Flash Flood Watch 3, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
212 PM MST Sun Dec 14 2025

.DISCUSSION...

KEY MESSAGES:

 - Renewed Flooding Threat: Another Atmospheric River returns
   Monday through Wednesday. Rain falling on saturated soils will
   likely cause rapid rises on streams and creeks, with
   rockslides possible in steep terrain.

 - Strong Cold Front Wednesday: A sharp transition from rain to
   snow is expected Wednesday. This brings a threat of damaging
   winds and busts of snow or snow squalls.

FORECAST SUMMARY:

Another Atmospheric River is set to impact the Northern Rockies
starting Monday morning. This system is again warmer and wetter
than typical December storms, driving snow levels above 7000 feet.
Moderate to heavy rain will stress an already saturated
hydrologic system. By Wednesday, a powerful cold front will sweep
through, crashing snow levels to valley floors and introducing a
period of strong, potentially damaging winds.

Tonight & Monday: Expect patchy fog and low stratus to redevelop,
especially in northwest Montana. Freezing fog could create slick
spots on bridges and overpasses early Monday.

 - Precipitation: Light precipitation begins in northwest Montana
   Monday morning, spreading south and intensifying by Monday
   night.

 - Snow Levels: Warm air advection will push snow levels above
   7000 feet by Monday afternoon. This means widespread rain for
   the valleys and lower mountain slopes.

 - Winds: A strong low-level jet develops Monday night. Expect
   gusty southwest winds (30-40 mph) over the terrain and up to 50
   mph in the Camas Prairie region.

Wednesday through Sunday:

Wednesday (The Cold Front): This is the most dynamic period of
the forecast. A sharp cold front will cross the Divide during the
day.

 - Impacts: Temperatures will plunge, causing rain to change
   rapidly to snow. Winds will shift to the west-northwest and
   could gust 45-60 mph in the valleys and even higher in the
   mountains, posing a risk of downed trees in saturated soil.
   Snow accumulations over mountain passes and bursts of snow or
   snow squalls will cause periods of travel impacts.

 - Thursday - Friday: Global models are split on the evolution of
   a secondary wave. Currently, the WPC favors a solution that
   keeps impactful moisture in the region, so we are maintaining
   high chances for precipitation, though likely as snow rather
   than rain.

 - The Weekend: A trend toward cooler, more seasonable
   temperatures is expected as the main moisture feed finally cuts
   off.

&&

.AVIATION...The primary aviation impacts through 15/18Z is the
development of patchy fog and low stratus overnight. Visibility
may drop below 1/2SM at times tonight through Monday morning,
mainly north of Highway 12 in Idaho and western Montana.
Conditions should improve to MVFR by Monday afternoon as high
clouds increase. South of Highway 12, VFR conditions will prevail
initially, but expect lowering ceilings Monday evening.

Trend: Widespread mountain obscuration will develop after 15/12Z
as the next system approaches. Mechanical turbulence and Low-Level
Wind Shear (LLWS) will become significant hazards Monday night
into Tuesday as a 50kt jet streak moves overhead.

&&

.HYDROLOGY...The hydrologic response to the incoming Atmospheric
River is the primary concern for this forecast package.

 - Soil Conditions: Soils across Lincoln, Mineral, Sanders and
   Flathead Counties are already near saturation from last week's
   precipitation. This means the ground has very little capacity
   to absorb new rainfall.

 - Rain Amounts: With freezing levels rising to above 7000 feet
   Monday and Tuesday, we expect efficient rainfall of around 1.5
   to 4.0 inches in the mountains.

Specific Impacts:

 - Small Streams: Expect rises on smaller creeks and streams,
   especially in northwest Montana beginning Monday night.

 - Debris Flows: The risk for rock and mudslides is elevated,
   particularly across northwest Montana.

&&

.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.

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

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.

&&