500
FXUS65 KMSO 100847
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
247 AM MDT Fri Oct 10 2025

.DISCUSSION...

KEY MESSAGES:

- Near record warmth today, with scattered showers and isolated
  thunderstorms by late afternoon and evening.

- Widespread mountain snow begins late Saturday night into Sunday
  morning, with heaviest amounts above 4,500-5,000 feet.

- Snow levels drop to valley floors along the divide and in
  northwest Montana Sunday night into Monday morning.

GOES Water Vapor imagery early this morning shows a closed low off
the OR/WA coast with southerly flow moving moisture northward
associated Tropical Storm Priscilla, located south of California.
The closed low will begin moving onshore this afternoon, with
southerly winds increasing areawide. The strongest gusts (20-35
mph) will be felt across southern Idaho and Lemhi Counties, and
along the divide. Mid-level moisture and instability will kick off
scattered showers and isolated thunderstorms in central Idaho by
early afternoon with activity tracking northeastward through
western Montana through the early evening. Storms will be capable
of producing isolated areas of outflow winds in excess of 30 mph.

The low will swing across the Northern Rockies Saturday afternoon
and evening, with precipitation initially focusing along and south
of the I-90 corridor as a surface low deepens east of the divide
in southwest Montana. Snow levels will fall near 5,500-6,000 feet
Saturday evening, with snow beginning to accumulate across the
mountain ranges in Lemhi County and across the Bitterroot,
Sapphire, and Anaconda-Pintlar Ranges in southwest Montana.

Winter weather impacts will become more widespread early Sunday
morning as snow levels fall further as a secondary trough of low
pressure drops southward from Canada. A cold front will bank along
the divide in northwest Montana Sunday morning, with snow levels
dropping from 4,500-5,000 feet to 2,000-2,500 feet in northwest
Montana, 3000-3,500 feet near and along the I-90 corridor and near
4,500-5,000 feet south of I-90 through Monday morning as the
front tracks across the divide. Global ensembles suggest the front
will stall across northwest Montana, with the coldest air
impacting the West Glacier Region, the Flathead Valley, and the
Kootenai/Cabinet Region. Forecast models continue to suggest an
overrunning scenario across northwest Montana, with deepening
southwest aloft sending Pacific moisture over cold air moving
along and west of the divide. This will be an abnormally cold
airmass, ranking in the 5th percentile of climatology for mid-
October, providing more confidence for winter weather impacts,
especially across the West Glacier Region, where there`s >80% for
6 inches or more of snow above 4,000 feet, including US Highway 2
over Marias Pass. Furthermore, the Mission, Swan, Cabinets,
Whitefish Mountain Ranges, and mountains of Glacier National Park
(including Logan Pass) have a >60% chance for 12 inches or more
above 5,000 feet.

The threat for valley snow will focus Sunday night into Monday
morning, primarily across northwest Montana. The highest
probabilities for 1 inch or more of snow for valleys, are
centered across the Flathead Valley, US-93 from Whitefish to
Eureka, and US-2 west of Kalispell, reaching 50-70%. Probabilities
drop <20% for Libby and areas south of Flathead Lake, where
confidence is lower for the interface between cold air and Pacific
moisture to occur.

Bottom line, those heading into the backcountry or traveling
across area mountain passes, should prepare for winter-like
conditions beginning Saturday night into Sunday morning. Winter
weather impacts will escalate Sunday night into Monday as colder
air arrives, setting the stage for roadways to freeze, especially
along the divide and in northwest Montana.

&&

.AVIATION...An east-southeast gradient this morning will become
southerly this afternoon as our next storm system moves onshore in
the Pacific Northwest. Mid-level moisture and instability will
increase, kicking off scattered showers and thunderstorms across
central Idaho this afternoon, with activity pushing northeastward
through western Montana this evening. Storms will be capable of
producing isolated areas of outflow winds in excess of 30kts.
South winds will increase south of the I-90 corridor and along the
divide this afternoon, with gusts of 15-25kts, including KSMN,
KBTM, and KHRF.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from Saturday evening through Monday morning
     for Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region...Kootenai/Cabinet Region...Lower Clark Fork
     Region...Potomac/Seeley Lake Region...West Glacier Region.

ID...Winter Storm Watch from Saturday afternoon through Monday
     morning for Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Storm Watch from Saturday evening through late Sunday
     night for Eastern Lemhi County...Western Lemhi County.

&&

$$