Area Forecast Discussion
Issued by NWS Missoula, MT

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673
FXUS65 KMSO 091905
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
1205 PM MST Sat Nov 9 2024

.DISCUSSION...

KEY MESSAGES:

-  Pacific moisture arrives tonight, bringing wet snow above
   4,500-5,000 feet along the ID/MT border and northwest MT

-  Active Pacific storm track beginning Monday night, bringing
   snow to mountain passes by Tuesday morning.

The lower valleys in north-central Idaho and western Montana are
currently trapped under inversions, with fog and low clouds
dominating the weather. Model soundings indicate that these
inversions will persist, though some afternoon improvement in fog
is expected before it redevelops in the evening. Above the
inversion layer, there`s a mix of sun and clouds along with mild
temperatures.

Looking to the west, satellite imagery shows increasing cloud
cover ahead of an approaching shortwave trough in the southwest
flow aloft. Snow levels are projected to remain steady around
4,500-5,000 feet, which should limit travel impacts on mountain
passes to minimal, with only up to an inch of slushy, wet snow
accumulation.

The next Pacific system is expected to move onshore by Monday,
spreading precipitation along the Idaho-Montana border by mid-
afternoon. Snow levels will rise to 5,000-6,000 feet with
southwesterly flow ahead of this system, dropping to around 4,000
feet overnight into Tuesday morning as a cold front moves through.
Winter driving conditions are likely in mountain passes by
Tuesday morning, with Lookout and Lolo Passes seeing a 70% chance
of 4 inches or more of snow, and Lost Trail and Marias Passes a
40% chance.

A broad trough will remain over the western U.S. from Wednesday
through Friday, bringing intermittent mountain snow with snow
levels fluctuating between 4,000 and 5,000 feet. None of these
days are expected to bring particularly high travel impacts to
mountain passes and higher elevations. However, ensemble models
suggest a trend toward a split-flow pattern during this period,
with more concentrated precipitation moving south into California
and the Great Basin. This pattern warrants close monitoring,
especially for those planning backcountry activities or travel
through the passes.

&&

.AVIATION...Stable conditions under high pressure has caused
valley fog to remain persistent across the Northern Rockies, with
the lowest visibility and ceiling occurring at KMSO and KGPI.
Some locations have broken out of the fog, such as KHRF, but with
conditions expected to remain stagnant, redevelopment of fog is
expected this evening and overnight. This afternoon, minimal
improvements to visibility and ceiling are possible between
09/2200Z and 10/0100Z for KGPI and KMSO, before lowering
thereafter. With incoming cloud cover and moisture from the
Pacific Coast, improvements will be more likely later in the
morning Sunday or early afternoon Sunday.


&&

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

$$