471
FXUS65 KMSO 051004
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
304 AM MST Fri Dec 5 2025

.DISCUSSION...

KEY MESSAGES:

-  A prolonged period of winter weather lasting through at least
   the weekend

-  Heavy mountain snow, along with a wintry mix in the valleys
   will make for difficult travel conditions

-  Gusty westerly winds on Saturday

-  Unsettled winter weather will continue into next week

This morning will start with fog and low stratus, this is
important because over western Montana this layer could be 4 to
5000 feet thick, limiting solar heating this afternoon. Next, the
models suggest 700 mb winds are the strongest right now until
after sunset this evening, meaning what little valley downsloping
that is ongoing will not increase until this evening. Why is all
of this important? The models are outputting a precipitation type
of rain today for valleys, so this shift has hedged the forecast
to snow.

The reason for the hedge to snow is early afternoon the
atmosphere becomes saturated and unstable. The models are hinting
at convective showers with potential banding. The lowest levels
are isothermal, near freezing, through the first 2000 feet and
cold air advection aloft is expected after noon, for western
Montana. North-central ID is a different story as minor warm air
advection is expected through early evening.

Between 2 to 4 pm, expect the atmosphere to destabilize and
moderate showers develop. The models are depicting band like
showers developing in the vicinity of I-90 around 3 pm. These
bands will be capable of 0.50 to 1" of snow per hour, rapidly
reducing visibility and creating changeable road conditions. This
shift will extend the current winter weather advisory for the
Missoula and Bitterroot Valleys due to the uncertainty of exactly
where or if the bands develop. Short term snow squall warnings
during the afternoon will be a better tool to demonstrate where
the evening commute could be severely impacted by winter
conditions. Another component to the weather today will be breezy
conditions, generally 20 mph or less. However, in these heavier
showers or bands winds could gust as high as 40 mph further
reducing visibilities.

Westerly winds increase after sunset and now valley downsloping
will increase. At some point this evening precipitation intensity
will no longer be able to overcome low level warming and snow
transitions to rain. Snow levels generally do not climb higher
than 4500 to 5000 feet and above these levels snow will be
measured between 1 to 2 feet. Expect moderate to high impacts to
travel over area mountain passes through Saturday afternoon. Wind
gusts 30 to 40 mph overnight tonight will create further
reductions in visibility in the mountains, along with blowing
snow.

Precipitation will decrease but not stop through Sunday when the
next slug of moisture impacts the region. Snow levels which lower
below 4000 feet late Saturday, quickly rise above 4500 feet Sunday
evening. An additional 3 to 6 inches of snow is possible at
Lookout and Marias Passes, while other regional passes will see
lesser amounts. This will be a high density snow on top of
previous lower density, avalanche conditions will need to be
monitored in areas such as US-12 west of Lolo, MT and tight
canyons of northwest MT, west of the Flathead Valley.

The next system arrives Tuesday and snow levels quickly rise to
between 5500 and 7000 feet. This is a change from earlier model
runs that had a range of 3500 to 7000, meaning the models are
coming into better agreement. Another interesting change is how
quickly cooler air works into the system. Models now indicate
while moisture remains available, snow levels could crash below
4000 feet overnight Tuesday, currently 50% of the models represent
this scenario. More to come on the system as uncertainty on snow
levels late in the system are resolved. The active weather period
does not end with this system, stay tuned.





&&

.AVIATION...A lull in precipitation is expected through midday
Friday, however IFR conditions continue with fog and low stratus
at most terminals. Another surge of moisture pushes into the
region after 05/1900z. Ridgetop winds will increase out of the
west and southwest through the day on Friday and into Saturday.
Surface wind gusts are expected up to 20-35 knots. Snow will
transition to rain overnight into Saturday. Snow levels may be
temporarily driven back down to valley floors during this time
under any heavy convective bands that set up.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning until 5 PM MST Saturday for
     Bitterroot/Sapphire Mountains...Lower Clark Fork Region...
     Potomac/Seeley Lake Region.

     Winter Storm Watch from this afternoon through Sunday morning
     for West Glacier Region.

     Winter Weather Advisory until 11 PM MST this evening for
     Butte/Blackfoot Region...Missoula/Bitterroot Valleys.

ID...Winter Storm Warning until 4 PM PST Saturday for Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     Winter Weather Advisory until 11 AM MST this morning for Eastern
     Lemhi County...Western Lemhi County.

&&

$$