083
FXUS65 KMSO 082151
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
251 PM MST MON DEC 8 2025
DISCUSSION
KEY MESSAGES:
- SIGNIFICANT ATMOSPHERIC RIVER, POTENTIALLY HISTORIC, 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 5000-8000 FEET THIS WEEK.
THE PREVIOUS FORECAST DISCUSSION CAPTURED THE DETAILS WELL WITH
ONLY MINOR ADDITIONS.
A SIGNIFICANT, POTENTIALLY HISTORIC, LONG- DURATION ATMOSPHERIC
RIVER EVENT HAS BEGUN TO IMPACT THE NORTHERN ROCKIES. A DEEP
PLUME OF SUBTROPICAL MOISTURE IS SURGING INTO THE REGION THIS
AFTERNOON, CONTINUING IN WAVES THROUGH AT LEAST THURSDAY. BEFORE
WARM AIR MOVES IN LATE THIS AFTERNOON AND TONIGHT, SNOW WILL FALL
AT PASS LEVELS ALONG THE IDAHO/MONTANA BORDER BRINGING SLUSHY
ROAD CONDITIONS. AS WARM AIR MOVES IN, SNOW LEVELS WILL RISE TO
NEAR 7,000 FEET LATE TONIGHT, LEADING TO A CONCERNING "RAIN- ON-
SNOW" SCENARIO FOR MID- ELEVATIONS. THE AMOUNT OF MOISTURE CARRIED
BY THIS SYSTEM IS FORECAST TO BE NEAR ALL-TIME MAXIMUMS FOR EARLY
DECEMBER, ESSENTIALLY DIRECTING A FIREHOSE OF WARM, SUBTROPICAL
AIR INTO WESTERN MONTANA AND NORTH CENTRAL IDAHO. THIS SETUP IS
EXPECTED TO PRODUCE 4 TO 8 INCHES OF RAINFALL OVER THE HIGHER
TERRAIN OF NORTH CENTRAL IDAHO AND NORTHWEST MONTANA AND 2 TO 4
INCHES IN THE VALLEYS BY THURSDAY MORNING, A CRITICAL THRESHOLD
THAT SIGNIFICANTLY HEIGHTENS THE RISK OF FLOODING. THE PRIMARY
CONCERN IS NOT JUST THE HEAVY RAIN, BUT THE "RAIN-ON- SNOW"
EFFECT. THE WARM NATURE OF THIS SYSTEM WILL LIKELY MELT EXISTING
LOW-TO- MID ELEVATION SNOWPACK, COMBINING RUNOFF WITH RAINFALL TO
RAPIDLY SWELL CREEKS, STREAMS, AND RIVERS. RESIDENTS IN STEEP
TERRAIN AND NEAR WATERWAYS SHOULD PREPARE FOR SIGNIFICANT RISES
AND POTENTIAL FLOODING, AS THE UNUSUAL DURATION AND INTENSITY OF
THIS EVENT WILL STRESS RIVER BASINS FAR BEYOND TYPICAL WINTER
NORMS. MINOR FLOODING IN URBAN AND POOR-DRAINAGE AREAS ARE
POSSIBLE. STORM DRAINS AND DITCHES MAY BECOME CLOGGED WITH DEBRIS.
YOU MAY WANT TO PREEMPTIVELY SHOVEL SNOW AWAY FROM YOUR HOMES.
WATCH BASEMENTS FOR MOISTURE INTRUSION.
BEYOND THE HEAVY PRECIPITATION, THIS SYSTEM WILL GENERATE GUSTY
WEST-SOUTHWEST WINDS, PARTICULARLY ACROSS NORTHWEST MONTANA
WHERE WIND ADVISORIES ARE IN PLACE FOR TONIGHT INTO TUESDAY
MORNING. PROBABILISTIC GUIDANCE FROM HIGH RESOLUTION MODELS SHOWS
A 40 TO 60 PERCENT CHANCE OF WINDS GUSTING OVER 40 MPH FOR VALLEYS
OF WESTERN MONTANA TONIGHT, WITH STRONGER WINDS IN THE TERRAIN.
THE COMBINATION OF WIND AND SATURATED GROUND INCREASES THE RISK OF
DOWNED TREES AND POWER OUTAGES.
