687
FXUS65 KMSO 181828
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
1228 PM MDT FRI SEP 18 2026
DISCUSSION
KEY MESSAGES:
- THUNDERSTORMS THIS AFTERNOON/EVENING WITH LOCALLY HEAVY RAIN,
MINOR RUNOFF CONCERNS NEAR BURN SCARS AND SUSCEPTIBLE TERRAIN.
- STEADY PRECIPITATION TONIGHT INTO SATURDAY SOUTH OF I-90.
- WARMING AND DRYING TREND SUNDAY INTO THE FIRST PART OF NEXT
WEEK.
THIS AFTERNOON THROUGH SATURDAY AFTERNOON: BROAD-SCALE LIFT AND
PARTIAL CLEARING NORTH OF AN UPPER LOW OVER SOUTHWEST IDAHO WILL
TRIGGER ROUNDS OF SHOWERS AND THUNDERSTORMS THIS AFTERNOON AND
EVENING, PRIMARILY FROM I-90 SOUTHWARD. WEAK STEERING FLOW
COMBINED WITH ANOMALOUS MOISTURE (PRECIPITABLE WATER 150-200% OF
NORMAL) WILL MAKE LOCALLY HEAVY RAINFALL THE PRIMARY HAZARD.
TYPICAL RAIN RATES OF 0.25-0.50 IN/HR ARE EXPECTED, WITH LOCALIZED
CORES EXCEEDING 0.75 IN/HR. THIS POSES A LOCALIZED RISK FOR
STANDING WATER, RAPID RUNOFF, AND DEBRIS FLOWS OVER BURN SCARS AND
SUSCEPTIBLE TERRAIN.
THE UPPER LOW WILL SLIDE ACROSS SOUTHERN IDAHO TONIGHT INTO
SATURDAY, SPREADING A BAND OF WRAP-AROUND STRATIFORM RAIN WITH
EMBEDDED CONVECTION FROM I-90 SOUTH INTO LEMHI COUNTY.
PRECIPITATION WILL TAPER OFF LATE SATURDAY AFTERNOON AND EVENING.
STORM-TOTAL RAINFALL WILL RANGE FROM 0.50-1.50 INCHES FROM
NORTHERN LEMHI COUNTY EAST TO BUTTE, WITH ISOLATED TOTALS
EXCEEDING 2.0 INCHES.
SUNDAY THROUGH NEXT WEEKEND: CLEARING SKIES SATURDAY NIGHT WILL
FAVOR VALLEY FOG AND LOW STRATUS, PARTICULARLY IN AREAS THAT
RECEIVED SUBSTANTIAL RAINFALL. UPPER-LEVEL RIDGING BUILDS ACROSS
THE REGION SUNDAY AND MONDAY, ESTABLISHING A WARMING AND DRYING
TREND.
WHILE DETERMINISTIC MODELS DIVERGE ON THE DETAILS MID-TO-LATE
NEXT WEEK, ENSEMBLE CLUSTERS FAVOR SOUTHWESTERLY FLOW, MILD
TEMPERATURES, INCREASED AFTERNOON BREEZES, AND A GRADUAL RETURN OF
ISOLATED AFTERNOON THUNDERSTORM CHANCES STARTING AS EARLY AS
TUESDAY. BY SUNDAY (SEP 27), ENSEMBLE GUIDANCE (60-70% CONFIDENCE)
POINTS TOWARD A DEEPER PACIFIC NORTHWEST TROUGH PUSHING INLAND,
BRINGING COOLER TEMPERATURES AND HIGHER SHOWER CHANCES.
AVIATION
MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS WILL IMPACT
REGIONAL TERMINALS SOUTH OF I-90 THIS AFTERNOON AND EVENING AS
PARTIAL AFTERNOON CLEARING ALLOWS INSTABILITY TO INCREASE IN
CONJUNCTION WITH BROAD SCALE LIFT. PRIMARY HAZARDS INCLUDE ERRATIC
CONVECTIVE OUTFLOW GUSTS OF 20-30 KT (LOCALLY HIGHER WITH A STORM
OR TWO) AND BRIEF HEAVY DOWNPOURS DRIVING LOCALIZED MVFR
VISIBILITY REDUCTIONS.
PRECIPITATION TURNS STRATIFORM TONIGHT THROUGH SATURDAY LEADING
TO INTERMITTENT MVFR/IFR CEILINGS/VISIBILITY AND TERRAIN
OBSCURATIONS. PRECIPITATION DIMINISHES ON SATURDAY AFTERNOON WITH
CLEARING SATURDAY NIGHT ALLOWING FOR FOG AND SOME LOW STRATUS FOR
VALLEY LOCATIONS.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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