089
FXUS65 KMSO 061012
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
312 AM MST Sat Dec 6 2025

.DISCUSSION...

KEY MESSAGES:

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

- Significant rain and high elevation snow Monday through
  Wednesday may lead to hydrologic issues

Current radar analysis depicts persistent shower activity with
embedded banding features continuing across western Montana and
north central Idaho this morning. A distinct southerly flow
component has established overnight, facilitating weak warm
advection, particularly in lower elevations. Consequently, snow
levels are hovering near 4000 feet. While most valleys are seeing
rain or a rain/snow mix, orographic enhancement is driving
moderate to heavy snowfall rates in the higher terrain and across
mountain passes.

High-resolution guidance indicates a resurgence of banded
precipitation this afternoon and evening. Forecast soundings
suggest that heavy precipitation rates within these stronger bands
may facilitate dynamic cooling, potentially lowering snow levels
to valley floors locally. Winter Storm Warnings remain in effect
for the higher terrain of western Montana and north central Idaho
through this afternoon. Travel in the backcountry and over
mountain passes will remain hazardous due to the combination of
high snowfall rates and gusty westerly winds. By Sunday afternoon
into Monday, a weak disturbance embedded in northwest flow will
traverse the Northern Rockies, reinforcing shower activity and
maintaining accumulating snow over mountain passes.

Confidence is high regarding a significant pattern shift beginning
late Monday. Ensembles and deterministic models have aligned on a
robust Atmospheric River (AR) event impacting the Northern
Rockies through at least Wednesday. This system is characterized
by anomalous moisture transport for this time of year.

Atmospheric profiles indicate rising snow levels with the moisture
plume, reaching approximately 5000 feet across northwest Montana
and up to 7000 feet in the southern areas (Lemhi County/southwest
Montana) by Tuesday morning. While a brief lowering of snow levels
(approx. 1000 feet) is anticipated Wednesday morning, heights
rebound by Thursday.

The National Blend of Models (NBM) 72-hour QPF (Monday AM-Thursday AM)
signals significant precipitation accumulation. Probability
guidance highlights a 90 percent chance of at least 2 inches of
liquid equivalent, and a 10 percent chance of up to 5 inches of
liquid equivalent, with the axis of heaviest precipitation focused
over Idaho and Clearwater counties into northwest Montana. Valley
locations in Lemhi County and southwest Montana are progged for
0.25 to 1.50 inches. Statistical analysis suggests an Average
Return Interval (ARI) for these high end liquid amounts of
approximately 20 years.

Hydrologic impacts are the primary concern for this period.
The coincidence of heavy rainfall on an existing snowpack and
elevated freezing levels will lead to rapid runoff in low
elevations, ponding of water in low-lying areas, and significant
rises on area waterways. Furthermore, the saturation of soils
combined with runoff increases the risk of rock and mudslides,
particularly in steep terrain. Areas remaining above snow level
(generally >6000 feet) will see substantial snow accumulations,
with 1 to 3 feet of new snow likely.

&&

.AVIATION...Widespread terrain obscuration will persist across the
Northern Rockies through the forecast period. Terminals (KMSO,
KGPI, KBTM, KSMN) can expect periods of MVFR/IFR conditions
associated with passing showers. A resurgence of precipitation
intensity is expected this afternoon/evening, which may briefly
lower ceilings and visibility further. LLWS and mechanical
turbulence are likely near terrain features due to the active
southerly to westerly flow aloft, approaching 40 kts at ridge top
level. Gusty westerly winds will increase into Saturday afternoon
for valley locations up to 20 kts, helping mix out some of the
lowest stagnant layers.

&&

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

     Winter Weather Advisory until 5 PM MST this afternoon for West
     Glacier Region.

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

&&

$$