textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Atmospheric River: Widespread precipitation, high elevation snow, and gusty winds through Wednesday.

- Mountain Snow: Snow accumulations will primarily impact high elevations above 6,000 feet, followed by minor travel impacts on mountain passes, mainly Wednesday night.

- Wind and Fire Risk: Strong winds arrive Wednesday. Combined with low humidity, confidence is growing for critical fire weather conditions east of the divide in Southwest Montana.

- Mild Weekend: A warming trend begins Friday, bringing above- normal temperatures by Saturday.

Atmospheric River (Today-Tonight): A brief Atmospheric River arrives this afternoon, bringing 0.5" to 1.5" of liquid precipitation to the Idaho/Montana border and northwest Montana. Snow levels will rise sharply to near 7,000 feet tonight. While mainstem rivers should remain below action stage, creeks in Lincoln, Sanders, and Mineral counties will see further rises but overall the concern for flooding is minor. Rock and mudslides remain a possibility along terrain-adjacent roadways.

Cold Front & Snow Levels (Wednesday-Thursday): A potent cold front crosses the region Wednesday morning.

- Northwest Montana: Snow levels will crash to 3,500 feet by Wednesday afternoon. While Lookout and Marias Passes may see several inches of accumulation Wednesday night into Thursday morning, travel impacts appear minor.

- Southwest Montana/Lemhi County: Levels will rise to 7,000 feet ahead of the front before falling region-wide Wednesday night.

Wind Hazards (Tonight-Wednesday): A 40-60 kt low-level jet will produce widespread wind concerns.

- Peak Gusts: 50-70% chance of gusts > 40 mph in Lemhi County and Southwest Montana; 25-50% chance of 30-40 mph gusts for most western Montana valleys (though higher probabilities in NW Montana).

- High Terrain: Gusts up to 60 mph along the Continental Divide.

- Impacts: Expect choppy lake conditions, the potential for localized tree damage and power outages.

Critical Fire Weather (Wednesday): A Red Flag Warning is in effect for the East Beaverhead (Southwest Montana) Wednesday afternoon. The combination of peak winds and relative humidity dropping into the teens will create critical fire conditions ahead of the frontal passage.

Extended Outlook: Temperatures drop below seasonal normals Thursday following the front. A drying and warming trend begins Friday as a ridge builds, with temperatures returning to above- normal levels by Saturday. Showers return as early as Sunday afternoon. Ensemble clusters are in good agreement (80-90%) on upper level troughing (widespread cooler/unsettled weather) returning to the Northern Rockies by the second half of next week.

AVIATION

A fast-moving Atmospheric River arrives this afternoon, bringing widespread rain and terrain-obscuring clouds to north- central Idaho and northwest Montana. Ceilings and visibility reductions will become more common by mid afternoon.

Ceilings/Visibility: VFR conditions will deteriorate to areas of MVFR this afternoon (approx. 21Z-00Z) across north-central Idaho and northwest Montana as widespread precipitation begins. Expect persistent mountain obscuration along the Montana/Idaho border and the Continental Divide through the period.

Wind/Turbulence: Low-level wind shear and mechanical turbulence will become a hazard this evening through Wednesday. Surface gusts of 25-40 kt are expected in western Montana valleys by late Wednesday morning as a cold front passes. By Wednesday afternoon, gusts will exceed 40 kt over ridges and through wind- prone canyons in southwest Montana and Lemhi County (near KBTM and KSMN).

MSO WATCHES/WARNINGS/ADVISORIES

MT...Wind Advisory from 11 AM to 7 PM MDT Wednesday for Butte/Blackfoot Region.

Wind Advisory from 6 AM to 7 PM MDT Wednesday for Kootenai/Cabinet Region.

Wind Advisory from 8 AM to 7 PM MDT Wednesday for Flathead/Mission Valleys...West Glacier Region.

Red Flag Warning from noon to 9 PM MDT Wednesday for East Beaverhead.

ID...Wind Advisory from 11 AM to 7 PM MDT Wednesday for Eastern Lemhi County...Western Lemhi County.