406
FXUS65 KMSO 111916
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
116 PM MDT THU SEP 11 2025
DISCUSSION
KEY MESSAGES:
- AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS ARE EXPECTED
TODAY AND FRIDAY, BRINGING THREATS OF GUSTY OUTFLOW WINDS UP TO
45 MPH, LOCALIZED HEAVY DOWNPOURS, SMALL HAIL, AND FREQUENT
LIGHTNING.
- A LOW-END DEBRIS FLOW THREAT CONTINUES FOR SENSITIVE BURN SCARS
DUE TO THE POTENTIAL FOR LOCALIZED HEAVY RAIN.
- AN UNSETTLED, COOLER PATTERN PERSISTS THROUGH THE WEEKEND AND
INTO EARLY NEXT WEEK, WITH ANOTHER STORM SYSTEM EXPECTED TO
BRING WIDESPREAD SHOWERS AND POTENTIALLY MUCH COOLER
TEMPERATURES BY MONDAY.
AN UPPER-LEVEL LOW CONTINUES TO SPIN SLOWLY EASTWARD ACROSS THE
NORTHERN ROCKIES, KEEPING OUR REGION UNDER A MOIST AND UNSTABLE
AIRMASS. CONVECTION ALLOWING MODELS AGREE ON SHOWERS AND
THUNDERSTORMS DEVELOPING BY LATE THIS AFTERNOON AND CONTINUE INTO
THE EVENING.
THE PRIMARY CONCERNS WITH TODAY'S STORMS WILL BE GUSTY OUTFLOW
WINDS, BRIEF HEAVY RAIN, AND LIGHTNING. WITH PRECIPITABLE WATER
VALUES REMAINING HIGH, SOME STORMS COULD PRODUCE LOCALIZED
RAINFALL RATES THAT ELEVATE THE DEBRIS FLOW RISK OVER SUSCEPTIBLE
BURN SCARS, SUCH AS THE DALY BURN SCAR. THE POTENTIAL FOR SMALL
HAIL ALSO EXISTS, PARTICULARLY ACROSS NORTH-CENTRAL IDAHO AND
SOUTHWEST MONTANA, WHERE INSTABILITY IN THE HAIL GROWTH ZONE IS
SLIGHTLY HIGHER, WITH CAPE VALUES APPROACHING 500 J/KG. THE HRRR
MODEL SUGGESTS SOME OF THE STRONGER CELLS COULD PRODUCE HAIL UP TO
PENNY SIZE (0.75 INCHES). STORMS WILL DIMINISH BY LATE IN THE
EVENING. PATCHY FOG MAY DEVELOP OVERNIGHT IN VALLEYS THAT
RECEIVED SIGNIFICANT RAINFALL.
THE SLOW-MOVING LOW WILL KEEP THE REGION UNSETTLED ON FRIDAY,
WITH ANOTHER ROUND OF SCATTERED SHOWERS AND THUNDERSTORMS
EXPECTED. MODEL GUIDANCE SUGGESTS THE FOCUS FOR HEAVIER SHOWERS
MAY SHIFT TO NORTHWEST MONTANA AND TOWARD THE MOUNTAINS OF GLACIER
NATIONAL PARK AND THE SALISH MOUNTAINS. THIS COULD BRING PERIODS
OF HEAVY RAIN TO THE GOING TO THE SUN ROAD. THREATS OF GUSTY
WINDS AND LIGHTNING WILL PERSIST. BY FRIDAY NIGHT AND INTO
SATURDAY MORNING, PARTIAL CLEARING COMBINED WITH MOIST GROUND
COULD ONCE AGAIN LEAD TO PATCHY VALLEY FOG.
BY SATURDAY, THE UPPER LOW FINALLY SHIFTS INTO EASTERN MONTANA.
OUR REGION WILL REMAIN UNDER CYCLONIC FLOW, SUPPORTING SCATTERED
SHOWERS AND A FEW ISOLATED THUNDERSTORMS, PRIMARILY ACROSS WESTERN
MONTANA. SUNDAY MAY OFFER A BRIEF LULL WITH SOME MORNING SUN
BEFORE CLOUDS INCREASE AHEAD OF OUR NEXT APPROACHING WEATHER
SYSTEM.
CONFIDENCE IS INCREASING ON ANOTHER PACIFIC TROUGH WILL MOVE INTO
THE REGION SUNDAY AFTERNOON INTO MONDAY, USHERING IN ANOTHER
ROUND OF WIDESPREAD SHOWERS AND COOLER TEMPERATURES, AND INCREASED
WESTERLY WINDS.
ENSEMBLE MODEL GUIDANCE PRESENTS TWO PRIMARY SCENARIOS:
A "CUT-OFF" LOW (AROUND A 70% PROBABILITY): THE TROUGH DEEPENS
AND SLOWS DOWN OVER THE REGION. THIS WOULD RESULT IN A WETTER AND
COLDER PERIOD, WITH PROLONGED RAINFALL AND VALLEY HIGH
TEMPERATURES POTENTIALLY STRUGGLING TO GET OUT OF THE 50S AND LOW
60S ON MONDAY.
A PROGRESSIVE TROUGH (AROUND 30% PROBABILITY): THE SYSTEM MOVES
THROUGH MORE QUICKLY WITHOUT CUTTING OFF FROM THE MAIN JET STREAM.
THIS WOULD LEAD TO A DRIER AND BREEZIER OUTCOME WITH LESS OVERALL
PRECIPITATION.
WE WILL CONTINUE TO MONITOR THIS SYSTEM CLOSELY AS IT DEVELOPS.
FOR NOW, PLAN ON A COOL AND WET START TO THE WORK WEEK.
AVIATION
CONVECTIVE SHOWERS AND THUNDERSHOWERS POSSIBLE AGAIN
THIS AFTERNOON AND EVENING BETWEEN 11/2000Z AND 12/0400Z. SPOTTY
IFR/LIFR WEATHER CONDITIONS EXPECTED WITH REDUCED VISIBILITIES AND
CEILINGS. ISOLATED AREAS OF STRONG GUSTY OUTFLOW WINDS TO 40
KNOTS, HEAVY RAINFALL INTENSITIES THAT MAY CAUSE PONDING ON
RUNWAYS AND AIRFIELDS, AND SMALL HAIL POSSIBLE. THE HIGHEST RISK
AREAS ARE MOST LIKELY NEAR KMSO, KHRF, AND TO A LESSER EXTENT KBTM
AND KGPI. OVERNIGHT PATCHY VALLEY FOG AND LINGERING LOWERED
CEILINGS/STRATUS ARE POSSIBLE THROUGH ABOUT 12/1400Z.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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