592
FXUS65 KMSO 131933
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
1233 PM MST SAT DEC 13 2025
DISCUSSION
KEY MESSAGES:
- AREAS OF FOG AND LOW CLOUDS IN VALLEYS EACH MORNING THROUGH
MONDAY
- RENEWED HYDRO + WIND CONCERNS TUESDAY-WEDNESDAY
- PASS LEVEL TRAVEL IMPACTS WEDNESDAY AS SNOW LEVELS TO VALLEY
FLOORS
THROUGH SUNDAY NIGHT: HIGH PRESSURE RIDGING WILL DOMINATE BRINGING
STABLE CONDITIONS, LIGHT WINDS, MILD TEMPERATURES, AND PARTLY
CLOUDY SKIES. VALLEY FOG IS EXPECTED TO DEVELOP DURING THE LATE
NIGHT AND MORNING HOURS THROUGH MONDAY. TRAVELERS SHOULD BE ALERT
FOR LOCALLY DENSE FOG, WHICH MAY CAUSE SUDDEN REDUCTIONS IN
VISIBILITY.
MONDAY THROUGH WEDNESDAY: AN ATMOSPHERIC RIVER WILL INTRODUCE
LOWER CLOUD COVER LATE SUNDAY, WITH LIGHT SHOWERS POSSIBLE ALONG
THE IDAHO/MONTANA BORDER AS EARLY AS MONDAY MORNING. PRECIPITATION
WILL BECOME MORE WIDESPREAD MONDAY AFTERNOON AS SNOW LEVELS RISE
ABOVE 7,000 FEET. BY TUESDAY, SNOW LEVELS ARE EXPECTED TO SETTLE
AROUND 5,000 FEET IN NORTHWEST MONTANA AND 6,000 FEET SOUTH OF
I-90.
A ROBUST SURGE OF MOISTURE ARRIVES TUESDAY NIGHT INTO WEDNESDAY MORNING
AHEAD OF A STRONG COLD FRONT.
-PRECIPITATION TOTALS MONDAY-WEDNESDAY: EXPECT 1-2 INCHES IN
THE VALLEYS OF NORTHWEST MONTANA AND 2-4 INCHES IN THE
MOUNTAINS. AREAS SOUTH OF I-90 WILL LIKELY SEE ABOUT HALF
THESE AMOUNTS.
-HYDROLOGIC CONCERNS: ALTHOUGH FORECAST PRECIPITATION TOTALS
ARE LOWER THAN THE PREVIOUS MID-WEEK ATMOSPHERIC RIVER EVENT,
THERE IS A CONTINUED RISK FOR RIVER RISES, MINOR FLOODING,
AND DEBRIS FLOWS/MUDSLIDES, PARTICULARLY ACROSS NORTHWEST
MONTANA.
STRONG TO POTENTIALLY DAMAGING WINDS ARE ALSO A PRIMARY CONCERN
WITH THE FRONTAL PASSAGE TUESDAY NIGHT INTO WEDNESDAY.
-IMPACTS: GIVEN THE SATURATED AND UNFROZEN SOILS IN MANY
VALLEYS, THE RISK FOR TREEFALLS BLOCKING ROADS AND CAUSING
SIGNIFICANT POWER OUTAGES IS ELEVATED.
-WIND SPEEDS: EARLY GUIDANCE INDICATES MEAN WIND GUSTS OF 45-55
MPH IN MANY VALLEYS, WHICH IS SUFFICIENT TO PRODUCE THE
IMPACTS DESCRIBED ABOVE.
BEHIND THE WEDNESDAY MORNING FRONT, SNOW LEVELS WILL DROP TO VALLEY FLOORS.
WINTER TRAVEL IMPACTS ARE LIKELY AT PASS LEVELS DUE TO SLICK ROADS AND
REDUCED VISIBILITY. WHILE VALLEY ACCUMULATIONS ARE LESS CERTAIN, SLICK
ROADWAYS MAY DEVELOP UNDER HEAVIER POST-FRONTAL SHOWERS.
EXTENDED OUTLOOK (THURSDAY INTO CHRISTMAS WEEK): ANOTHER
ATMOSPHERIC RIVER WILL FOLLOW CLOSELY, IMPACTING THE REGION
THURSDAY AND FRIDAY. FORECAST CONFIDENCE IS LOWER REGARDING THE
MOISTURE PLUME'S TRAJECTORY (CURRENT CLUSTERS ARE SPLIT 50/50
BETWEEN A DIRECT HIT OVERHEAD VERSUS A TRACK SOUTH OF THE NORTHERN
ROCKIES). REGARDLESS OF THE EXACT TRACK, WINTRY ROAD IMPACTS AT
PASS LEVELS ARE LIKELY, POTENTIALLY EVEN AT VALLEY FLOORS.
THE OVERALL ACTIVE PATTERN PERSISTS HEADING INTO CHRISTMAS WEEK, WITH
ATMOSPHERIC RIVERS IMPACTING THE WEST COAST AND PERIODIC ARCTIC AIR BANKING
AGAINST THE CONTINENTAL DIVIDE. AT THE VERY LEAST, THIS WILL
RESULT IN PERIODIC PASS-LEVEL SNOW IMPACTS AND A WINTRY MIX IN THE
VALLEYS.
AVIATION
HIGH PRESSURE RIDGING DOMINATES INTO SUNDAY WITH VFR CONDITIONS
FOR MOST OF THE AREA INTO THIS EVENING. STABLE CONDITIONS, LIGHT WIND AND
MOSTLY CLEAR SKIES OVERNIGHT WILL BE A DECENT SETUP FOR VALLEY FOG
DEVELOPMENT ESPECIALLY FROM I-90 NORTHWARD INCLUDING KMSO AND KGPI. ONCE
ANY FOG BREAKS UP ON SUNDAY MORNING, VFR CONDITIONS WILL ONCE AGAIN BE
PRESENT.
HYDROLOGY
A PROLONGED ATMOSPHERIC RIVER IS EXPECTED TO IMPACT WESTERN MONTANA
AND NORTH-CENTRAL IDAHO FROM MONDAY THROUGH WEDNESDAY, BRINGING
MODERATE TO HEAVY PRECIPITATION. SNOW LEVELS ARE ANTICIPATED TO
START AROUND 7,000 FEET ON MONDAY BEFORE FALLING TO VALLEY FLOORS
BEHIND A COLD FRONT EARLY WEDNESDAY. THE MOST LIKELY RAINFALL
SCENARIO FORECASTS 1 TO 2 INCHES FOR THE VALLEYS OF LINCOLN,
MINERAL, SANDERS IN WESTERN MONTANA AND CLEARWATER, AND IDAHO
COUNTIES IN IDAHO. 2 TO 4 INCHES EXPECTED IN THE MOUNTAINS OF
NORTHWEST MONTANA AND NORTH-CENTRAL IDAHO. AREAS FURTHER SOUTH AND
EAST IN WESTERN MONTANA WILL LIKELY SEE 1 INCH OR LESS. A HIGH-
END SCENARIO COULD BRING TOTALS 1 TO 2 INCHES HIGHER THAN THE
ESTIMATES GIVEN ABOVE.
DUE TO ALREADY SATURATED GROUND, PARTICULARLY IN NORTHWEST MONTANA,
THIS RAINFALL INCREASES THE RISK OF ROCK AND LANDSLIDES, STREAM AND
RIVER RISES, AND MINOR FLOODING. CONTINUE TO MONITOR THE LATEST
FORECAST UPDATES AS THIS SYSTEM APPROACHES.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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