389
FXUS65 KTFX 172350
AFDTFX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
450 PM MST TUE DEC 17 2024
AVIATION SECTION UPDATED.
KEY MESSAGES
- STRONG, POTENTIALLY HIGH-END WINDS ACROSS MUCH OF THE REGION FROM
EARLY WEDNESDAY INTO WEDNESDAY EVENING.
- HEAVY SNOW AND AREAS OF BLOWING AND DRIFTING SNOW ALONG THE
NORTHERN ROCKY MOUNTAIN FRONT TONIGHT INTO WEDNESDAY.
- MILD AND LARGELY DRY AFTER WEDNESDAY FOR MUCH OF THE REGION.
DISCUSSION
/ISSUED 142 PM MST TUE DEC 17 2024/
- METEOROLOGICAL OVERVIEW:
WEAK, BRIEF UPPER LEVEL RIDGING IS SHIFTING EASTWARD TOWARD FROM THE
PACIFIC NORTHWEST TOWARDS THE NORTHERN ROCKIES IN WAKE OF YESTERDAY
EVENINGS AND LAST NIGHTS LIGHT SNOW. LIGHT NORTHERLY FLOW ACROSS THE
PLAINS HAS RESULTED IN PERSISTENT LOW-LEVEL CLOUDS AND AREAS OF FOG.
THIS LOOKS TO CONTINUE INTO THE EVENING. ADDITIONALLY, FURTHER SOUTH
IN CENTRAL AND SOUTHWEST MONTANA VALLEYS, CLEARING SKIES EARLIER
TODAY COMBINED WITH NEAR SURFACE MOISTURE AND LIGHT WINDS TO RESULT
IN AT LEAST PATCHY FOG. THIS TREND WILL CONTINUE IN SPOTS INTO AT
LEAST EARLY EVENING.
AS QUICKLY AS THE AFOREMENTIONED RIDGING MOVES INTO THE REGION, IT
WILL BE USHERED OUT JUST AS QUICKLY BY A ROBUST TROUGH EJECTING FROM
A MORE BROAD TROUGH ACROSS THE FAR EASTERN PACIFIC OCEAN THIS
EVENING AND TONIGHT. THIS WILL INITIALLY INTRODUCE CHANCES FOR SNOW,
MAINLY ALONG THE ROCKY MOUNTAIN FRONT, BUT ACROSS THE HI-LINE AS
WELL TONIGHT. AMOUNTS ON THE PLAINS LOOK TO BE AROUND AN INCH OR
LESS WITH THIS SYSTEM. THERE IS A LOW-END CHANCE FOR FREEZING
DRIZZLE ACROSS THE PLAINS LATE TONIGHT INTO THE DAY WEDNESDAY AHEAD
OF THE WINDS. CONFIDENCE IN IMPACTS IS NOT HIGH AT THIS TIME,
HOWEVER, AS BUFKIT SOUNDINGS ARE ONLY MARGINALLY CONDUCIVE TO THIS
POTENTIAL THREAT. HEAVER SNOW WILL BE CONFINED TO THE NORTHERN ROCKY
MOUNTAIN FRONT, WHERE INCOMING MOISTURE LOOKS MOST IMPRESSIVE.
MID-LEVEL WESTERLY WIND LOOKS QUITE IMPRESSIVE WITH THIS SYSTEM,
WITH BUFKIT SOUNDINGS SHOWING 60 TO 80 KT WINDS WITHIN ~5K FT OF THE
SURFACE WEDNESDAY ACROSS MUCH OF THE PLAINS. DAYTIME MIXING WILL
HELP TRANSLATE THESE STRONG WINDS TO THE SURFACE. IN ADDITION TO
STRONG, POTENTIALLY HIGH-END WIND GUSTS, THERE EXISTS A THREAT FOR
BLOWING AND DRIFTING SNOW ALONG THE NORTHERN ROCKY MOUNTAIN FRONT
WHERE THE HEAVIEST SNOW WILL OVERLAP WITH THE STRONG WINDS FROM LATE
TONIGHT THROUGH THE DAY WEDNESDAY.
