469
FXUS65 KMSO 121944
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
144 PM MDT FRI SEP 12 2025
DISCUSSION
KEY MESSAGES:
- AFTERNOON THUNDERSTORMS ARE EXPECTED TODAY, ESPECIALLY IN
NORTHWEST MONTANA AND ALONG THE DIVIDE WITH GUSTY WINDS AND
LOCALIZED HEAVY RAIN BEING THE PRIMARY THREATS.
- UNSETTLED CONDITIONS CONTINUE SATURDAY WITH PATCHY MORNING
VALLEY FOG, FOLLOWED BY SCATTERED AFTERNOON SHOWERS AND
THUNDERSTORMS UNDER A NORTHWESTERLY FLOW.
- SIGNIFICANT UNCERTAINTY PERSISTS FOR A WEATHER SYSTEM EARLY
NEXT WEEK, WITH MODEL GUIDANCE SHOWING A WIDE RANGE OF
POSSIBILITIES FROM VERY LITTLE RAIN TO WIDESPREAD, BENEFICIAL
PRECIPITATION.
THE SLOW-MOVING UPPER-LEVEL LOW THAT HAS INFLUENCED OUR WEATHER
FOR DAYS IS FINALLY BEGINNING TO EXIT THE REGION. HOWEVER, IT WILL
PROVIDE ONE LAST ROUND OF SCATTERED SHOWERS AND THUNDERSTORMS
THIS AFTERNOON AND EVENING. THE BEST FOCUS FOR THIS ACTIVITY WILL
BE ACROSS NORTHWEST MONTANA, PARTICULARLY IN A CORRIDOR FROM
MARION THROUGH THOMPSON FALLS, AND HIGHER TERRAIN OF NORTHWEST
MONTANA INCLUDING THE BOB MARSHALL WILDERNESS, GLACIER NATIONAL
PARK, SALISH MOUNTAINS AND CABINET RANGE, WHERE INSTABILITY AND
MOISTURE WILL BE MOST CONCENTRATED. THESE STORMS WILL BE CAPABLE
OF PRODUCING GUSTY OUTFLOW WINDS, BRIEF HEAVY DOWNPOURS, AND
LIGHTNING.
MEANWHILE, DRIER AIR IS ALREADY STARTING TO FILTER INTO WEST-
CENTRAL MONTANA. THIS WILL LIKELY LIMIT THUNDERSTORM DEVELOPMENT
AND COVERAGE IN AREAS FARTHER SOUTH AND EAST AS THE AFTERNOON
PROGRESSES. STORMS WILL DIMINISH THIS EVENING, AND AS SKIES
PARTIALLY CLEAR OVERNIGHT, SOME PATCHY VALLEY FOG COULD DEVELOP IN
LOCATIONS THAT HAVE SEEN RECENT RAINFALL.
HIGH PRESSURE WILL ATTEMPT TO BUILD INTO THE REGION, BUT A
PERSISTENT NORTHWESTERLY FLOW ALOFT WILL KEEP THE PATTERN SOMEWHAT
UNSETTLED, PARTICULARLY ON SATURDAY. DURING THE AFTERNOON,
DAYTIME HEATING WILL COMBINE WITH THE COOL, UNSTABLE NORTHWESTERLY
FLOW TO TRIGGER SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS.
BY SUNDAY, THE FLOW BECOMES MORE WESTERLY AND STABLE, WHICH
SHOULD RESULT IN A DRIER AND SUNNIER FIRST HALF OF THE DAY FOR
MOST. CLOUDS WILL BEGIN TO INCREASE SUNDAY AFTERNOON AS THE NEXT
WEATHER SYSTEM APPROACHES FROM THE PACIFIC. THIS SYSTEM IS
FORECAST TO MOVE INTO THE NORTHERN ROCKIES ON MONDAY, BRINGING A
RETURN TO COOLER AND WETTER CONDITIONS. HOWEVER, THERE IS
CONSIDERABLE UNCERTAINTY IN HOW THIS SYSTEM WILL EVOLVE AND,
CONSEQUENTLY, HOW MUCH RAIN WILL FALL AND WHERE.
ENSEMBLE MODEL CLUSTERS HAVE DIVERGED SIGNIFICANTLY AND ARE NOW
SPLIT NEARLY INTO THIRDS ON THE POTENTIAL OUTCOME:
- SCENARIO 1 (DRIEST): THE SYSTEM TRACKS IN A WAY THAT BRINGS
LIGHT RAIN, BUT MOSTLY DRY IN NORTHWEST MONTANA.
- SCENARIO 2 (MODERATE/TARGETED RAIN): THE SYSTEM DELIVERS MODEST
AND FOCUSED RAINFALL, PRIMARILY IMPACTING NORTH-CENTRAL IDAHO
AND WEST-CENTRAL MONTANA.
- SCENARIO 3 (WIDESPREAD RAIN): A MORE ORGANIZED AND SLOWER-
MOVING SYSTEM PROVIDES BENEFICIAL, WIDESPREAD RAINFALL ACROSS
MOST OF THE FORECAST AREA.
GIVEN THIS UNCERTAINTY, IT IS TOO EARLY TO PINPOINT SPECIFIC
IMPACTS, BUT RESIDENTS SHOULD BE PREPARED FOR THE POSSIBILITY OF A
COOL AND WET START TO THE WORK WEEK.
LOOKING FURTHER AHEAD, MODEL CLUSTERS ARE BEGINNING TO TREND
TOWARD ANOTHER TROUGH IMPACTING THE REGION TO END NEXT WEEK,
SUGGESTING THE ACTIVE, UNSETTLED PATTERN MAY CONTINUE.
AVIATION
AN UPPER-LEVEL LOW WILL CONTINUE TO PROMOTE THE
DEVELOPMENT OF SHOWERS AND THUNDERSTORMS THIS AFTERNOON AND
EVENING, PRIMARILY IMPACTING THE MOUNTAINOUS TERRAIN SURROUNDING
THE TERMINALS. THE MAIN THREATS WITH ANY STORM MOVING OVER AN
AIRFIELD WILL BE BRIEF PERIODS OF MVFR VISIBILITY IN HEAVY RAIN,
ERRATIC WIND GUSTS UP TO 40 KNOTS, SMALL HAIL, AND LIGHTNING. THE
HIGHEST CONFIDENCE IN THUNDERSTORM ACTIVITY REMAINS AT KGPI, WITH
A LOWER, BUT STILL POSSIBLE, THREAT AT KMSO AND KBTM. CONVECTIVE
ACTIVITY IS EXPECTED TO DIMINISH AFTER 0600Z SATURDAY. FOLLOWING
THE CONVECTION, AREAS OF PATCHY VALLEY FOG AND LOW STRATUS
CEILINGS ARE POSSIBLE, PARTICULARLY AT KGPI WHERE MORE MOISTURE IS
PRESENT. THESE CONDITIONS COULD LEAD TO PERIODS OF MVFR OR IFR
BETWEEN 0800Z AND 1400Z SATURDAY BEFORE IMPROVING. VFR CONDITIONS
ARE EXPECTED TO PREVAIL FOR MOST OF THE TAF PERIOD OUTSIDE OF ANY
SHOWER AND STORM ACTIVITY.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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