436
FXUS65 KMSO 291007
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
407 AM MDT FRI MAY 29 2026
DISCUSSION
KEY MESSAGES:
- TODAY: ISOLATED STRONG TO SEVERE THUNDERSTORMS WITH LOCALIZED
FLASH FLOODING POSSIBLE, ESPECIALLY IN NORTH-CENTRAL IDAHO AND
NORTHWEST MONTANA.
- SATURDAY THROUGH MONDAY: A SLOW MOVING WEATHER SYSTEM WILL
BRING PROLONGED, HEAVY RAINFALL. EXPECT SIGNIFICANT RIVER RISES,
POTENTIAL MINOR FLOODING, AND A RETURN TO WINTER-LIKE
CONDITIONS IN THE HIGH BACKCOUNTRY.
AN INCOMING PACIFIC LOW-PRESSURE SYSTEM WILL BRING INCREASED
PRECIPITATION FROM SOUTHEAST TO NORTHWEST TODAY, WITH
THUNDERSTORMS EXPECTED TO GET AN EARLIER START THAN RECENT DAYS,
FIRING UP AS EARLY AS 1 TO 2 PM. LINGERING WARMTH AND STRONG WINDS
ALOFT (WIND SHEAR) WILL FAVOR A FEW STRONG TO SEVERE STORMS, WITH
THE HIGHEST RISK ACROSS NORTHWEST MONTANA WHERE WINDS OVER 50 MPH
AND HAIL ARE THE PRIMARY THREATS. ADDITIONALLY, STORMS MOVING
FROM SOUTH TO NORTH MAY REPEATEDLY PASS OVER THE SAME AREAS. THIS
TRAINING OF STORMS CREATES A LOCALIZED FLASH FLOODING CONCERN,
PARTICULARLY FOR THE US-12 AND US-95 CORRIDORS IN IDAHO UNDER
HEAVY DOWNPOURS.
BY SATURDAY, THE LOW-PRESSURE SYSTEM IS PROJECTED TO MOVE VERY
SLOWLY THROUGH THE REGION, TAPPING INTO DEEP, UNSEASONABLY HIGH
MOISTURE AND LOCKING IN A PERSISTENT AREA OF HEAVY RAIN ACROSS
WESTERN MONTANA THROUGH AT LEAST MONDAY. THE HEAVIEST RAINFALL IS
EXPECTED FROM HIGHWAY 93 EASTWARD TO THE CONTINENTAL DIVIDE, WHERE
THERE IS A 70% TO 90% CHANCE OF TOTALS EXCEEDING 1 INCH. HIGHER
TERRAIN ALONG THE DIVIDE, ESPECIALLY IN NORTHWEST MONTANA, COULD
SEE SIGNIFICANT ACCUMULATIONS OF 2 TO 4 INCHES. WE ARE ALSO
WATCHING A MINOR TREND WHERE ABOUT 20 PERCENT OF WEATHER MODELS
SLOW THIS SYSTEM DOWN EVEN FURTHER. IF THIS SLOWER TREND CONTINUES
TO GAIN TRACTION, RAINFALL COULD EXTEND INTO TUESDAY, POTENTIALLY
DRIVING TOTAL PRECIPITATION AMOUNTS EVEN HIGHER FOR NORTHWEST
MONTANA.
BECAUSE RECENT RAINFALL HAS ALREADY SATURATED SOILS, THIS PROLONGED
HEAVY RAIN COMBINED WITH ONGOING HIGH-ELEVATION SNOWMELT WILL
CAUSE SHARP RISES ON AREA WATERWAYS. THE CLARK FORK RIVER ABOVE
MISSOULA AND THE FLATHEAD RIVER AT COLUMBIA FALLS ARE CURRENTLY
FORECAST TO REACH MINOR FLOOD STAGE LATE SUNDAY OR EARLY MONDAY.
MINOR FLOODING AND WATER PONDING ON ROADS ARE ALSO A CONCERN FOR
LOW-LYING AND URBAN AREAS, PARTICULARLY WITHIN MISSOULA AND
FLATHEAD COUNTIES. RESIDENTS IN FLOOD-PRONE LOCATIONS SHOULD CLEAR
STORM DRAINS AND GUTTERS AHEAD OF TIME. IN ADDITION TO WATER
IMPACTS, STEEP CANYON TERRAIN WILL FACE AN ELEVATED RISK OF
ROCKFALLS AND MUDSLIDES. MEANWHILE, SHARP COOLING ALOFT WILL CAUSE
SNOW LEVELS TO DROP TO 6,000 TO 7,000 FEET ALONG THE CONTINENTAL
DIVIDE BY SUNDAY MORNING, BRINGING RAW, WINTER-LIKE CONDITIONS AND
HAZARDOUS TRAVEL TO BACKCOUNTRY USERS.
ONCE THE STORM SYSTEM EXITS, WEATHER MODELS FAVOR A TRANSITION BACK
TO A HIGH-PRESSURE RIDGE BY MID-TO-LATE NEXT WEEK. THIS WILL
BRING A RAPID RETURN TO DRY CONDITIONS AND ABOVE-NORMAL
TEMPERATURES, OFFERING A WELCOME REPRIEVE FROM THE WET HOLIDAY-
WEEKEND PATTERN.
AVIATION
WIDESPREAD VFR CONDITIONS WILL GIVE WAY TO EXPANDING
CONVECTIVE ACTIVITY EARLY THIS AFTERNOON, WITH THUNDERSTORMS
EXPECTED TO INITIATE BETWEEN 19Z AND 20Z (1 TO 2 PM MDT). INITIAL
DEVELOPMENT WILL TRACK SOUTH-TO-NORTH ACROSS NORTH-CENTRAL IDAHO
AND WEST-CENTRAL MONTANA, SPREADING INTO NORTHWEST MONTANA BY LATE
AFTERNOON. TERMINALS, MOST NOTABLY KGPI AND KMSO, WILL FACE
PERIODS OF MVFR/IFR CONDITIONS DUE TO TORRENTIAL DOWNPOURS, SUDDEN
VISIBILITY REDUCTIONS, AND LOW CEILINGS. THE PRIMARY HAZARD TO
AVIATION WILL BE STRONG, ERRATIC CONVECTIVE OUTFLOW WINDS
EXCEEDING 45 KNOTS AND LOCALIZED HAIL, WITH THE HIGHEST
PROBABILITY FOR SEVERE CRITERIA CENTERED OVER NORTHWEST MONTANA
(KGPI). ADDITIONALLY, HEAVY RAIN FROM TRAINING STORMS ALONG THE
IDAHO CORRIDORS COULD PRODUCE PROLONGED LOCALIZED FLIGHT
RESTRICTIONS AND MOUNTAIN OBSCURATIONS.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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