690
FXUS65 KTFX 210906
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
306 AM MDT TUE JUL 21 2026
KEY MESSAGES
- SCATTERED SHOWER AND THUNDERSTORM ACTIVITY DEVELOPS OVER
SOUTHWEST MONTANA THIS AFTERNOON AND EVENING.
- SHOWERS AND THUNDERSTORMS BECOME MORE WIDESPREAD OVER CENTRAL
AND SOUTHWEST MONTANA ON WEDNESDAY WITH SOME STORMS PRODUCING
TORRENTIAL DOWNPOURS CAPABLE OF LOCALIZED FLOODING, ESPECIALLY
FOR BURN SCARS, URBAN AREAS, AND OTHER FLOOD PRONE LOCALES.
- MOST LOCATIONS WILL SEE WIDELY SCATTERED STORMS ON THURSDAY.
THE STRONGER CELLS MAY CONTAIN STRONG, GUSTY WINDS, HAIL, AND
DOWNPOURS IN ADDITION TO CLOUD TO GROUND LIGHTNING.
- SUMMER HEAT BUILDS IN FOR THE SECOND HALF OF THE WEEK WITH
LOCALIZED ELEVATED FIRE WEATHER CONCERNS FRIDAY AND SATURDAY.
DISCUSSION
- METEOROLOGICAL OVERVIEW:
MONSOON MOISTURE CONTINUES TO STREAM EASTWARD ON THE NORTHERN
PERIPHERY OF A STRENGTHENING RIDGE OF HIGH PRESSURE CENTERED OVER
THE GREAT BASIN. WHILE THE MOISTURE ALOFT THICKENS FROM TOP TO
BOTTOM, VARIABLE MID- AND HIGHER LEVEL CLOUDS AND A FEW SPOTTY
LIGHT SHOWERS OR VIRGA WILL BE THE MAIN IMPACT THIS MORNING.
WIDELY SCATTERED SHOWERS AND THUNDERSTORMS WILL BEGIN CREEPING
BACK NORTHWARD INTO THE SOUTHWEST THIS AFTERNOON AND EVENING;
OTHERWISE, MOSTLY DRY AND WARM CONDITIONS WILL PREVAIL ELSEWHERE.
A SUBTLE SHORTWAVE WILL MOVE THROUGH THE RIDGE ON WEDNESDAY AND
WILL PROVIDE SUFFICIENT FORCING FOR A MORE CONCENTRATED AREA OF
SHOWERS AND EMBEDDED THUNDERSTORMS OVER SOUTHWEST AND PORTIONS OF
CENTRAL MONTANA. WHILE ML CAPE VALUES LOOK TO BE AROUND OR LESS
THAN 500 J/KG, THE COMBINATION OF FORCING AND PWATS WELL OVER AN
INCH WILL INCREASE INSTANCES OF EXCESSIVE DOWNPOURS CAPABLE OF
LOCALIZED FLOODING, PARTICULARLY FOR RECENT BURN SCARS, URBAN
AREAS, AND OTHER FLOOD PRONE LOCALES. HIGHER CAPE VALUES AROUND
500 TO 800 J/KG LOOK TO BE PRESENT ON THE NORTHERN FRINGE OF THIS
AREA OF PRECIPITATION, MAINLY FOR AREAS ALONG THE CENTRAL MT
HIGHWAY 87/200 CORRIDOR SOUTHWARD TO I90. THESE AREAS WILL SEE
SPOTTIER RAINFALL, BUT HIGHER OCCURRENCES OF STRONG, TO SEVERE
STORMS WITH STRONG, GUSTY WINDS, HAIL, AND DOWNPOURS IN ADDITION
TO CLOUD TO GROUND LIGHTNING.
THE EXITING SHORTWAVE WILL PUSH THE RICHEST MOISTURE TO THE EAST
AND LEAVE A DRIER BOUNDARY LAYER IN ITS WAKE ON THURSDAY; HOWEVER,
ANOTHER SHORTWAVE MOVES IN FROM THE SOUTHWEST AND BRINGS MORE
SCATTERED SHOWERS AND STORMS TO MUCH OF THE FORECAST AREA.
FORECAST SOUNDINGS SHOW INVERTED V SOUNDINGS, A KEY INDICATOR OF
STRONGER THUNDERSTORM OUTFLOW WINDS. HAIL, DOWNPOURS, AND CLOUD TO
GROUND LIGHTNING WILL ALSO BE CONCERNS WITH THURSDAY'S STORMS.
THE COMBINATION OF THE DRYING CONDITIONS AND THE BUILDING RIDGE
WILL ALSO WARM AFTERNOON TEMPERATURES WELL INTO THE 90S AND EVEN
SOME 100S FOR MOST LOWER ELEVATION LOCATIONS.
