Advisory until 11 AM MST Thursday West Glacier
     Region.

     Winter Weather Advisory until 5 AM MST Thursday Kootenai/Cabinet
     Region.

     Winter Weather Advisory until 11 AM MST Thursday Potomac/Seeley
     Lake Region.

     Winter Weather Advisory until 11 AM MST Thursday Lower Clark
     Fork Region.

ID...     Winter Weather Advisory from 7 AM this morning to 10 AM PST
     Thursday Southern Clearwater Mountains.

     Winter Weather Advisory until 10 AM PST this morning Lower Hells
     Canyon/Salmon River Region.

     Winter Weather Advisory until 10 AM PST this morning
     Orofino/Grangeville Region.

     Winter Weather Advisory until 10 AM PST Thursday Northern
     Clearwater Mountains.

&&