798
FXUS65 KMSO 181830
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
1230 PM MDT SAT JUL 18 2026
DISCUSSION
KEY MESSAGES:
- STRONG TO SEVERE THUNDERSTORMS AFTERNOON HOURS AND EVENING,
BRINGING THE 5% THREAT OF STRONG WINDS, LARGE HAIL, AND HEAVY
RAIN FROM NORTH-CENTRAL IDAHO INTO SOUTHWEST MONTANA.
- ELEVATED FIRE WEATHER THIS WEEKEND FROM HOT, DRY, AND BREEZY
CONDITIONS, ESPECIALLY IN NORTHWEST MONTANA.
- ANOTHER SURGE OF MONSOON MOISTURE EXPECTED TO ARRIVE BY
TUESDAY. MORE CLOUDS AND/OR THUNDERSTORM CHANCES TO INCREASE.
WE MAY EXPERIENCE FURTHER MOISTURE INCREASE FROM REMNANTS OF
EAST PACIFIC HURRICANE ELIDA BY WEDNESDAY OR THURSDAY.
WATER VAPOR IMAGERY SHOWS A NICE AREA OF MONSOON MOISTURE EMBEDDED
IN SOUTHWEST FLOW OVER EASTERN OREGON INTO SOUTHERN IDAHO. THIS
MOISTURE INTERACTED WITH THE VERY WARM AIR MASS THAT WAS IN PLACE
AND TRIGGERED MORNING THUNDERSTORMS. THEIR MOVEMENT HAS BEEN
RATHER FAST, GENERALLY 30 TO 35 MPH TO THE NORTHEAST WHICH CAN
INFER THAT THERE IS A DECENT MID-LEVEL JET STREAK ASSOCIATED WITH
THE MOISTURE. CURRENT AND FORECAST EFFECTIVE SHEAR DEPICTS 40 TO
50 KNOTS COULD BE POSSIBLE OVER IDAHO COUNTY AND WEST-
CENTRAL/SOUTHWEST MONTANA INTO THIS AFTERNOON. CURRENT CONDITIONS
OVER IDAHO COUNTY WOULD SUGGEST THAT THERE COULD BE A COUPLE OF
SUPERCELL/SEVERE THUNDERSTORMS THAT COULD FORM, CAPABLE OF
PRODUCING LARGE HAIL OR DAMAGING WINDS. CURRENT BOUNDARY LAYER
MOISTURE LEVELS ARE BEING UNDERDONE BY THE HIGH RESOLUTION MODELS
WHICH COULD BE PART OF THE REASON THEY ARE NOT RESOLVING THAT MANY
STORMS.
FURTHER TO THE EAST, THE RAP IS SHOWING A BULLS-EYE OF 1000 TO
1500 JOULES/KG OF MIXED LAYER CAPE/INSTABILITY OVER THE BITTERROOT
VALLEY AND POINTS EASTWARDS. THIS IS THE REGION WHERE WE HAVE
HIGHER CONFIDENCE FOR STORM THREATS.
ONE CAVEAT TO TODAY'S STORMS IS THE FACT THAT THERE IS A DRY
LAYER FOUND OVER NORTHWEST MONTANA TOWARDS CENTRAL IDAHO. THIS IS
DEPICTED BY LOOKING AT THE 24-HOUR RELATIVE HUMIDITY CHANGE ON OUR
WEATHER AND HAZARDS DATA VIEWER. RH'S ARE GENERALLY 10 TO 30
PERCENT LOWER FROM A LINE FROM DICKEY LAKE SOUTH AND WEST TOWARDS
GRANGEVILLE. THIS DRY AIR MAY SHIFT FURTHER EAST WITH TIME TODAY
AS THE WESTERLY WIND GRADIENT TIGHTENS A BIT. WITH THAT SAID, A
FEW ISOLATED SHOWERS OR THUNDERSTORMS COULD INITIATE OFF THE
MOUNTAINS EAST OF THIS LINE (I.E. BLACKTAIL MOUNTAIN, MISSION
MOUNTAINS AND POINTS SOUTH). IF THEY WERE TO DEVELOP, THEY COULD
BECOME VERY STRONG, VERY QUICKLY AND PRODUCE HAIL AND GUSTY
OUTFLOWS.
