557
FXUS65 KMSO 152115
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
215 PM MST Mon Dec 15 2025

.DISCUSSION...

KEY MESSAGES:

 - Atmospheric River Impacts: An atmospheric river will bring
   widespread rain and high-elevation snow through Tuesday.
   Saturated soils will likely cause rises on streams and
   rockslides in steep terrain, particularly in northwest Montana
   and north-central Idaho.

 - Strong Cold Front Wednesday: A sharp cold front will sweep
   across the region on Wednesday. Expect a rapid transition from
   rain to snow, and damaging winds with gusts of 50-70 mph in
   valleys (higher in mountains).

 - Winter Conditions Return: Following the front, cooler and
   unsettled weather will prevail for the remainder of the week,
   with snow levels dropping to valley floors at times and periods
   of mountain snow affecting travel.

The primary weather story for the next 48 hours is a classic
"one-two punch" involving a moisture-laden atmospheric river
followed immediately by a sharp cold front.

A strong southwesterly flow is currently directing a plume of
subtropical moisture into the Northern Rockies. With 700mb
temperatures rising, snow levels have ascended above 7000 feet,
meaning precipitation will fall primarily as rain. The Weather
Prediction Center (WPC) highlights a continued influx of moisture,
with high-resolution ensembles suggesting 24-hour rainfall totals
of 1.5 to 3.0 inches in the higher terrain of the Cabinets and
Clearwaters by Tuesday morning. This rainfall on top of existing
snowpack and saturated soils has elevated the risk for rockslides,
mudslides, and rises on creeks and small streams.

Winds tonight: A powerful upper-level jet streak is moving
overhead. While an inversion may protect some protected valleys
from the strongest winds initially, expect gusty southwesterly
winds to mix down in favorable corridors. Wind advisories are in
effect for west-central and northwest Montana for tonight into
Tuesday morning for gusts 40-50 mph.

The Wednesday Front: Model agreement is high regarding a potent
cold front passage on Wednesday. This feature will be dynamic and
impactful.

Temperature Crash: As the front passes (likely during the
morning/mid-day hours), temperatures will plummet with a rapid
change to snow.

Damaging Winds: The pressure gradient along the front is tight.
Widespread wind gusts of 50 to 70 mph are probable in valleys,
with gusts exceeding 80 mph on ridgetops. This creates a risk of
downed trees (made easier by sodden soil), power outages, and
damage to infrastructure.

Snow Squalls: The transition from rain to snow will be abrupt.
Bursts of heavy snow or snow squalls could lead to near-zero
visibility and difficult travel conditions immediately following
the frontal passage. Higher valleys, mountain passes, and
locations along the Divide have the highest risk for these heavy
bursts of snow and snow squalls.

Thursday through Sunday: The upper-level pattern will remain
active but shift to a cooler regime behind Wednesday`s front.

The jet stream will remain positioned over the Pacific Northwest,
maintaining a feed of moisture into the region, though the
airmass will be colder. Troughing over the region will keep
temperatures closer to normal for the end of the week.

With colder air in place, snow levels will hover near valley
floors on Thursday, creeping slightly upward on Friday. This
period will be characterized by frequent mountain snow showers and
valley rain/snow mix. While the intensity will not match the
early-week atmospheric river, the accumulation of snow on mountain
passes (Lookout, Lolo, Marias) will likely necessitate winter
travel precautions. A continued unsettled weather pattern with
light to moderate precipitation accumulation is expected through
the weekend.


&&

.AVIATION...Another atmospheric river will impact the Northern
Rockies airspace. This instance will be focused on winds as
opposed to precipitation. With some valleys inverted and stable,
initially there could be strong low-level wind shear as mid level
winds increase 40 to 50 kts. All terminals are expected to mix out
and gusts 30 to 40 kts at most terminals are expected.
Precipitation will remain all rain, with only minor reductions to
visibility forecasted. Winds will remain elevated Tuesday with
gusts 15 to 25 kts common.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...  High Wind Watch from late Tuesday night through Wednesday
  afternoon for Bitterroot/Sapphire Mountains... Butte/Blackfoot
     Region...Flathead/Mission Valleys... Kootenai/Cabinet
     Region...Lower Clark Fork Region... Missoula/Bitterroot
     Valleys...Potomac/Seeley Lake Region.

     Wind Advisory from 5 PM this afternoon to 3 AM MST Tuesday for
     Kootenai/Cabinet Region...Lower Clark Fork Region.

     Wind Advisory from 7 PM this evening to 5 AM MST Tuesday for
     Flathead/Mission Valleys...Potomac/Seeley Lake Region...
     West Glacier Region.

     Winter Storm Watch from Tuesday evening through Wednesday
     evening for West Glacier Region.

     Wind Advisory from 11 PM this evening to 8 AM MST Tuesday for
     Missoula/Bitterroot Valleys.

ID...High Wind Watch from Tuesday evening through Wednesday afternoon
     for Lower Hells Canyon/Salmon River Region...Northern
     Clearwater Mountains...Orofino/Grangeville Region...
     Southern Clearwater Mountains.

     High Wind Watch from late Tuesday night through Wednesday
     afternoon for Eastern Lemhi County...Western Lemhi County.

&&

$$