636
FXUS65 KTFX 161129
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
529 AM MDT SAT MAY 16 2026
AVIATION SECTION UPDATED.
KEY MESSAGES
- LOCALLY BREEZY CONDITIONS DEVELOP SATURDAY AHEAD OF A COLDER
SPRING SYSTEM MOVING IN LATE SATURDAY INTO SUNDAY.
- MOUNTAIN SNOW, WITH A MIX OF LOWER ELEVATION RAIN AND SNOW
SATURDAY NIGHT INTO SUNDAY.
DISCUSSION
/ISSUED 1253 AM MDT SAT MAY 16 2026/
- METEOROLOGICAL OVERVIEW:
LONGWAVE TROUGHING OVER MUCH OF THE WESTERN CONUS WILL BRING WELL
BELOW NORMAL TEMPERATURES AND WIDESPREAD PRECIPITATION TO THE
NORTHERN ROCKIES THROUGH THE WEEKEND, WITH BELOW NORMAL TEMPERATURES
LINGERING INTO THE FIRST HALF OF THE UPCOMING WORK WEEK ALONG WITH
SCATTERED SHOWERS. HIGH TEMPERATURES IN THE 50S TODAY WILL FALL INTO
THE UPPER 30S TO MID-40S BY SUNDAY THANKS TO A STRONG COLD FRONT
PUSHING SOUTH FROM CANADA TONIGHT THROUGH SUNDAY MORNING. THIS COLD
FRONT COMBINED WITH INCREASING SYNOPTIC LIFT WILL HELP TO SUPPORT A
SHIELD OF PRECIPITATION (MOUNTAIN SNOW AND LOWER ELEVATION
RAIN/SNOW) DEVELOPING FROM LATE TONIGHT THROUGH THE DAY ON SUNDAY.
PRIOR TO THIS SHIELD OF PRECIPITATION A LINE OF SHOWERS AND ISOLATED
THUNDERSTORMS IS EXPECTED TO DEVELOP BY THE EARLY MORNING HOURS
TODAY, WITH THESE SHOWERS AND STORMS SLOWLY SLIDING EAST THROUGH THE
AFTERNOON AND EARLY EVENING HOURS, ESPECIALLY FOR AREAS ALONG AND
SOUTH OF THE MONTANA HWY 200 CORRIDOR. BY THE LATE AFTERNOON/EARLY
EVENING HOURS TONIGHT SNOW LEVELS WILL BEGIN TO FALL ACROSS
SOUTHWEST AND CENTRAL MONTANA, WHICH WILL ALLOW PRECIPITATION TO
CHANGE OVER TO SNOW FOR ELEVATIONS ABOVE 5000FT. SNOW LEVELS WILL
CONTINUE TO FALL INTO THE DAY ON SUNDAY THANKS TO THE AFOREMENTIONED
COLD FRONT PUSHING SOUTH; HOWEVER, THE EFFECTS OF A HIGH MAY SUN
ANGLE IS LIKELY TO LIMIT SNOWFALL ACCUMULATION ACROSS LOWER
ELEVATIONS, ESPECIALLY BETWEEN THE HOURS OF 10 AM AND 8 PM SUNDAY.
PRECIPITATION WILL GRADUALLY DECREASE IN AREAL COVERAGE AND
INTENSITY FROM WEST TO EAST SUNDAY NIGHT, BUT DAILY CHANCES FOR
SHOWERS WILL LINGER THROUGH TUESDAY. SEVERAL NIGHTS, MAINLY SUNDAY
NIGHT AND MONDAY NIGHT, ARE LIKELY TO SEE LOW TEMPERATURES FALL INTO
THE UPPER TEENS TO 20S ACROSS ALL ELEVATIONS. - MOLDAN
- FORECAST CONFIDENCE & SCENARIOS:
PRECIPITATION (RAIN OR SNOW) THROUGH SUNDAY NIGHT...
