788
FXUS65 KMSO 200913
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
313 AM MDT THU AUG 20 2026
DISCUSSION
KEY MESSAGES:
- RIDGE OF HIGH PRESSURE AND ABNORMALLY HOT TEMPERATURES IN
CONTROL THROUGH END OF THE WEEK PEAKING FRIDAY.
- ELEVATED CONVECTION SPREADING NORTHWARD FRIDAY AFTERNOON INTO
THE OVERNIGHT BRINGING LOCALIZED LIGHTNING AND GUSTY OUTFLOW
WINDS.
- SHOWERS AND THUNDERSTORMS WITH HEAVY RAIN POTENTIAL FOR
SATURDAY.
SURFACE CONDITIONS THIS MORNING SHOW LOCALIZED INCREASES IN
SURFACE WILDFIRE SMOKE ACROSS WESTERN MONTANA, PARTICULARLY IN
WEST-CENTRAL MONTANA NEAR MISSOULA WHERE SURFACE VISIBILITY HAS
DROPPED AND AIR QUALITY IS NOTICEABLY POOR. OVERNIGHT MAXIMUM
RELATIVE HUMIDITY RECOVERIES ARE VERY POOR WITH TEENS AND 20
PERCENT RH OBSERVED ON MID AND UPPER ELEVATION SLOPES ACROSS THE
CWA. NORTHWEST MONTANA HAS SEEN SLIGHTLY BETTER RECOVERIES WITH
MOST LOCATIONS MOISTENING INTO AT LEAST THE 40 PERCENT RH RANGE.
HIGH PRESSURE OVER THE NORTHERN ROCKIES WILL MAINTAIN HOT AND DRY
CONDITIONS THROUGH THE END OF THE WEEK. THE HOTTEST DAY REMAINS
EXPECTED ON FRIDAY WITH HIGH TEMPERATURES IN THE 90S FOR MANY
VALLEY LOCATIONS. TRIPLE-DIGIT HEAT AND LIMITED OVERNIGHT COOLING
RELIEF REMAINS EXPECTED FOR THE LOWER RIVER VALLEYS OF NORTH-
CENTRAL IDAHO. WHILE NOCTURNAL INVERSIONS ENHANCE THE ENTRAPMENT
OF SURFACE SMOKE IN VALLEY LOCATIONS, DAYTIME THERMAL MIXING WILL
ONLY IMPROVE SURFACE AIR QUALITY AND VISIBILITY IF CLEANER AIR
EXISTS ALOFT. WHERE ELEVATED SMOKE PLUMES PERSIST, MIXING WILL
ONLY CONTINUE TO DRAW MORE SMOKE DOWN TO THE SURFACE, MAINTAINING
DEGRADED VISIBILITY AND AIR QUALITY.
MONSOONAL MOISTURE ADVECTION BEGINS FRIDAY AFTERNOON IN LEMHI
COUNTY AND SOUTHWEST MONTANA TRIGGERING HIGH-BASED WIDELY-SCATTERED
SHOWERS AND THUNDERSTORMS CAPABLE OF STRONG CONVECTIVE OUTFLOW
WINDS (GUSTS 30-45 MPH). THIS ELEVATED CONVECTION RISK SPREADS
FURTHER NORTHWARD THROUGH FRIDAY EVENING INTO THE OVERNIGHT AND
SATURDAY MORNING INCREASING THE AREAL THREAT FOR ISOLATED DRY-
LIGHTNING AND GUSTY OUTFLOW POTENTIAL. A WEAK, SHALLOW SURFACE
FRONT ORIGINATING FROM EAST OF THE CONTINENTAL DIVIDE WILL ENHANCE
AN EASTERLY PRESSURE GRADIENT AND RESULTING WINDS LATE FRIDAY
INTO SATURDAY MORNING MOST NOTICEABLE IN THE USUALLY-PRONE EAST-
WEST CANYONS.
AN INCOMING PACIFIC NORTHWEST UPPER-LEVEL TROUGH, WEST OF THE
REGION, LIFTS NORTHEASTWARD ON SATURDAY PHASING WITH A
SIGNIFICANT MONSOONAL MOISTURE SURGE. PRECIPITABLE WATER VALUES
EXCEEDING 1.0 INCHES WILL SUPPORT INCREASING COVERAGE OF SHOWERS
AND THUNDERSTORMS CAPABLE OF HEAVY RAINFALL RATES AND LOCALIZED
CONCERNS FOR EXCESSIVE RUNOFF AND FLASH FLOODING. A COLD FRONT
CROSSES THROUGH FROM THE WEST BY SATURDAY EVENING INTO SUNDAY
MORNING ENDING PRECIPITATION AND CONVECTION CHANCES FOR MOST
LOCATIONS AND USHERING IN A MUCH DRIER AIRMASS. DAYTIME HIGH
TEMPERATURES WILL FALL BACK TO NEAR SEASONAL NORMS (STILL QUITE
WARM) FOR SUNDAY WITH WIDESPREAD ELEVATED WESTERLY WINDS
PERSISTING.
AVIATION
WILDFIRE SMOKE HAS NOTICEABLY INCREASED IN DENSITY
ACROSS WESTERN MONTANA (PARTICULARLY NEAR KMSO) THIS MORNING
REDUCING VISIBILITY TO NEAR 6SM. DAYTIME MIXING MAY IMPROVE
VISIBILITY PENDING UPSTREAM SMOKE PLUME EVOLUTION THROUGH THE
AFTERNOON HOURS. HOT TEMPERATURES WILL PEAK FRIDAY AFTERNOON
MAXIMIZING DENSITY ALTITUDES ACROSS THE REGION. ELEVATED
CONVECTION FRIDAY AFTERNOON NEAR KSMN & KBTM WILL SPREAD NORTHWARD
THROUGH THE EVENING AND OVERNIGHT BRINGING THE THREAT FOR
LOCALIZED LIGHTNING AND TEMPORARY ERRATIC OUTFLOW WINDS TO 25-40
KNOTS.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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