869
FXUS65 KMSO 140919
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
219 AM MST SUN DEC 14 2025
DISCUSSION
KEY MESSAGES:
- AREAS OF FOG AND LOW CLOUDS IN VALLEYS THIS MORNING AND AGAIN ON MONDAY
- RENEWED FLOODING AND WIND CONCERNS TUESDAY-WEDNESDAY
- PASS LEVEL TRAVEL IMPACTS WEDNESDAY AS SNOW LEVELS DROP TO VALLEY
FLOORS
HIGH PRESSURE WILL PERSIST TODAY OVER THE NORTHERN ROCKIES.
EXPECT DRY CONDITIONS, MILD TEMPERATURES, AND LIGHT WINDS TODAY
AND TONIGHT. VALLEY FOG HAS ALREADY FORMED IN THE FLATHEAD VALLEY
EARLY THIS MORNING, AND WILL BE IN THE FORECAST AGAIN FOR MONDAY.
UPPER LEVEL FLOW WILL FLATTEN OUT AND BECOME WESTERLY THIS
AFTERNOON. A STRONG LOW PRESSURE CIRCULATION OFF THE COAST OF
BRITISH COLUMBIA WILL START TO PUSH MOISTURE ONSHORE IN THE FORM
OF AN ATMOSPHERIC RIVER. THAT PACIFIC MOISTURE WILL REACH THE
NORTHERN ROCKIES, NORTHWEST MONTANA IN PARTICULAR, ON MONDAY
MORNING, KICKING OFF A WEEK OF WET AND POTENTIALLY IMPACTFUL
WEATHER.
PRECIPITATION WILL BECOME MORE WIDESPREAD MONDAY AFTERNOON AS
SNOW LEVELS RISE ABOVE 7,000 FEET. WINDS WILL PICK UP ON MONDAY
NIGHT AS AN UPPER LEVEL JET STREAK MOVES OVERHEAD. 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 CONTINUES THIS MORNING. BEST CHANCES FOR
FOG WILL BE AT KGPI, WHERE DECREASED VISIBILITY AND LOWERED
CEILINGS ARE ALREADY PRESENT EARLY THIS MORNING. CLOUD COVER WILL
PERSIST OVER NORTHWEST MONTANA, WHILE AVIATORS SHOULD EXPECT
DECREASING CLOUDS ALONG AND SOUTH OF ABOUT INTERSTATE 90. CLOUD
COVER INCREASES AGAIN TONIGHT AHEAD OF MORE UNSETTLED WEATHER
OVERNIGHT AND INTO MONDAY MORNING.
HYDROLOGY
A PROLONGED ATMOSPHERIC RIVER EVENT WILL IMPACT
WESTERN MONTANA AND NORTH-CENTRAL IDAHO FROM MONDAY THROUGH
WEDNESDAY, BRINGING MODERATE TO HEAVY PRECIPITATION. SNOW LEVELS
WILL 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, AND SANDERS COUNTIES 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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