689
FXUS65 KMSO 091023
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
323 AM MST Tue Dec 9 2025

.DISCUSSION...

KEY MESSAGES:

- Significant Atmospheric River, potentially historic, to bring
  heavy rain, leading to hydrological concerns and flood risk.

- Gusty winds this morning across northwest Montana, with risk
  for power outages from downed trees.

- Brief burst of accumulating snow late tonight into Wednesday
  morning for northwest Montana valleys near the Continental
  Divide

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

The first plume of anomalous moisture from an ongoing atmospheric
river event is currently peaking over western Montana and north
central Idaho. Rainfall rates of 1.50 to 2.00 inches in 6 hours
have been noted in the higher terrain of north central Idaho
overnight, with notable rises in small streams and creeks. This
first moisture plume will sag southwards through the morning as a
cold front passes through the region. While this introduces a
brief lull in the heaviest precipitation intensities for northwest
Montana, widespread precipitation will continue further south,
especially through Idaho County. Gusty southwest to west winds
will develop this morning as the cold front passes, with
widespread gusts to 40 mph still anticipated, mainly for valleys
of western Montana, with higher speeds in exposed terrain. Snow
levels will briefly lower across the region to roughly 3000 feet
in northwest Montana and 5000 feet in north central Idaho.

The next surge of anomalous moisture will arrive into the region
late tonight and continue into Thursday. This second surge looks
similarly strong to the ongoing wave, however it appears to be
more prolonged, peaking for a 24 hr period (or possibly longer).
Before warm air scours out the cold air Wednesday afternoon, snow
levels will dip low enough (3000-4000 feet) to produce winter
impacts late tonight into Wednesday morning. High-resolution
guidance indicates a quick burst of moderate to heavy snow is
possible for northwest Montana, including Seeley Lake and Marias
Pass. Snowfall rates of 0.5" to 1" per hour may create slick
conditions and reduced visibility for the Wednesday morning
commute. Even the Flathead Valley could see a brief transition to
wet snow.

Looking ahead, confidence is increasing that anomalous moisture
will persist through the remainder of the week (Thursday-
Saturday). A complicating factor is an arctic airmass that is
forecast to bank up along the Northern Continental Divide. While
strong westerly winds aloft will likely keep the arctic front from
crossing the divide, this setup introduces a classic overrunning
scenario, potentially creating winter conditions (snow and ice)
that extend into the valleys of northwest Montana by Thursday
morning. The interaction between this dense, shallow cold air and
the overriding subtropical moisture will be a critical focal point
for the extended forecast.


&&

.AVIATION...Widespread mountain obscuration will persist through
the forecast period as deep moisture continues to stream into the
Northern Rockies. While the heaviest precipitation from the
initial atmospheric river plume is shifting south into Idaho
County this morning, widespread low ceilings (MVFR) remain across
the terminals.

A cold front sweeping through this morning will turn surface winds
to the west-southwest. Expect widespread gusts of 25 to 35 kts at
valley terminals (KMSO/KGPI), with gusts exceeding 40-50 kts over
exposed terrain. This interaction with the terrain will generate
moderate to severe mechanical turbulence and areas of Low Level
Wind Shear (LLWS).

A brief lull in precip intensity is possible this afternoon, but
ceilings will struggle to lift significantly. Conditions
deteriorate rapidly again late tonight (after 06Z Wednesday) as
the second moisture surge arrives. Snow levels will briefly crash
to between 3000-4000 feet late tonight. This will introduce a
period of IFR conditions due to heavy, wet snow, particularly for
KGPI and locations along the Northern Continental Divide. Snowfall
rates of 1 inch/hour are possible in these bands before warm air
scours out the cold air Wednesday afternoon.

&&

.HYDROLOGY...Recent rainfall has already primed regional soils and
triggered rises on waterways, and the incoming second surge will
further exacerbate these conditions. This next wave is forecast to
deliver an additional 2 to 5 inches of liquid precipitation to
the terrain of north-central Idaho and northwest/west-central
Montana, with valleys receiving between 0.50 and 2.00 inches.
Consequently, cumulative storm totals by Thursday morning are
projected to reach localized amounts of 5 to 9 inches of liquid.

To place this in context: if these forecast 72-hour totals verify,
this would represent a historic rainfall event with an Average
Recurrence Interval (ARI) of 50 to 100 years (a 1% to 2% chance of
occurring in any given year).

The primary concern, however, extends beyond pure rainfall intensity.
This system is driven by a warm, subtropical airmass that will
drive freezing levels well above 6,000 to 8,000 feet by Wednesday.
This will induce a significant rain-on-snow event, rapidly
ripening and melting the existing low-to-mid elevation snowpack.
The combination of heavy, prolonged rainfall and snowmelt
contribution is expected to overwhelm small creeks and streams
first, followed by significant rises on mainstem rivers.

Given this setup, numerous Flood Watches remain in effect. Residents
in steep terrain should also remain alert for rock and mudslides,
particularly in north-central Idaho and northwest Montana where
the ground is deeply saturated.

&&

.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.

     Wind Advisory until 8 AM MST this morning for Flathead/Mission
     Valleys...Kootenai/Cabinet Region...Lower Clark Fork Region.

     Wind Advisory until 11 AM MST this morning for West Glacier
     Region.

     Winter Weather Advisory from midnight tonight to 11 AM MST
     Wednesday for Potomac/Seeley Lake Region...West Glacier
     Region.

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

&&

$$