A COLD FRONT WILL SWEEP THROUGH TUESDAY MORNING, BRIEFLY LOWERING
SNOW LEVELS, ESPECIALLY ACROSS NORTHWEST MONTANA. THIS LOWERING
MAY BE ENOUGH TO BRING A PERIOD OF ACCUMULATING SNOW ON MOUNTAIN
PASSES, SUCH AS MARIAS LOOKOUT AND LOLO. A BRIEF LULL IN
PRECIPITATION IS EXPECTED TUESDAY AFTERNOON BEFORE A SECOND SURGE
OF MOISTURE ARRIVES TUESDAY NIGHT THROUGH THURSDAY. FORECAST
CONFIDENCE DECREASES SLIGHTLY REGARDING SNOW LEVELS DURING THIS
SECOND PHASE; COLDER AIR MAY FILTER IN, POTENTIALLY DROPPING THE
TRANSITION FROM RAIN TO SNOW DOWN TO THE 3,500 TO 4,500 FOOT
RANGE. REGARDLESS OF THE EXACT SNOW LEVEL, HEAVY PRECIPITATION
WILL FOCUS ONCE AGAIN ALONG NORTH-CENTRAL IDAHO AND THE
MONTANA/IDAHO BORDER.
IMPACTS SUMMARY:
-HYDROLOGY: FLOOD WATCHES REMAIN IN EFFECT FOR IDAHO AND
CLEARWATER COUNTIES (ID) AND MINERAL COUNTY (MT). LINCOLN,
FLATHEAD, SANDERS, AND LAKE COUNTIES HAVE BEEN ADDED TO THE
WATCHES. RAINFALL TOTALS OF 2 TO 4 INCHES ARE EXPECTED IN
VALLEYS, WITH 4 TO 8 INCHES, LOCALLY HIGHER, IN THE
MOUNTAINS. BE ALERT FOR SHARP RISES ON SMALL STREAMS, PONDING
OF WATER IN LOW- LYING AREAS, AND AN INCREASED RISK OF ROCK
AND MUDSLIDES IN STEEP TERRAIN. THERE COULD BE ISSUES WITH
WATER INTO BASEMENTS, BE SURE TO SHOVEL SNOW AWAY FROM YOUR
HOME.
-BACKCOUNTRY: ABOVE 6,000 FEET, HEAVY, WET SNOW AND STRONG
WINDS WILL CREATE CHANGEABLE AVALANCHE CONDITIONS. BACKCOUNTRY
USERS SHOULD MONITOR FORECASTS AT WWW.AVALANCHE.ORG. MOUNTAIN
PASSES MAY SEE PERIODIC BURSTS OF SNOW, MOST LIKELY TUESDAY
MORNING-WEDNESDAY MORNING AND POSSIBLY AGAIN ON THURSDAY.
FINALLY, ROUGHLY 20 PERCENT 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 ITS
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, ESPECIALLY IN NORTHWEST MONTANA AND
NEAR THE CONTINENTAL DIVIDE.
AVIATION
A SIGNIFICANT PATTERN CHANGE BEGINS THIS AFTERNOON AS
A POTENT ATMOSPHERIC RIVER IMPACTS THE NORTHERN ROCKIES.
CONDITIONS WILL DETERIORATE RAPIDLY FROM WEST TO EAST THIS
AFTERNOON AND EVENING.
-CEILINGS AND VISIBILITY: WIDESPREAD MVFR AND AREAS OF IFR WILL
FILL IN FROM WEST TO EAST THIS AFTERNOON AS HEAVY MOISTURE
SURGES INTO THE REGION. WIDESPREAD MOUNTAIN OBSCURATION IS
EXPECTED THROUGH THE PERIOD. WITH SNOW LEVELS RISING TO NEAR
7,000 FEET, PRECIPITATION AT ALL MAIN TERMINALS (KMSO, KGPI,
KBTM, KSMN, KHRF) WILL FALL AS RAIN.
-WINDS AND SHEAR: STRONG WEST-SOUTHWEST WINDS WILL DEVELOP
ALOFT AND MIX DOWN TO THE SURFACE TONIGHT. LLWS IS A PRIMARY
CONCERN AT ALL TERMINALS THIS EVENING INTO TUESDAY MORNING,
PARTICULARLY BEFORE SURFACE GUSTS FULLY DEVELOP. SURFACE
GUSTS OF 35-45 KTS ARE PROBABLE, ESPECIALLY ACROSS NORTHWEST
MONTANA (IMPACTING KGPI) AND LOCALIZED VALLEYS.
