022
FXUS65 KMSO 311036
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
336 AM MST Fri Jan 31 2025

.DISCUSSION...

KEY MESSAGES:

*Friday-Saturday: valley freezing rain and heavy mountain snow
  creating hazardous travel conditions across Northern Rockies.

*Saturday afternoon-evening: snow squall threat across west
  central and southwest Montana.

*Sunday-Monday: arctic airmass will move west of the Continental
  Divide.

*Middle of next week: model agreement on significant snow
  accumulation event across southwest Montana and Lemhi County.

While inversions have decreased in depth, they do remain across
the region supported by the warm air that has moved in between
roughly 4000 and 5500 feet. Poor air quality will remain through
most of the morning even though our advisories will be dropped in
the next few hours. Our messaging about impactful winter weather
systems is now front and center for what may be the most impactful
week of winter weather yet this season.

Our confidence in mixed precipitation has increased. This is due
to temperature observations around the region demonstrating a
near 1500 foot depth of the atmosphere, above valleys, that is 5
to 10 degrees above freezing. Timing for onset of precipitation is
morning from Camas Prairie to far northwest MT and late afternoon
to evening US-93 and eastward. Frozen precip that falls through
this area will initially evaporate moistening up the atmosphere,
then subsequently melt into rain. As long as cold pools remain at
the surface the rain will freeze creating icy conditions in
valleys. The highest confidence for these conditions is north of
I-90 and east of Missoula, as the Missoula and Bitterroot Valleys
are expected to be down sloped and precipitation limited. Valleys
of north-central ID will be all rain at onset, currently
Whitebird Hill along US RTE 95 is above freezing and rain is the
more likely precipitation type. Be prepared for travel conditions
in valleys of western MT to deteriorate suddenly. Heavy snow is
expected in the mountains, travel across passes will be difficult
at times.

Saturday a cold trough will settle over region allowing for
dynamic cooling throughout the atmosphere creating instability
aloft. Heavy snow showers or snow bands will be possible. These
banded snow events can support snow squalls, when the combination
of intense snow, low visibility, and rapidly changing road
conditions can lead to multi-vehicle pileups.

Finally on Sunday, an Arctic air mass is expected to come across
the Continental Divide. Although there is a strong gradient
pushing the cold air, westerly winds at ridgetop will resist this
air mass slowing its advance initially. After sunset, the cold air
if stalled will surge towards the Missoula Valley. Left over
moisture will allow for the potential of a brief period of
intense snow along the Arctic boundary as it advances.

Run to run the models are shifting where the next surge of
moisture will be positioned in relation to the Arctic boundary
Monday through Wednesday. This will greatly affect which locations
receive moderate to heavy snow with blowing and drifting,
currently that is south of Missoula, southwest MT, and the
Anaconda Mountains.



&&

.AVIATION...High pressure over the Northern Rockies air space
will weaken through Friday, ahead of a significant, multi-day
winter storm cycle (beginning Friday morning in north-central ID).
Winds from the west- southwest at ridgetop will be greater than
40 knots through Saturday. Clouds will also increase over the
region on Friday, as a surge of subtropical moisture moves into
the air space. Terrain obscurations will return after 31/1500Z.
Mixed precipitation is expected to begin after 01/0000z at KGPI
and KHRF, and 01/0700z at KMSO. KSMN and KBTM will begin to
receive precipitation Saturday morning.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 11 AM Friday to 5 PM MST Saturday for
     Bitterroot/Sapphire Mountains.

     Winter Storm Watch from late Saturday night through late Tuesday
     night for Bitterroot/Sapphire Mountains.

     Air Stagnation Advisory until 5 AM MST Friday for
     Kootenai/Cabinet Region...Lower Clark Fork Region...
     Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

     Winter Weather Advisory from 5 PM Friday to 5 PM MST Saturday
     for Butte/Blackfoot Region...Missoula/Bitterroot Valleys.

     Winter Storm Warning from 11 AM Friday to 5 AM MST Saturday for
     Flathead/Mission Valleys...Kootenai/Cabinet Region...West
     Glacier Region.

     Winter Storm Watch from Sunday morning through Monday afternoon
     for Kootenai/Cabinet Region...West Glacier Region.

     Winter Storm Watch from Sunday morning through Tuesday evening
     for Flathead/Mission Valleys.

     Winter Storm Warning from 11 AM Friday to 11 AM MST Saturday for
     Lower Clark Fork Region.

     Winter Storm Watch from Sunday morning through late Tuesday
     night for Butte/Blackfoot Region...Lower Clark Fork
     Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake
     Region.

     Winter Weather Advisory from 11 AM Friday to 5 PM MST Saturday
     for Potomac/Seeley Lake Region.

ID...Winter Storm Warning from 10 AM Friday to 4 PM PST Saturday for
     Northern Clearwater Mountains...Southern Clearwater
     Mountains.

     Winter Storm Watch from late Saturday night through late Tuesday
     night for Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Air Stagnation Advisory until 5 AM MST /4 AM PST/ Friday for
     Eastern Lemhi County...Orofino/Grangeville Region.

     Winter Weather Advisory from 10 AM Friday to 10 AM PST Saturday
     for Orofino/Grangeville Region.

     Winter Weather Advisory from 5 PM Friday to 5 PM MST Saturday
     for Eastern Lemhi County...Western Lemhi County.

&&

$$