716
FXUS65 KMSO 081036
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
336 AM 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 5000-7000 FEET THIS WEEK.
TODAY WILL MARK THE ARRIVAL OF A SIGNIFICANT, POTENTIALLY
HISTORIC, LONG-DURATION ATMOSPHERIC RIVER EVENT FOR THE NORTHERN
ROCKIES. A DEEP PLUME OF SUBTROPICAL MOISTURE WILL SURGE INTO THE
REGION THIS AFTERNOON, CONTINUING IN WAVES THROUGH AT LEAST
THURSDAY. AS WARM AIR MOVES IN WITH THIS FEATURE, SNOW LEVELS WILL
RISE TO NEAR 7,000 FEET 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 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 RIVERS, CREEKS, AND STREAMS. 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.
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 HAVE BEEN ISSUED FOR IDAHO AND
CLEARWATER COUNTIES (ID) AND MINERAL COUNTY (MT). RAINFALL
TOTALS OF 2 TO 4 INCHES ARE EXPECTED IN VALLEYS, WITH 4 TO 8
INCHES 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.
-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 TODAY AS A POTENT
ATMOSPHERIC RIVER IMPACTS THE NORTHERN ROCKIES. CONDITIONS WILL
DETERIORATE RAPIDLY FROM WEST TO EAST THIS AFTERNOON AND EVENING.
-CEILINGS AND VISIBILITY: EXPECT VFR CONDITIONS TO GIVE WAY TO
WIDESPREAD MVFR AND AREAS OF IFR BY 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,
PARTICULARLY ACROSS THE CLEARWATER RIVER BASIN IN NORTH CENTRAL
IDAHO. 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 7,000 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 IN NORTH
CENTRAL IDAHO, LEADING TO EFFICIENT RUNOFF. ROCK/MUDSLIDES AND
PONDING OF WATER IN LOW LYING/URBAN AREAS WILL ALSO BE A CONCERN.
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. FLOOD WATCHES ARE IN EFFECT FOR IDAHO/CLEARWATER AND
MINERAL COUNTIES.
MSO WATCHES/WARNINGS/ADVISORIES
MT...FLOOD WATCH FROM THIS AFTERNOON THROUGH LATE WEDNESDAY NIGHT FOR
LOWER CLARK FORK 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 THIS AFTERNOON THROUGH LATE WEDNESDAY NIGHT FOR
LOWER HELLS CANYON/SALMON RIVER REGION...NORTHERN
CLEARWATER MOUNTAINS...OROFINO/GRANGEVILLE REGION...
SOUTHERN CLEARWATER MOUNTAINS.
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