LATEST ECMWF EFIS SUPPORT THE POTENTIAL FOR AN UNUSUAL PRECIPITATION
EVENT (SOME SNOW MIXING IN AT TIMES ACROSS LOWER ELEVATION) WITH
RESPECT TO CLIMATOLOGY ALONG THE NORTHERN SLOPES OF THE
GALLATIN/MADISON MOUNTAINS AND GALLATIN VALLEY, PORTIONS OF THE
MONTANA HWY 200/US HWY 87 CORRIDOR FROM GREAT FALLS TO LEWISTOWN,
NORTHERN SLOPES OF THE BEARS PAW MOUNTAINS, AND EASTERN SLOPES OF
THE ROCKY MOUNTAIN FRONT. BELOW IS THE NBM5.0 PROBABILITIES FOR
RAINFALL ACCUMULATIONS FROM 6AM SATURDAY THROUGH 6AM MONDAY FOR
SELECT CITIES FROM NORTH CENTRAL THROUGH SOUTHWEST MONTANA.
48 HOUR RAINFALL PROBABILITIES
LOCATION 0.10" | 0.25" | 0.50 " | 1.00"
BROWNING 85% | 65% | 15% | <5%
CUT BANK 75% | 40% | 10% | <5%
HAVRE 90% | 85% | 60% | 25%
GREAT FALLS 90% | 80% | 55% | 20%
LEWISTOWN 100% | 95% | 95% | 55%
HELENA 90% | 60% | 25% | 10%
BOZEMAN 95% | 90% | 40% | 10%
DILLON 55% | 35% | 15% | <5%
ENNIS 95% | 95% | 75% | 20%
WEST YELLOWSTONE 50% | 15% | 5% | 0%
ACCUMULATING SNOW FROM TONIGHT THROUGH SUNDAY EVENING...
LATEST ECMWF EFIS SUPPORT THE POTENTIAL FOR AN UNUSUAL SNOWFALL
EVENT WITH RESPECT TO CLIMATOLOGY FOR THE NORTHERN SLOPES OF THE
LITTLE BELT MOUNTAINS AND EASTERN SLOPES OF THE ROCKY MOUNTAIN FRONT
AND IMMEDIATE EASTERN PLAINS OUT TO THE US HWY 89 CORRIDOR FROM
CHOTEAU TO THE PIEGAN PORT OF ENTRY. HERE EFIS RANGE FROM 0.5 TO
0.75, WITH SHIFT OF TAILS (SOTS) OF 1. FURTHER EAST OVER THE PLAINS
NEAR THE NORTHWESTERN SLOPES OF THE BEARS PAW MOUNTAINS INCLUDING
THE HAVRE AREA THE SOTS CLIMB TO AROUND 2.5, WHICH INDICATES THAT AT
LEAST SEVERAL ENSEMBLE MEMBERS ARE SUGGESTING THE POTENTIAL FOR AN
"EXTREME" EVENT WITH RESPECT TO CLIMATOLOGY (MAY CLIMATOLOGY VS
ANYTIME DURING THE WINTER). A CLOSER LOOK AT THE HAVRE AREA REVEALS
THAT 5 ECMWF ENSEMBLE MEMBERS DO SUPPORT SNOWFALL ACCUMULATIONS OF 2
INCHES OR MORE HERE, WITH THE MOST EXTREME MEMBER SUGGESTING UPWARDS
OF 6 INCHES THROUGH THE TIMEFRAME. WHILE ACCUMULATING SNOW CAN'T BE
RULED OUT FOR THE HAVRE AREA, ESPECIALLY THE MID-SLOPES OF THE BEARS
PAW MOUNTAINS, THE OVERALL TIMING OF THE SNOWFALL (DURING MOST OF
THE DAY ON SUNDAY) DOES NOT SEEM FAVORABLE FOR SNOWFALL
ACCUMULATIONS ON ROAD SURFACES TO IMPACT TRAVEL. FOR THIS REASON A
WINTER WEATHER ADVISORY WAS NOT ISSUED. FURTHER WEST AND SOUTH
ACROSS THE PREVIOUSLY MENTIONED ROCKY MOUNTAIN FRONT AND LITTLE BELT
MOUNTAIN AREAS A WINTER WEATHER ADVISORY HAS BEEN ISSUED FOR THE
MOUNTAINS AS CONFIDENCE IS HIGH ENOUGH THAT SNOWFALL ACCUMULATIONS
WILL AMOUNT TO ENOUGH TO IMPACT TRAVEL AND OUTDOOR RECREATION,
ESPECIALLY THROUGH SUNDAY MORNING. OVER THE ADJACENT PLAINS OUT TO
THE US HWY 89 CORRIDOR CONFIDENCE WAS NOT HIGH ENOUGH TO ISSUE AN
ADVISORY AT THIS TIME, BUT FUTURE SHIFTS MAY CONSIDER TO DO SO
SHOULD MODELS TREND HIGHER WITH SNOWFALL AMOUNTS AND COLDER WITH
TEMPERATURES.