THE STRONGER STORMS COULD ALSO PRODUCE TORRENTIAL RAINFALL. THE
PRECIPITABLE WATER VALUES(PWAT, MEASUREMENT OF HOW MUCH MOISTURE
IS IN THE ATMOSPHERE) ARE GENERALLY AROUND 0.90" WHICH IS +2
STANDARD DEVIATIONS FOR THIS TIME OF YEAR. YESTERDAY ONE OF THE
STORMS PRODUCED 0.54" IN 15 MINUTES, 0.86" IN 30 MINUTES JUST
SOUTHWEST OF CLEARWATER JUNCTION.
FOR SUNDAY, THERE IS A BIT MORE DRYING ACROSS THE REGION, BUT THE
SOUTHERN IDAHO COUNTY TO SOUTHWEST MONTANA COULD SEE SHOWERS AND
ISOLATED THUNDERSTORMS DEVELOP. WORKING AGAINST THE SHOWER
DEVELOPMENT COULD BE MODEST WARMING OCCURRING ALOFT.
ON MONDAY, THE UPPER RIDGE BUILDS MORE OVER THE WESTERN U.S. AS IT
DOES THIS, IT WILL ALSO CONTINUE TO DRAW UP MONSOON MOISTURE. SOME
OF THIS COULD MAINLY BE HIGH-LEVEL CLOUD COVER, WHILE OTHER
LOCATIONS TOWARDS LEMHI COUNTY COULD GET ENOUGH HEATING TO HELP
BRING THE "CAP" OR CAPPING INVERSION AND ALLOW STORMS TO FIRE OFF.
ON TUESDAY, NORTH TO EAST WINDS MAY DEVELOP. THE LATEST RRFS SMOKE
MODEL SHOWS SOME OF THE CANADIAN SMOKE MOVING INTO THE GLACIER
REGION. THE UPPER LEVEL RIDGE AXIS MAY BE PRESENT FROM COLORADO
NORTH AND WEST TO SPOKANE. USUALLY IN THIS PATTERN, THE ELEVATED
WARMTH WILL SUPPRESS OR PREVENT SHOWERS FROM GROWING VERY WELL
(MORE STABLE). BUT WITH PWATS APPROACHING 1.00", ALL IT COULD TAKE
IS A WEAK PUSH OF MID-LEVEL MONSOON MOISTURE AND ASSOCIATED
SHORTWAVE OR MOUNTAIN INITIATION TO FIRE OFF A FEW STORMS.
MODELS DEPICT THAT THE REMNANTS OF HURRICANE ELIDA MAY TRACK
NORTHWARDS, WEST OF CALIFORNIA BY TUESDAY. THIS COULD BRING EVEN
MORE SUBTROPICAL MOISTURE INTO THE NORTHERN ROCKIES. HOW THIS
EVOLVES ON WEDNESDAY IN TERMS OF SHOWERS OR THUNDERSTORMS WILL
DEPEND ON TIMING OF AND MAGNITUDE OF CLOUD COVER.
AVIATION
THUNDERSTORMS ARE EXPECTED AGAIN TODAY, MAINLY IN
SOUTHWEST MONTANA AFFECTING TERMINALS KBTM, KSMN, AND KHRF. THE
HREF ENSEMBLE MODEL HAS SIGNIFICANT CONVECTIVE INITIATION STARTING
STARTING BETWEEN 18/19Z AND 18/20Z THIS AFTERNOON BETWEEN
TERMINALS KSMN AND KHRF, AND ALL HREF MEMBERS SHOW STORMS OUT OF
THE AREA BY 19/04Z. HEAVY RAIN AND HAIL HAVE BEEN THE MAIN IMPACTS
FROM THE STORMS IN THIS AIR MASS THE LAST FEW DAYS, AND THAT'S
WHAT WE EXPECT AGAIN TODAY AS WELL AS WIND GUSTS AROUND 40 MPH.
MODELS ARE TRENDING DOWNWARD ON THUNDERSTORM POTENTIAL FOR SUNDAY
THE 19TH.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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