381
FXUS65 KMSO 212120
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
220 PM MST Thu Nov 21 2024

.DISCUSSION...

KEY MESSAGES:

-  Valley snow impacts focused across northwest Montana and
   mountain passes Saturday into Sunday.

-  Highest snow accumulations focused along the divide in Glacier
   Park region and higher elevations across Lincoln County.

Satellite imagery from this afternoon shows a broad trough of low
pressure off the Oregon/Washington coast, with a Pacific jet
stream and atmospheric river making landfall in northern
California. Temperatures across the Northern Rockies have
moderated under a mild south-southwest flow, with snow levels
rising above 6,000 feet south of the I-90 corridor and near 5,000
feet in northwest Montana. Cold air from Canada remains banked
along the divide and in Glacier Park, where temperatures are in
the teens and low 20s.

Pulses of Pacific moisture associated with the atmospheric river
will continue to bring precipitation to the region through
Saturday. The next widespread surge of precipitation arrives
Friday morning, bringing snow to Lincoln County above 4,000 feet
and winter driving conditions along US-2, especially from Essex to
Marias Pass. Snow levels along and south of the I-90 corridor
will remain above pass level.

A second surge of precipitation will impact the area late Friday
night into Saturday, as a shortwave trough breaks away from the
broad Pacific low and moves inland. Guidance suggests a surface
low will deepen across eastern Washington and move eastward into
western Montana by Saturday morning. The track of this low is
critical to the forecast, as it will determine how quickly the
cold air banked along the divide moves westward into northwest
Montana. About 70% of the guidance brings the cold air into the
Flathead Valley and Lincoln County by Saturday evening, causing
precipitation to change over to snow. The remaining 30% tracks the
surface low more over the I-90 corridor, sweeping the cold air
across the divide Saturday morning and changing precipitation to
snow in valley areas of Lincoln, Flathead, and Lake Counties.
Confidence for the highest winter impacts is focused on the West
Glacier region, where the clash between Pacific moisture and
colder air will lead to the heaviest snow accumulation (greater
than 12 inches by Sunday). This will be particularly true above
4,000 feet in the Swan and Whitefish Ranges, as well as for the
Essex to Marias Pass area along US-2. At present, snow rates are
expected to be closer to advisory levels, so no winter storm
watches or warnings have been issued yet.

Winter travel impacts will extend across all mountain passes by
Saturday night into Sunday morning as snow levels drop to around
3,000 feet with the passage of a Pacific cold front. Valley areas
south of I-90 will experience light snow showers, with minor
accumulations of less than one inch.

A low-amplitude ridge of high pressure will slide over the
Northern Rockies Sunday into Monday, leading to a decrease in
shower activity. By mid-week, the large low pressure system off
the coast is expected to move southeast into the Great Basin. This
will place the Northern Rockies in a northwesterly upper-level
flow pattern for Thanksgiving and into the following weekend,
bringing cooler-than-average temperatures and the potential for
light snow showers. /AL

&&

.AVIATION...A milder southwesterly flow aloft is raising snow
levels, leading to more scattered precipitation with valley rain
and mountain snow. These showers are expected to persist
overnight, causing periodic terrain obscurations. The next wave
of widespread precipitation arrives by Friday morning, with snow
levels generally between 5000-6000 feet. According to the NBM, north-
central Idaho and western Montana have a 50-70 percent chance of
ceilings below 3000 feet with this upcoming moisture.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$