WINDS SLOWLY DIMINISH HEADING INTO WEDNESDAY NIGHT AS UPPER LEVEL
RIDGING BUILDS IN. MILD AND BENIGN CONDITIONS LARGELY LOOK TO
PERSIST THROUGH EARLY NEXT WEEK, THOUGH WEAK IMPULSES TRAVERSING
THROUGH THE RIDGE WILL RESULT IN PERIODS OF BREEZY WINDS AND LIGHT,
MAINLY MOUNTAIN PRECIPITATION. -AM
- FORECAST CONFIDENCE & SCENARIOS:
WIND EARLY WEDNESDAY THROUGH WEDNESDAY NIGHT:
THE AFOREMENTIONED SHORTWAVE WILL BRING STOUT MID AND EVEN LOW LEVEL
WESTERLY WINDS ACROSS THE REGION DURING THE DAY WEDNESDAY. THE
PROBABILITY FOR H85 WINDS OF 55+KTS IS NEARLY 100% ACROSS THE ENTIRE
PLAINS DURING THE DAY WEDNESDAY. DAYTIME MIXING WILL HELP TRANSLATE
THESE WINDS TO THE SURFACE. IT IS WORTH NOTING THAT THE 10TH
PERCENTILE FOR MAX GUSTS (I.E 90% CHANCE FOR A STRONGER MAX GUST) IS
BETWEEN 45 AND 55 MPH FOR CUT BANK (55 MPH), GREAT FALLS (50 MPH),
HELENA (45 MPH), AND LEWISTOWN (45 MPH). THIS GIVES HIGH CONFIDENCE
THAT AN IMPACTFUL PERIOD OF WIND WILL MOVE ACROSS MUCH OF THE REGION
WEDNESDAY. THERE IS A BIT OF UNCERTAINTY WITH RESPECT TO HOW FAR
SOUTH THE STRONGEST WINDS MAKE IT DURING THE AFTERNOON WEDNESDAY.
HENCE, A HIGH WIND WATCH REMAIN IN EFFECT THERE. -AM
ACCUMULATING/BLOWING/DRIFTING SNOW FROM TONIGHT THRU WEDNESDAY :
INCREASING ZONAL FLOW AHEAD OF A COMPACT, BUT POTENT, SHORTWAVE WILL
TRANSPORT PACIFIC MOISTURE UP TO AND JUST EAST OF THE CONTINENTAL
DIVIDE, ESPECIALLY NORTH OF THE MONTANA HWY 200 CORRIDOR, FROM THIS
EVENING THROUGH WEDNESDAY MORNING. BY WEDNESDAY AFTERNOON THE
AFOREMENTIONED SHORTWAVE WILL BEGIN TO MOVE OVER THE NORTHERN
ROCKIES, WHICH WILL BRING A REDUCTION IN OVERALL SNOWFALL RATES AND
SUBSEQUENT AMOUNTS TO THE DIVIDE BUT CONTINUED STRONG AND GUSTY WEST
WINDS. NBM 25TH AND 75TH SNOWFALL PROBABILITIES FOR MARIAS PASS
THROUGH WEDNESDAY AFTERNOON GENERALLY RANGE FROM 3 TO 5", WITH
VALUES QUICKLY FALLING OFF TO BETWEEN 0.5" TO 1.5" NEAR EAST GLACIER
PARK. AS WIND SPEEDS INCREASE OVERNIGHT AND INTO THE DAY ON
WEDNESDAY SURFACE TEMPERATURES ARE EXPECTED TO WARM INTO THE LOW TO
MID-30S AT MARIAS PASS AND UPPER 30S TO LOW 40S NEAR EAST GLACIER
PARK. HOW HIGH TEMPERATURES ACTUALLY WARM ON WEDNESDAY WILL
ULTIMATELY IMPACT HOW MUCH BLOWING AND DRIFTING SNOW OCCURS, WITH