BELOW IS THE NBM5.0 PROBABILITIES FOR SNOWFALL ACCUMULATIONS FROM
12AM SATURDAY THROUGH 12AM SUNDAY FOR SELECT MOUNTAIN PASSES ACROSS
NORTH CENTRAL THROUGH SOUTHWEST MONTANA.
24 HOUR SNOWFALL PROBABILITIES
LOCATION 2" | 4" | 6" | 8"
MARIAS PASS 60% | 15% | 5% | <5%
ROGERS PASS 80% | 55% | 25% | 10%
KINGS HILL PASS 75% | 35% | 15% | 10%
LEWISTOWN DIVIDE 30% | 15% | 5% | <5%
MACDONALD PASS 40% | 15% | 10% | 5%
DEEP CREEK PASS 10% | <5% | <5% | <5%
BOULDER HILL 25% | 5% | <5% | <5%
ELK PARK PASS 10% | <5% | <5% | <5%
HOMESTAKE PASS 10% | <5% | <5% | <5%
BOZEMAN PASS 15% | 5% | <5% | <5%
CHIEF JOSEPH PASS 30% | 10% | 5% | <5%
BIG HOLE PASS 10% | 5% | <5% | <5%
MONIDA PASS <5% | 0% | 0% | 0%
RAYNOLDS PASS <5% | <5% | 0% | 0%
TARGHEE PASS <5% | 0% | 0% | 0%
- MOLDAN
AVIATION
16/12Z TAF PERIOD
A LARGE UPPER LEVEL DISTURBANCE MOVING ACROSS THE WESTERN US WILL
BRING LOWERING CIGS AND INCREASING PRECIPITATION COVERAGE TO THE
NORTHERN ROCKIES THROUGH THE TAF PERIOD, WITH CIGS FALLING FROM
LOW-VFR/VFR THIS MORNING TO IFR/MVFR BY THE EVENING HOURS TONIGHT
TO EARLY MORNING HOURS ON SUNDAY. MOUNTAIN OBSCURATION WILL OCCUR
THROUGHOUT MUCH OF THE 1612/1712 TAF PERIOD, ESPECIALLY BEYOND
16/21Z TO 17/03Z. A COLD FRONT DIVING SOUTH FROM CANADA LATE
TONIGHT THROUGH SUNDAY MORNING WILL BRING A WIND SHIFT TO THE
NORTH AND GUSTY WINDS. - MOLDAN
PRELIMINARY POINT TEMPS/POPS
GTF 55 33 40 28 / 40 40 90 70
CTB 55 30 38 25 / 10 60 80 10
HLN 50 33 42 31 / 80 40 80 50
BZN 58 29 45 28 / 80 90 30 30
WYS 52 25 35 20 / 30 60 30 20
DLN 57 26 44 22 / 30 40 50 30
HVR 59 35 43 29 / 30 50 80 70
LWT 53 31 39 27 / 80 90 80 90
TFX WATCHES/WARNINGS/ADVISORIES
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO MIDNIGHT MDT
SUNDAY NIGHT FOR BIG BELT, BRIDGER AND CASTLE MOUNTAINS-EAST
GLACIER PARK REGION-ELKHORN AND BOULDER MOUNTAINS-LITTLE BELT
AND HIGHWOOD MOUNTAINS-SOUTHERN ROCKY MOUNTAIN FRONT-UPPER
BLACKFOOT AND MACDONALD PASS.
WINTER WEATHER ADVISORY FROM 5 PM THIS AFTERNOON TO 6 AM MDT
SUNDAY FOR GALLATIN AND MADISON COUNTY MOUNTAINS AND CENTENNIAL
MOUNTAINS.
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