THE MOISTURE PROFILE WILL CONTINUE TO THIN OUT FROM WEST TO EAST
ON FRIDAY, BUT THERE MAY BE ENOUGH LINGERING INSTABILITY AND
MOISTURE FOR SOME SHOWERS AND STORMS OVER THE EASTERN PORTIONS OF
THE FORECAST AREA. OTHERWISE, STRENGTHENING DRY SOUTHWESTERLY FLOW
WILL INCREASE SURFACE WINDS FOR AT LEAST THE FIRST HALF OF THE
WEEKEND WHILE TEMPERATURES REMAIN HOT. THIS MAY INTRODUCE ELEVATED
FIRE WEATHER CONCERNS IF THE MID- WEEK RAINS ARE TOO LIGHT OR
SCATTERED IN NATURE. - RCG
- FORECAST CONFIDENCE & SCENARIOS:
ENSEMBLE GUIDANCE HAS BEEN PERSISTENT IN NARROWING DOWN THE
HEAVIER RAIN THREAT TO SOUTHWESTERN AREAS SOUTH OF I90. NBM
PROBABILISTIC GUIDANCE HAS INCREASED THE CHANCES FOR A HALF INCH
OF RAIN FOR THE 24 HOUR PERIOD ENDING 6 AM THURSDAY UP TO AROUND
30% FOR LOWER ELEVATIONS AND 60 TO 80% OVER HIGHER TERRAIN. THIS
IS SIGNIFICANT BECAUSE THESE PROBABILITIES TYPICALLY DO NOT
REFLECT THE LOCALIZED HEAVIER RAINFALL FROM THUNDERSTORMS. THE
MAIN UNCERTAINTY IS THE LOWER CAPE AND WHETHER THESE RAINFALL
AMOUNTS WILL FALL IN A TIMEFRAME LESS THAN SIX HOURS IN ORDER TO
PRODUCE HIGHER LOCALIZED FLASH FLOODING OF BURN SCARS, URBAN
CENTERS, AND OTHER FLOOD PRONE AREAS.
RAINFALL BECOMES MUCH MORE SCATTERED IN NATURE FOR CENTRAL AND
SOUTHWESTERN LOCATIONS NORTH OF I90, BUT HIGHER CAPE AND MOISTURE
LEVELS MAY RESULT IN MUCH HIGHER RAINFALL RATES IN SHORT PERIODS
OF TIME FOR MORE IMPACTFUL FLOODING, ALBEIT IN A HIGHLY LOCALIZED
FASHION. THE WEATHER PREDICTION CENTER HAS RECENTLY RAISED THE
FLOOD RISK TO THE 'SLIGHT' LEVEL (5 TO 15% CHANCE) FOR AREAS SOUTH
OF I90 WHILE MAINTAINING A 'MARGINAL' RISK LEVEL FOR CENTRAL AND
SOUTHWESTERN AREAS NORTH OF I90 AND SOUTH OF THE CENTRAL MT
HIGHWAY 87/200 CORRIDOR. THIS SITUATION WILL CONTINUE TO BE
MONITORED FOR POTENTIAL FLOOD PRODUCTS OVER THE NEXT 24 HOURS.
- RCG
AVIATION
21/06Z TAF PERIOD
WILDFIRE SMOKE ALOFT WILL LINGER AROUND THROUGH THE EVENING WITH
THE PRIMARY IMPACT BEING TO SLANTWISE VISIBILITY DURING
ASCENT/DESCENT. AT SURFACE LEVEL, NO SIGNIFICANT REDUCTIONS IN
VISIBILITY OR CEILINGS ARE EXPECTED. MID-LEVEL CLOUDS WILL
CONTINUE TO FILL IN ACROSS THE REGION THROUGH THE NIGHT WITH SOME
OBSCURATION OF THE HIGHEST MOUNTAIN PEAKS POSSIBLE.
ISOLATED SHOWERS AND THUNDERSTORMS ARE POSSIBLE TUESDAY AFTERNOON,
MAINLY ALONG THE CONTINENTAL DIVIDE AND SOUTH OF THE I-90
CORRIDOR. KWYS AND KEKS HAVE THE HIGHEST CHANCES FOR THUNDERSTORMS
WITH THE MAIN THREATS BEING LIGHTNING AND GUSTY WINDS. PERIODS OF
MVFR CIGS AND MOUNTAIN OBSCURATION ARE POSSIBLE ACROSS SOUTHWEST
MONTANA THROUGH THE EARLY EVENING HOURS ASSOCIATED WITH
THUNDERSTORM DEVELOPMENT. -THOR
PRELIMINARY POINT TEMPS/POPS
GTF 85 60 90 62 / 10 0 10 20
CTB 80 56 88 59 / 20 10 0 10
HLN 88 63 90 63 / 10 10 30 60
BZN 88 58 90 59 / 10 10 40 70
WYS 87 51 83 51 / 40 30 70 70
DLN 84 57 85 56 / 30 20 50 80
HVR 85 59 93 61 / 10 0 0 10
LWT 82 56 86 57 / 10 10 20 30
TFX WATCHES/WARNINGS/ADVISORIES
NONE.
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