THE BLOW/DRIFT-ABILITY OF ANY FALLING/FALLEN SNOW DRASTICALLY
FALLING TO NEAR ZERO AS TEMPERATURES RISE ABOVE 36 DEGREES. WITH
NBM10TH (LOW END) WIND GUSTS VALUES RANGING FROM 45-60 KTS BETWEEN
MARIAS PASS AND EAST GLACIER THE PROBABILITY FOR VISIBILITY BELOW
1/2 MILE DUE TO FALLING OR FALLEN SNOW RANGES FROM 70-100% GIVEN
THAT TEMPERATURES REMAIN BELOW 36 DEGREES. WITH CONFIDENCE HIGH IN
TEMPERATURES REMAINING BELOW THIS CRITICAL TEMPERATURE THRESHOLD
POINTS WEST OF EAST GLACIER PARK A WINTER STORM WARNING HAS BEEN
ISSUED TO ADDRESS THE IMPACTS FROM THE BLOWING AND DRIFTING SNOW
DESPITE OVERALL SUB-WARNING LEVEL SNOWFALL AMOUNTS. - MOLDAN
RIDGING LATE WEEK INTO EARLY NEXT WEEK:
ALTHOUGH CLUSTER GUIDANCE AND ENSEMBLE AVERAGES FAVOR RIDGING AND
WARM CONDITIONS THURSDAY THROUGH EARLY NEXT WEEK, THERE DOES LOOK TO
BE A FEW EMBEDDED FEATURES TOWARD THE WEEKEND AND EARLY NEXT WEEK
THAT WILL BRING ABOUT BREEZY PERIODS AND PERIODS OF MAINLY LIGHT
MOUNTAIN PRECIPITATION. OVERALL, IMPACTS LOOK TO BE LOW OVER THIS
TIMEFRAME. -AM
AVIATION
18/00Z TAF PERIOD
VFR CONDITIONS WILL PREVAIL AT THE KBZN, KEKS, AND KCTB TERMINALS
DURING THIS TAF PERIOD. HOWEVER, AT THE KCTB TERMINAL THERE IS A 20 -
30% CHANCE FOR SNOW BETWEEN 04Z AND 10Z AND A 20 - 30% CHANCE FOR A
RAIN/SNOW MIX BETWEEN 15Z AND 21Z. THERE WILL BE FOG AND LOW
CEILINGS AROUND THE KHLN, KGTF, AND KLWT TERMINALS FOR A LEAST THE
NEXT COUPLE OF HOURS. DETERMINISTIC AND PROBABILISTIC GUIDANCE
DIFFER ON WHEN THE WIND WILL SHIFT AND INCREASE ENOUGH TO CLEAR THE
FOG AND LOW CEILINGS. AT THE KHVR TERMINAL THERE WILL BE SNOW
BETWEEN 11Z AND 13Z WHICH WILL REDUCE VISIBILITY TO IFR LEVELS. AT
THE KLWT TERMINAL THERE IS A 20 - 30% CHANCE FOR RAIN/SNOW BETWEEN
09Z AND 13Z AND BETWEEN 16Z AND THE END OF THE TAF PERIOD. AT THE
KGTF TERMINAL THERE IS A 20 - 30% CHANCE FOR RAIN AND SNOW BETWEEN
05Z AND 11Z. AT THE KHLN TERMINAL THERE IS A 20% CHANCE FOR SNOW
BETWEEN 07Z AND 09Z AND A 20 - 30% CHANCE FOR RAIN BETWEEN 15Z AND
23Z. AT THE KBZN AND KEKS TERMINALS THERE IS A 20 - 30% CHANCE FOR
RAIN/SNOW BETWEEN 19Z AND 23Z. ANOTHER BIG AVIATION CONCERN DURING
THIS TAF PERIOD IS STRONG TO VERY STRONG WINDS AT ALL TERMINALS.
