372
FXUS65 KMSO 222026
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
126 PM MST Wed Jan 22 2025

.DISCUSSION...

KEY MESSAGES:

-  Light to moderate snow event Friday, followed by another arctic
   over the weekend.

-  Prolonged high pressure ridging next week may lead to air
   stagnation concerns.

Benign weather is expected through Thursday evening with cold
mornings and near normal afternoon temperatures, which may feel
warm under blue skies and bountiful sunshine.

Clouds increase late Thursday as a short wave drops south out of
Canada. While this system is moisture starved a quick burst of up
to 2 inches of snow is possible before the morning commute
Friday. Plan accordingly for the potential of slippery roadways,
for this reason winter weather advisories may be added to the
forecast package Thursday.

This wave is expected to meander southwest and cutoff over central
California, helping to pull a deep Arctic air mass into western
Montana over the weekend. This will be a relatively quick hitting
Arctic, that said Sunday morning lows will be cold but likely not
as cold as the previous Arctic. Dress accordingly Sunday, along
the Continental Divide, as wind chills could range from -5 to
-15F.

Almost as quickly as the low cutoffs over central California, a
positively tilted high pressure ridge folds over the Northern
Rockies and is prognosticated to endure through the 30th. For
this reason concerns are increasing that air stagnation could
become an issue, please see our `air stagnation` section for
specifics. Under this strong ridge, cold air is expected to become
trapped in valleys. Diurnal temperature ranges are expected to
narrow, with valleys potentially experiencing temperatures 8 to
12 degrees below normal, while mountains areas see warming. Our
temperature forecast does not currently reflect this scenario and
edits will likely be made if future confidence increases.



&&

.AVIATION...Satellite imagery this afternoon shows stratus along
the Divide retreating from north to south as drier air moves into
the region, driven by a ridge of high pressure. Forecast soundings
indicate that valley inversions will reestablish tonight under
mostly clear skies across much of north-central Idaho and western
Montana. In northwest Montana, sufficient low-level moisture
remains to support patchy fog or lowered ceilings by Thursday
morning. Cloud cover is expected to increase and lower throughout
the day on Thursday ahead of an approaching trough. Terrain winds
are forecast to strengthen in advance of this system, with a
greater than 90% probability of gusts reaching or exceeding 22
knots.


&&

.AIR STAGNATION...The combination of cold air in valleys and strong
high pressure developing Monday (27 Jan) is leading to concerns
of air stagnation. The longevity of high pressure may allow for
increasing low level contamination without adequate valley
ventilation. Those with respiratory problems should monitor local
air stagnation guidance and future forecasts.



&&

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

$$