147
FXUS65 KTFX 180719
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
119 AM MDT FRI SEP 18 2026
KEY MESSAGES
- THUNDERSTORMS, A FEW OF WHICH LOOK TO BE ON THE STRONGER SIDE, DEVELOP
THROUGH THE DAY TODAY, MOSTLY ACROSS CENTRAL AND SOUTHWEST
MONTANA.
- AREAS OF PRECIPITATION CONTINUE INTO SATURDAY, RESULTING IN
INCREASED CONCERNS FOR LOCALIZED FLOODING.
- A DRIER AND WARMER STRETCH LOOKS TO ENSUE LATE WEEKEND THROUGH
MUCH OF NEXT WEEK.
DISCUSSION
- METEOROLOGICAL OVERVIEW:
A SOMEWHAT ELONGATED UPPER LEVEL LOW ACROSS THE PACIFIC NW AND
VICINITY WILL SLOWLY SHIFT EASTWARD THROUGH THE DAY TODAY. ALTHOUGH
FORCING FROM THIS APPROACHING UPPER SYSTEM WILL RESULT IN SHOWERS
AND THUNDERSTORMS DEVELOPING RATHER EARLY IN THE CONVECTIVE CYCLE,
COOL TEMPERATURES ALOFT WILL STILL YIELD SUFFICIENTLY STEEP LAPSE
RATES FOR A FEW STRONGER THUNDERSTORMS. AREAS WHERE SHEAR ENDS UP
BEING MAXIMIZED WILL HAVE A THREAT FOR MARGINALLY LARGE HAIL.
OTHERWISE THE MAIN THREAT WILL BE FOR SLOW MOVING, HEAVY RAIN
PRODUCING THUNDERSTORMS THROUGH THE DAY TODAY.
DIFLUENT UPPER LEVEL FLOW ON THE NORTHEAST SIDE OF THE UPPER LEVEL
SYSTEM LOOK TO COMBINE WITH LINGERING INSTABILITY AND FORCING FROM
THE APPROACHING TROUGH TO RESULT IN AN EXTENDED STRETCH OF RAIN,
MOSTLY NEAR AND SOUTH OF A LINE FROM HELENA TO LEWISTOWN SOUTHWARD
TO A LINE FROM DILLON TO BIG SKY TONIGHT THROUGH THE DAY SATURDAY.
THIS PROLONGED STRETCH OF RAIN WILL RESULT IN CONCERN FOR EXCESSIVE
RAINFALL, PARTICULARLY OVER BURN SCARS, BUT FOR URBAN AREAS AS WELL.
PRECIPITATION SLOWLY EXITS EASTWARD LATE SATURDAY NIGHT AND SUNDAY
MORNING AS A MESSY/WEAK UPPER LEVEL RIDGE BUILDS IN SUNDAY INTO
MONDAY. MUCH OF NEXT WEEK LOOKS DRY, THOUGH WEAK DISTURBANCES CANNOT
BE RULED OUT, WHICH WOULD BE CAPABLE OF PRODUCING SPOTTY LIGHT
PRECIPITATION AT TIMES. -AM
- FORECAST CONFIDENCE & SCENARIOS:
RAIN THROUGH SATURDAY:
CONFIDENCE IS INCREASING IN A CONCENTRATED AREA OF MODERATE
RAINFALL, MAINLY ALONG AND SOUTH OF A HELENA TO LEWISTOWN LINE
SOUTHWARD THROUGH A DILLON TO BIG SKY LINE. OVER 48 HOURS ENDING
SUNDAY MORNING THE CORRIDOR ALONG I-90 EAST OF BUTTE THROUGH BOZEMAN
HAS AROUND A 90% PROBABILITY TO RECEIVE 1 INCH OF RAIN. THESE
PROBABILITIES SLOWLY DECREASE AS YOU GO FURTHER NORTH AND SOUTH OF
THIS CORRIDOR.