THE STRONGEST WINDS WILL OCCUR BETWEEN 12Z AND AT LEAST THE END
OF THIS TAF PERIOD WITH THE STRONGEST WINDS PEAKING FROM WEST TO
EAST DURING THAT TIME PERIOD. BY THE END OF THIS TAF PERIOD WINDS
AT THE KEKS, KGTF, AND KCTB TERMINALS WILL GUST UP TO 60 KTS. FROM
12Z ON THROUGH THE END OF THE TAF PERIOD ACROSS ALL TERMINALS
THERE WILL BE LOW-LEVEL WIND SHEAR. DURING MOST OF THIS TAF PERIOD
THERE WILL BE MOUNTAIN OBSCURATION. DURING THE MAJORITY OF THIS
TAF PERIOD THERE WILL BE MOUNTAIN WAVE TURBULENCE. -IG
REFER TO WEATHER.GOV/ZLC FOR MORE DETAILED REGIONAL AVIATION
WEATHER AND HAZARD INFORMATION.
PRELIMINARY POINT TEMPS/POPS
GTF 16 52 23 46 / 30 10 10 0
CTB 8 46 16 41 / 40 30 10 0
HLN 27 50 28 43 / 20 40 0 0
BZN 19 46 20 42 / 10 30 10 0
WYS 14 32 11 33 / 60 50 20 0
DLN 21 45 19 39 / 10 10 0 0
HVR 4 45 5 22 / 50 60 20 0
LWT 16 48 20 42 / 30 30 20 0
TFX WATCHES/WARNINGS/ADVISORIES
WINTER STORM WARNING FROM 9 PM THIS EVENING TO MIDNIGHT MST
WEDNESDAY NIGHT FOR EAST GLACIER PARK REGION.
HIGH WIND WARNING FROM 2 AM WEDNESDAY TO MIDNIGHT MST WEDNESDAY
NIGHT FOR EAST GLACIER PARK REGION-EASTERN GLACIER, WESTERN
TOOLE, AND CENTRAL PONDERA-NORTHERN HIGH PLAINS-SOUTHERN HIGH
PLAINS-SOUTHERN ROCKY MOUNTAIN FRONT.
DENSE FOG ADVISORY UNTIL 6 PM MST THIS EVENING FOR HELENA VALLEY.
HIGH WIND WARNING FROM 5 AM WEDNESDAY TO MIDNIGHT MST WEDNESDAY
NIGHT FOR CASCADE COUNTY BELOW 5000FT-EASTERN PONDERA AND
EASTERN TETON-EASTERN TOOLE AND LIBERTY-GATES OF THE MOUNTAINS-
JUDITH BASIN COUNTY AND JUDITH GAP-MADISON RIVER VALLEY-UPPER
BLACKFOOT AND MACDONALD PASS-WESTERN AND CENTRAL CHOUTEAU COUNTY.
HIGH WIND WATCH FROM WEDNESDAY MORNING THROUGH WEDNESDAY EVENING
FOR BEAVERHEAD AND WESTERN MADISON BELOW 6000FT-GALLATIN AND
MADISON COUNTY MOUNTAINS AND CENTENNIAL MOUNTAINS-NORTHWEST
BEAVERHEAD COUNTY-RUBY MOUNTAINS AND SOUTHERN BEAVERHEAD
MOUNTAINS.
HIGH WIND WARNING FROM 11 AM WEDNESDAY TO 5 AM MST THURSDAY FOR
BEARS PAW MOUNTAINS AND SOUTHERN BLAINE-FERGUS COUNTY BELOW
4500FT-HILL COUNTY-NORTHERN BLAINE COUNTY-SNOWY AND JUDITH
MOUNTAINS.
HIGH WIND WARNING FROM 11 AM WEDNESDAY TO MIDNIGHT MST WEDNESDAY
NIGHT FOR BIG BELT, BRIDGER AND CASTLE MOUNTAINS-CANYON FERRY
AREA-ELKHORN AND BOULDER MOUNTAINS-GALLATIN VALLEY-HELENA VALLEY-
LITTLE BELT AND HIGHWOOD MOUNTAINS-MEAGHER COUNTY VALLEYS-
MISSOURI HEADWATERS.
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