430
FXUS65 KMSO 100952
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
252 AM MST Wed Dec 10 2025

.DISCUSSION...

KEY MESSAGES:

- The second installment of a Significant Atmospheric River, will
  bring heavy rain back to the region through Friday, leading to
  hydrological concerns and flood risk.

- Strong winds return today, most likely from I-90 southwards;
  strong mountain wind continues through the period.

- Shallow Arctic air intrusion late Thursday into Friday may
  bring mixed wintry precipitation (snow/freezing rain) near
  Marias Pass and Essex, primarily impacting areas east of the
  Divide.

- Heavy, wet snow across backcountry and high elevations areas
  above 5000-8000 feet this week.

The axis of the incoming atmospheric river has shifted slightly
northward in recent model cycles. Consequently, forecasted
precipitation totals for this second weather system have been
trended downward for the southern tier of the forecast area
through Friday, particularly across southern Idaho County. The
focus of the strongest moisture transport and heaviest
precipitation is now expected to target Northwest Montana and the
Idaho Panhandle from Clearwater County northward. While this
second surge is still expected to be prolonged, peaking for a
24-hour period, snow levels will still fluctuate. Snow levels this
morning have dropped to between 3000-4000 feet across northwest
Montana. Periods of moderate snowfall are occurring in locations
such as Libby, Kalispell and Seeley Lake, potentially producing
slick and slushy conditions and reduced visibility (0.5" to 1" per
hour rates) for the morning commute. Snow will be relatively
short lived as warmer air surges into the region through the
morning hours. Precipitation totals today through Friday morning
in northwest Montana and Clearwater County will range from 1.50"
to 4.00" in the higher terrain and 0.75" to 2.00" for valleys.
Totals drop off significantly to the south and east, with
southwest Montana seeing the lowest amounts.

Wind Advisories have been issued for the Missoula and Bitterroot
valleys, as well as the Butte and Blackfoot regions for today.
Deterministic and ensemble guidance indicates high probabilities
for gusts in the 40 to 50 mph range in these areas. The mechanism
for these winds appears to be enhanced mixing south of the I-90
corridor, where stability profiles are less inhibited compared to
the northern valleys. In these well-mixed areas, temperatures
could peak in the 50s, up to 20 degrees above climatological
norms! In the higher terrain, strong gusty winds will persist into
Thursday.

Confidence is increasing that the approaching shallow arctic
air mass will effectively dam up along the Continental Divide
rather than spilling westward on Thursday into Friday. This trend
significantly reduces the probability of significant snowfall or
freezing precipitation for the majority of western Montana west of
the Divide. The primary exception remains the Marias Pass to
Essex corridor, where the cold pool may deepen sufficiently
Thursday into Friday to interact with overriding Pacific moisture.
Outside of this specific terrain feature, the threat for
widespread winter impacts from this arctic air intrusion has
diminished.

Overall, forecast models are trending drier for the weekend, but
the break in precipitation looks relatively short. Given the deep
saturation from antecedent rainfall, the setup favors the
development of low stratus and fog, which may result in persistent
low ceilings and reduced visibility even in the absence of
measurable precipitation. Temperatures are forecast to remain
mild, running 5 to 10 degrees above average. The large-scale
pattern over the eastern Pacific remains quasi-stationary into
next week, keeping the door open for subsequent moisture flux.

&&

.AVIATION...
For KGPI: The terminal is currently impacted by a surge of
moisture and lower snow levels (3000-4000 ft). Expect periods of
IFR conditions this morning with moderate snowfall rates reducing
visibility to less than 2 miles at times. A transition to rain is
expected by mid morning (14Z-16Z) as warmer air surges northward.
While visibility should improve to MVFR/VFR with the switch to
rain, ceilings will likely remain low (MVFR) through Wednesday
night due to continuous precipitation and terrain obscuration.

For KMSO / KBTM / KSMN/ KHRF: The primary concern for terminals
south of I-90 is strong surface winds. As mixing increases later
this morning, expect frequent gusts in the 30 to 40 kt range,
particularly at KHRF. Turbulence will be a significant factor in
the valleys and over area ridges. Precipitation mixed with snow
will briefly cause some lowered ceilings and visibility this
morning, but should improve quickly between 12-14z as warmer air
arrives. Precipitation will be lighter in these areas compared to
the north, allowing for VFR ceilings much of the day, though
mountain obscuration remains likely.

General: Widespread mountain obscuration is expected across
the Northern Rockies. Winds will begin to decrease in the valley
bottoms after 03Z Thursday, but strong winds aloft will persist,
introducing the threat of Low Level Wind Shear (LLWS) for all
terminals Wednesday night.

&&

.HYDROLOGY...
Antecedent rainfall has primed regional soils, resulting in
notable responses on area waterways earlier this period.
Specifically, Orofino Creek and the Selway River saw rapid rises;
however, hydrographs indicate these waterways have crested across
Idaho and Clearwater counties following a recent lull in
precipitation intensity.

The axis of the atmospheric river has shifted slightly further
north than indicated by previous model cycles. Consequently, the
focus of the strongest moisture transport, and the "brunt" of the
precipitation, is now targeted from Clearwater County northward
through the Idaho Panhandle and Northwest Montana. While rain-on-
snow processes remain a critical driver due to freezing levels
rising to 6,000-8,000 feet, projected precipitation totals have
been trended downward for the southern tier of the region,
particularly southern Idaho County. Conversely, the northern
target areas are still poised to receive an additional 2 to 5
inches of liquid precipitation, maintaining a high risk for runoff
issues in those specific basins.

Flood Watches remain in effect for the time being. However, given
the northward trend in the moisture plume and the stabilizing
river levels in the south, the risk profile for Idaho County is
decreasing. Future shifts will monitor if the precipitation
deficit in the south warrants an early cancellation of the Flood
Watch for Idaho County. Residents in steep terrain, particularly
in the updated target areas of Northwest Montana and northern
Idaho, should remain alert for rock and mudslides due to deep soil
saturation.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch through Thursday afternoon for Flathead/Mission
     Valleys...Kootenai/Cabinet Region...Lower Clark Fork
     Region...Potomac/Seeley Lake Region...West Glacier Region.

     Winter Weather Advisory until 11 AM MST this morning for
     Potomac/Seeley Lake Region...West Glacier Region.

     Wind Advisory from 8 AM this morning to 7 PM MST this evening
     for Butte/Blackfoot Region...Missoula/Bitterroot Valleys.

ID...Flood Watch through Thursday afternoon for Lower Hells
     Canyon/Salmon River Region...Northern Clearwater
     Mountains...Orofino/Grangeville Region...Southern
     Clearwater Mountains.

&&

$$