129
FXUS65 KTFX 210915
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
315 AM MDT THU MAY 21 2026
KEY MESSAGES
- AREAS OF RAIN AND MOUNTAIN SNOW DECREASE IN COVERAGE LATER THIS
MORNING, WITH MUCH WARMER AND DRIER CONDITIONS MOVING IN FOR
MOST IF NOT ALL OF THE HOLIDAY WEEKEND.
- AN APPROACHING COLD FRONT WILL BRING BREEZY TO WINDY CONDITIONS
AND INCREASED SHOWER AND THUNDERSTORM ACTIVITY MONDAY INTO
TUESDAY FOLLOWED BY A PERIOD OF COOLER TEMPERATURES, MOUNTAIN
SNOW, AND LOWER ELEVATION RAIN.
UPDATE
FORECAST IS PERFORMING WELL AND NO UPDATE IS PLANNED THIS MORNING.
DISCUSSION
/ISSUED 1209 AM MDT THU MAY 21 2026/
- METEOROLOGICAL OVERVIEW:
SHOWERS AND THUNDERSTORMS HAVE LARGELY TRANSITIONED TO AREAS OF
STRATIFORM RAIN THIS EVENING AND THIS ACTIVITY WILL CONTINUE TO
FOLLOW THE H500 FRONTOGENESIS SOUTHWARD DURING THE OVERNIGHT HOURS.
MOUNTAIN SNOW HAS ALREADY BEEN OBSERVED FOR LOCATIONS ABOVE 7000 FT
AND SNOW LEVELS WILL DROP AS LOW AS 5000 FT BY DAWN IN RESPONSE TO
H700 TEMPERATURES DIPPING TO AROUND -5C TO -8C. THE COLDER AIR ALOFT
IS SLIGHTLY DISJOINTED FROM THE PRECIPITATION SHIELD, BUT MOST
CENTRAL AND SOUTHWEST LOCATIONS ABOVE 6000 FT WILL SEE AT LEAST SOME
ACCUMULATING SNOW. THE CORE OF PRECIPITATION WILL MOVE INTO
SOUTHWEST MONTANA DURING THE COOLEST PART OF THE MORNING, RESULTING
IN THE GREATEST WINTER IMPACTS BEING EXERTED OVER THE HIGHER TERRAIN
OF MADISON AND GALLATIN COUNTIES, MOSTLY IN THE FORM OF COLD AND WET
CONDITIONS FOR OUTDOOR RECREATION.
LINGERING PRECIPITATION DIMINISHES LATER THIS MORNING AS THE H500
FORCING WEAKENS AND MOVES OFF TO THE EAST. A COOL AND MOIST NORTH TO
NORTHWESTERLY FLOW ALOFT DEVELOPS IN THE WAKE OF THE DEPARTING
SYSTEM AND MAINTAINS SCATTERED TO BROKEN CLOUD COVER, WIDELY
SCATTERED SHOWERS/THUNDER, AND CONTINUED SLIGHTLY BELOW AVERAGE
TEMPERATURES FOR THE REMAINDER OF TODAY.
NORTHWESTERLY FLOW ALOFT WILL SLOW THE WARMING PROCESS SOME FOR THE
NEXT COUPLE OF DAYS BEFORE WEAK RIDGING AND STRENGTHENING WESTERLY
FLOW USHERS IN VERY WARM, DRY, AND BREEZY TO WINDY CONDITIONS BY
SUNDAY AND MONDAY. LOWER ELEVATION TEMPERATURES WARM WELL INTO THE
80S DURING THIS PERIOD AND THERE MAY EVEN BE A DAY WITH HIGHS NEAR
90 DEGREES DEPENDING ON THE AMPLITUDE OF THE NEXT TROUGH DUE IN
EARLY NEXT WEEK. THIS NEXT SYSTEM WILL INITIALLY BRING WINDY
CONDITIONS, PRIMARILY TO SOUTHWEST MT, AND INCREASED SHOWER AND
THUNDERSTORM ACTIVITY. THERE WILL ALSO BE AT LEAST A PERIOD OF
COOLER TEMPERATURES AND INCREASED MOUNTAIN SNOW AND LOWER ELEVATION
RAIN ON THE BACKSIDE OF THIS SYSTEM HEADING TOWARDS MIDDLE AND END
OF NEXT WEEK. - RCG
- FORECAST CONFIDENCE & SCENARIOS:
AREAS OF RAIN AND MOUNTAIN SNOW THROUGH THIS MORNING...
