482
FXUS65 KMSO 072051
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
151 PM MST Sun Dec 7 2025

.DISCUSSION...

KEY MESSAGES:

- Burst of moderate to heavy snow this afternoon and evening
  across Lookout, Lolo, Marias and Lost Trail passes.

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

- Gusty winds Monday night into Tuesday morning across northwest
  Montana, with risk for power outages from downed trees.

- Heavy, wet snow across backcountry and high elevations areas
  above 5000-7000 feet this week.

While the main focus is the incoming atmospheric river Monday, we
are tracking a quick hitting Pacific moisture surge this
afternoon and evening. Mild temperatures are keeping snow levels
above 4,500 feet, with winter travel concerns being focused across
mountain passes, including: Lookout, Marias, Lost Trail, and Lolo
Passes. Travelers should expect periods of moderate to heavy snow
and slushy roadways. High-res models suggest convective snow
showers will develop later this evening as the wave passes,
therefore, the worst of the conditions should develop following
sunset.

Atmospheric River & Hydrologic Concerns for Monday-Thursday:

Meteorological Details: The weather pattern shifts dramatically
as a deep fetch of subtropical moisture, known as an Atmospheric
River, surges into the Northern Rockies beginning Monday
afternoon. This will be a long-duration event, with periods of
heavy precipitation continuing into Thursday. Snow levels will
rise Monday afternoon, with rain falling as high as 7000 feet
Monday night. This presents a concerning rain on snow situation.

A Pacific cold front will pass Monday night into Tuesday morning,
leading to sharp pressure rises and gusty west-southwest winds,
especially across northwest Montana, where wind advisories have
been issued. Wet ground conditions will lead to greater
vulnerabilities for downed trees and areas of power outages. A
second surge of moisture arrives Tuesday night into Thursday, with
heavy precipitation focusing along north-central ID and the
Montana/Idaho border.

The two main forecast challenges are 1) Duration of moisture
surge and 2) Snow levels, during this second phase of the
atmospheric river event. Model guidance suggests a sharp
temperature gradient aloft across the Northern Rockies as this
second phase occurs, leading to significant spread within snow
levels. Generally, snow levels will be lower during this event,
ranging from 4,000-5,000 feet across northwest Montana,
4,500-5,500 feet along I-90, and 5,500-7,000 feet across the
southern Clearwater, Bitterroot Mountains, Southwest Montana and
Lemhi County. It`s worth mentioning, the 10-90th percentile spread
between the NBM is on the order of 2000 feet for snow levels,
hence there`s still a lot of uncertainty within the location of
colder air aloft and where snow levels will end up during this
second surge. Finally, roughly 1/3 of ensemble models suggest the
moisture surge will continue Thursday night into Friday morning.
This is a recent trend within guidance, particularly the ECMWF and
it`s ensemble system. This solution would lead to higher
precipitation totals and greater mountain snow as snow levels
trend lower towards the end of the event.

*Hydrological Summary: Flood watches have been issued across
 Idaho and Clearwater Counties in Idaho and across Mineral County,
 in Montana, where our confidence is highest for the most
 significant precipitation totals through Thursday. Rises in small
 streams and creeks, rock and mudslides in areas of steep
 terrain, ponding of water and/or minor flooding in low lying and
 urban areas. By Thursday, significant precipitation totals are
 forecast, with 1 to 3 inches in the valleys and 4 to 8 inches in
 the mountains, with locally higher totals over mountain crests.
 Mountain areas in the Clearwater and Bitteroot Ranges will see
 the heaviest totals during this event.

*High elevation Snow: Significant wet, heavy snow will impact
 mountain areas, especially above 5,000-7,000 feet. The
 combination of high precipitation totals, moisture content, and
 strong winds will lead to changeable avalanche conditions. Those
 heading into the backcountry are encouraged to monitor avalanche
 forecasts at: www.avalanche.org.

&&

.AVIATION...The next surge of Pacific moisture is moving through
eastern Washington and the Idaho Panhandle this afternoon as of
07/2100Z. Widespread precipitation will impact the Northern
Rockies as this wave moves through, leading to mountain
obscurations, with snow levels generally above 4,500 feet. Area
terminals will experience temporary visibility and ceiling impacts
as the wave passes, generally from the 08/0000-0400Z timeframe. A
significant, long-duration atmospheric river event arrives Monday
afternoon, with widespread precipitation.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch from Monday afternoon through late Wednesday night
     for Lower Clark Fork Region.

     Wind Advisory from 8 PM Monday to 8 AM MST Tuesday for
     Kootenai/Cabinet Region...Lower Clark Fork Region.

     Wind Advisory from 11 PM Monday to 11 AM MST Tuesday for West
     Glacier Region.

     Wind Advisory from 11 PM Monday to 8 AM MST Tuesday for
     Flathead/Mission Valleys.

ID...Flood Watch from Monday afternoon through late Wednesday night
     for Lower Hells Canyon/Salmon River Region...Northern
     Clearwater Mountains...Orofino/Grangeville Region...
     Southern Clearwater Mountains.

&&

$$