-TURBULENCE: EXPECT MODERATE TO SEVERE MECHANICAL TURBULENCE
OVER TERRAIN DUE TO STRONG RIDGE-TOP WINDS INTERACTING WITH
THE RUGGED TOPOGRAPHY.
HYDROLOGY
A POTENTIALLY HISTORIC HYDROLOGIC EVENT IS UNFOLDING
AS A LONG-DURATION ATMOSPHERIC RIVER (AR) TAKES AIM AT THE
NORTHERN ROCKIES. MODEL GUIDANCE INDICATES A HIGHLY ANOMALOUS
MOISTURE SURGE, WITH PRECIPITABLE WATER (PWAT) VALUES FORECAST TO
REACH NEAR ALL-TIME MAXIMUMS FOR EARLY DECEMBER. THE SYNOPTIC
SETUP SUGGESTS A GENERAL STALL OF THE ANOMALOUS MOISTURE OVER
WESTERN MONTANA AND NORTH CENTRAL IDAHO, WITH ONLY BRIEF BREAKS
THROUGH AT LEAST THE NEXT 72 HOURS, IF NOT LONGER.
CONFIDENCE IS HIGH REGARDING EXCESSIVE PRECIPITATION TOTALS,
THERE IS PARTICULARLY ACROSS THE CLEARWATER RIVER BASIN IN NORTH
CENTRAL IDAHO AND ACROSS MUCH OF NORTHWEST MONTANA. 72 HOUR
PRECIPITATION AMOUNTS OF 4 TO 8 INCHES ARE LIKELY OVER HIGHER
TERRAIN. THE PRIMARY CONCERN EXTENDS BEYOND RAINFALL INTENSITY.
THIS IS A WARM, SUBTROPICAL AIRMASS THAT WILL DRIVE FREEZING
LEVELS WELL ABOVE 6,000 TO 8,000 FEET. 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 MAY OVERWHELM SMALL
CREEKS AND STREAMS FIRST, FOLLOWED BY SIGNIFICANT RISES ON
MAINSTEM RIVERS, THOUGH MAINSTEM RIVER FLOODING IS NOT CURRENTLY
FORECAST. SOILS ARE ALREADY SATURATED DUE TO THE RECENT
PRECIPITATION OF 2 TO 4 INCHES OVER THE PREVIOUS 3 DAYS, LEADING
TO EFFICIENT RUNOFF. ROCK/MUDSLIDES AND PONDING OF WATER IN LOW
LYING/URBAN AREAS WILL ALSO BE A CONCERN. THERE COULD BE ISSUES
WITH WATER INTO BASEMENTS, BE SURE TO SHOVEL SNOW AWAY FROM YOUR
HOME. THE POTENTIAL FOR MINOR TO MODERATE FLOODING IS ELEVATED.
PERSONS NEAR WATERWAYS OR BELOW STEEP SLOPES, PARTICULARLY IN
NORTH CENTRAL IDAHO, SHOULD PREPARE FOR RAPID FLUCTUATIONS IN
WATER LEVELS. NORTHWEST MONTANA HAS BEEN ADDED TO THE FLOOD
WATCHES, WATCHES CONTINUE FOR IDAHO AND CLEARWATER COUNTIES AS
WELL.
MSO WATCHES/WARNINGS/ADVISORIES
MT...FLOOD WATCH FROM 5 PM MST THIS AFTERNOON THROUGH THURSDAY
AFTERNOON FOR FLATHEAD/MISSION VALLEYS...KOOTENAI/CABINET
REGION...LOWER CLARK FORK REGION...POTOMAC/SEELEY LAKE
REGION...WEST GLACIER REGION.
WIND ADVISORY FROM 8 PM THIS EVENING TO 8 AM MST TUESDAY FOR
KOOTENAI/CABINET REGION...LOWER CLARK FORK REGION.
WIND ADVISORY FROM 11 PM THIS EVENING TO 11 AM MST TUESDAY FOR
WEST GLACIER REGION.
WIND ADVISORY FROM 11 PM THIS EVENING TO 8 AM MST TUESDAY FOR
FLATHEAD/MISSION VALLEYS.
ID...FLOOD WATCH FROM 4 PM PST THIS AFTERNOON THROUGH THURSDAY
AFTERNOON FOR LOWER HELLS CANYON/SALMON RIVER REGION...
NORTHERN CLEARWATER MOUNTAINS...OROFINO/GRANGEVILLE
REGION...SOUTHERN CLEARWATER MOUNTAINS.
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