ACCUMULATING SNOW LOOKS TO BE LARGELY BE CONFINED TO AREAS ABOVE
6000 TO 6500 FT AND MOST SIGNIFICANT OVER THE NORTHERN PORTIONS OF
THE MADISON/GALLATIN RANGES AND TO A LESSER DEGREE ALONG THE
CONTINENTAL DIVIDE, ROUGHLY FROM HOMESTAKE PASS NORTHWARD TO THE
SOUTHERN ROCKY MOUNTAIN FRONT. THE HIGHEST PROBABILITIES FOR 3
INCHES OF SNOW OR MORE ARE STILL RUNNING HIGHEST (ABOVE 80%) FOR THE
HIGHER PEAKS OF NORTHERN GALLATIN/MADISON COUNTIES WHERE THERE IS
AROUND A 30 TO 40% CHANCE FOR 6 INCHES OF SNOW OR MORE. ROAD IMPACTS
SHOULD FAIRLY BE ISOLATED GIVEN THE SHORT OPPORTUNITY FOR
ACCUMULATION AND WARM SURFACE CONDITIONS, BUT IT WILL BE COLD AND
WET/SLUSHY FOR THOSE WITH MOUNTAIN RECREATION PLANS. TOTAL
RAINFALL/LIQUID EQUIVALENT AMOUNTS WILL GENERALLY RANGE BETWEEN A
TENTH TO A QUARTER INCH FOR MOST CENTRAL AND SOUTHWEST LOWER
ELEVATIONS, THOUGH THERE WILL BE A SHARP DECREASING GRADIENT TO JUST
A FEW HUNDRETHS FOR THE DRIER SOUTHWEST VALLEYS ALONG AND WEST OF
I15. RAINFALL AMOUNTS IN EXCESS OF A HALF INCH WILL BE MORE COMMON
FOR AREAS OVER AND NEAR HIGHER TERRAIN, ESPECIALLY OVER THE
AFOREMENTIONED HIGHER PEAKS OF THE MADISON/GALLATIN RANGES.
PRECIPITATION DIMINISHES LATER THIS MORNING AND WILL BE REPLACED BY
ISOLATED SHOWER AND THUNDER ACTIVITY DURING THE AFTERNOON.
EARLY WEEK THUNDERSTORMS AND WINDY CONDITIONS FOLLOWED BY
MORE WIDESPREAD PRECIPITATION AND MOUNTAIN SNOW...
AN STRONGER AND UNSTABLE SOUTHWESTERLY FLOW ALOFT DEVELOPS AHEAD OF
THE APPROACHING TROUGH MONDAY INTO TUESDAY FOR INCREASED SHOWER AND
THUNDERSTORM ACTIVITY AND MORE BREEZY TO WINDY CONDITIONS. THE
CONCERN FOR STRONGER WINDS WILL PRIMARILY BE OVER SOUTHWEST MT WITH
SOME ENSEMBLES HIGHLIGHTING A HIGHER AMPLITUDE TROUGH AND H700 WINDS
APPROACHING 50 KTS MONDAY INTO TUESDAY. STRONG TO SEVERE
THUNDERSTORMS WILL ALSO BE A CONCERN, ESPECIALLY IF THE COLD FRONTAL
PASSAGE IS IN PHASE WITH DIURNAL HEATING AND THE TROUGH UNDERGOES
EXTENSIVE DEEPENING.
AS MENTIONED IN PREVIOUS DISCUSSIONS, CLOSED/SPLIT TROUGHS, SUCH AS
THIS ONE, OFTEN MOVE A LITTLE SLOWER THAN MODEL GUIDANCE
EXPECTATIONS. THIS MAY RESULT IN HOTTER AND DRIER WEATHER LINGERING
AS LATE AS TUESDAY. THERE WILL ALSO BE AT LEAST A PERIOD OF COOLER
TEMPERATURES AND MORE WIDESPREAD PRECIPITATION/MOUNTAIN SNOW ON THE
BACKSIDE OF THIS SYSTEM HEADING TOWARDS MIDDLE AND END OF NEXT WEEK,
PARTICULARLY IF THE MAIN CIRCULATION STAYS CLOSE ENOUGH TO EXERT A
NOTABLE INFLUENCE OVER THE NORTHERN ROCKIES. - RCG
AVIATION
21/06Z TAF PERIOD
IFR/MVFR/LOW-VFR CONDITONS WILL PREVAIL THROUGH 14-18Z THURSDAY
BEFORE IMPROVING TO MVFR/LOW-VFR, WITH ALL TERMINALS BECOMING VFR
BETWEEN 20/21 TO 21/03Z. SHOWER ACTIVITY WAS CONTINUING TO
DECREASE ACROSS NORTH CENTRAL AND PORTIONS OF CENTRAL MONTANA LATE
THIS EVENING, WITH SHOWER ACTIVITY EXPECTED TO PERSIST SOUTH OF A
KHLN TO KBZN LINE THROUGH 14-18Z THURSDAY. GIVEN THE RECENT
PRECIPITATION AND PARTIAL CLEARING SKIES THERE IS THE POTENTIAL
FOR PATCHY FOG TO DEVELOP NEAR SUNRISE ON THURSDAY, ESPECIALLY
OVER THE PLAINS OF CENTRAL AND NORTH CENTRAL MONTANA; HOWEVER,
LATEST PROBABILISTIC DATA SUPPORTS LESS THAN A 10% CHANCE OF THIS
OCCURRING AT ANY GIVEN TERMINAL. FOR NOW WE WILL MONITOR SATELLITE
TRENDS FOR FOG DEVELOPMENT AND AMEND ACCORDINGLY. - MOLDAN
PRELIMINARY POINT TEMPS/POPS
GTF 59 37 69 42 / 10 0 0 0
CTB 60 35 68 39 / 20 0 0 0
HLN 60 36 68 41 / 30 0 0 0
BZN 55 30 66 36 / 80 0 0 0
WYS 50 22 60 29 / 70 0 0 0
DLN 56 31 65 37 / 50 0 0 0
HVR 62 35 72 41 / 20 10 0 0
LWT 54 33 64 39 / 40 10 0 0
TFX WATCHES/WARNINGS/ADVISORIES
NONE.
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