THIS RAINFALL LOOKS TO BE PARTIALLY CONVECTIVE DRIVEN AND PARTIALLY
STRATIFORM IN NATURE. THE RISK FOR EXCESSIVE RAINFALL LEADING TO
FLASH FLOODING WILL BE HIGHEST WHILE CONVECTIVE PROCESSES ARE
DOMINANT, WHICH WILL BE PRIMARILY IN THE AFTERNOON AND EVENING. THAT
SAID, BURN SCARS WILL BE SUSCEPTIBLE TO FLASH FLOODING AT ANY TIME
GIVEN THE SIGNIFICANTLY LOWER PRECIPITATION AMOUNT THRESHOLDS. -AM
AVIATION
18/06Z TAF PERIOD
SHOWERS TRACKING NORTHEAST OF EASTERN ID MAY CLIP THE KWYS TERMINAL
OVERNIGHT, BRINGING SOME LOWER CLOUDS. OTHERWISE, MAINLY MID-
HIGH LEVEL CLOUD-COVER CONTINUES TO STREAM ACROSS THE REGION IN
SOUTH TO SOUTHWESTERLY FLOW ALOFT WITH A SMALL RISK FOR LOW CLOUD
AND/OR PATCHY FOG DEVELOPMENT NEAR THE KHVR TERMINAL LATE
TONIGHT/EARLY FRIDAY MORNING. COVERAGE OF SHOWERS AND
THUNDERSTORMS INCREASES ACROSS SW MT FRIDAY AFTERNOON. HOENISCH
HYDROLOGY
PRECIPITABLE WATER (PWATS) VALUES APPROACHING 1", WHICH ARE 2 TO 3
STANDARD DEVIATIONS ABOVE NORMAL, ARE FORECASTED TO RESIDE OVER
PORTIONS OF SOUTHWEST AND CENTRAL MONTANA FROM FRIDAY MORNING
THROUGH SATURDAY MORNING AS PACIFIC MOISTURE STREAMS NORTH TO
NORTHEAST AND ACROSS THE NORTHERN ROCKIES. ECMWF EFIS LARGELY
RANGING BETWEEN 0.75 AND 0.85 SOUTH OF A HELENA TO LEWISTOWN LINE
AND NORTH OF A DILLON TO BOZEMAN LINE, WITH SOTS APPROACHING 1.5
FROM A BUTTE TO WHITE SULPHUR SPRINGS LINE, SUPPORT THE POTENTIAL
FOR A VERY UNUSUAL RAIN EVENT WITH RESPECT TO MID-SEPTEMBER
CLIMATOLOGY THANKS TO THESE ANOMALOUS PWATS. DEVELOPING SHOWERS AND
THUNDERSTORMS OVER THIS TIMEFRAME WILL HAVE SLOW STORM MOTIONS OF
GENERALLY 15-25 MPH, WHICH WILL ALLOW FOR INCREASED RESIDENCE TIME
OVER A GIVEN AREA.
GIVEN ALL OF THESE INDICATORS (I.E. SLOW STORM MOTIONS, ANOMALOUS
PWATS, AND VERY UNUSUAL FORECASTED PRECIPITATION) THE NEED FOR FLASH
FLOOD WATCH FOR PORTIONS OF SOUTHWEST AND CENTRAL MONTANA HAS
INCREASED; MOST NOTABLY FOR AREAS THAT ARE HYDROPHOBIC. WHILE THE
POTENTIAL FOR URBAN FLOODING CAN'T BE RULED, ESPECIALLY IN THE
HELENA AND GALLATIN VALLEYS, CURRENT THINKING IS THAT RECENT BURN
SCARS (REGARDLESS OF SIZE)FROM THE 2026 SEASON AND THE HORSE GULCH
AREA WILL BE THE MOST SUSCEPTIBLE FOR FLASH FLOODING AND DEBRIS
FLOWS. FOR THIS REASON A FLASH FLOOD WATCH HAS BEEN ISSUED FOR
LITTLE BELT AND HIGHWOOD MOUNTAINS, BIG BELT, BRIDGER AND CASTLE
MOUNTAINS, ELKHORN AND BOULDER MOUNTAINS, AND NORTHWEST BEAVERHEAD
COUNTY TO ACCOUNT FOR THE RECENT/PAST BURN SCARS. GIVEN THAT HUNTING
SEASON IS WELL UNDERWAY ACROSS THE AFOREMENTIONED AREAS THOSE
RECREATING NEAR BURN SCARS SHOULD BE PREPARED TO SEEK HIGHER GROUND.
- MOLDAN
PRELIMINARY POINT TEMPS/POPS
GTF 73 51 66 47 / 10 40 40 20
CTB 70 46 67 44 / 10 0 0 0
HLN 74 51 64 47 / 60 90 80 30
BZN 72 46 59 42 / 70 90 90 70
WYS 66 37 57 34 / 90 80 70 50
DLN 68 46 60 43 / 80 70 90 60
HVR 76 48 71 45 / 10 0 10 10
LWT 73 48 59 46 / 30 70 80 80
TFX WATCHES/WARNINGS/ADVISORIES
FLASH FLOOD WATCH FROM THIS EVENING THROUGH SATURDAY EVENING FOR
BIG BELT, BRIDGER AND CASTLE MOUNTAINS-ELKHORN AND BOULDER
MOUNTAINS-GALLATIN VALLEY-LITTLE BELT AND HIGHWOOD MOUNTAINS.
FLASH FLOOD WATCH FROM THIS AFTERNOON THROUGH SATURDAY AFTERNOON
FOR NORTHWEST BEAVERHEAD COUNTY.
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