Special Weather Statement, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
259 AM MST Sat Dec 25 2021

.DISCUSSION...Surface observations show a shallow arctic frontal
boundary has pushed across the West Glacier region into the
Flathead valley this morning. Radar and area road webcams this
morning show widespread snow shower activity across western
Montana. Snow shower activity will focus across northwest Montana
this afternoon and evening with the passage of an upper level
trough. Accumulating snow will focus across the Kootenai/Cabinet
and West Glacier regions and across the Flathead Valley. A winter
storm warning remains in effect for the West Glacier region
through 1100 am MST this morning, where confidence is high for
periods of heavy snow as the arctic front interacts with pacific
moisture.

Model guidance has come into better agreement with the timing and
progression of the arctic front on Sunday. A broad surface low
will move into eastern Washington early Sunday morning, allowing
for arctic air to track south and west into northwest Montana. The
surface low will track across central Idaho, close to the I-90
corridor Sunday afternoon and evening, with the arctic front
reaching at least the Mission Valley and into the Potomac/Seeley
Lake region. A shortwave trough within southwesterly flow will
feed pacific moisture across the arctic boundary, potentially
leading to a period of difficult travel across portions of west-
central and northwest Montana. Therefore, a winter storm watch
remains in effect for the Kootenai/Cabinet region, the
Flathead/Mission valleys, and for the Butte/Blackfoot region.

The I-90 corridor around Missoula is a difficult area to assess
potential impacts Sunday afternoon and evening. Guidance is
struggling with the timing of the arctic boundary (Sunday morning
vs Sunday evening). If the boundary moves into Missoula Sunday
morning, a higher impact event is possible as pacific moisture
surges would lead to periods of moderate to heavy snowfall. This
scenario will be monitored closely.

The low pressure center will track into Wyoming and northern Utah
by Monday morning, drawing arctic air across the entire area.
Widespread snow will taper off by Monday morning, with scattered
snow showers continuing across north central Idaho. Model guidance
has trended away from worst-case scenario temperature
expectations the last few days. Nonetheless, the air mass will
still support low temperatures in the single digits across north
central Idaho and below zero readings across western Montana. The
primary reason for this trend towards less extreme temperatures
is due to a series of pacific waves which bring additional
periods of snowfall Wednesday into Friday next week. /Lukinbeal

&&

.AVIATION...Widespread terrain obscurations are expected with area
clouds and snow shower activity across the Northern Rockies
airspace for the foreseeable future. The bulk of accumulating snow
through 26/1200z is expected across northwest Montana and
Clearwater Mountains. A shallow arctic front has pushed across
Glacier Park region and into the Flathead valley as of 25/0800z.
Northeast winds will likely continue at KGPI through the day;
however, there is a chance for a southerly wind shift between
25/1500z and 26/0000z. Elsewhere, south to southwesterly breezes
will develop during after 25/1800z with gusts up to 15 knots.
/Lukinbeal

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from late tonight through Sunday evening
     Butte/Blackfoot Region.

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

     Winter Storm Watch from late tonight through Sunday evening
     Flathead/Mission Valleys.

     Winter Storm Warning until 11 AM MST this morning West Glacier
     Region.

     Winter Weather Advisory until 11 AM MST this morning
     Kootenai/Cabinet Region.

     Winter Storm Watch from late tonight through Sunday evening
     Kootenai/Cabinet Region.

ID...None.
&&

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