903
FXUS65 KRIW 051045
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
445 AM MDT MON MAY 5 2025
KEY MESSAGES
- TURNING MUCH COLDER TODAY WITH RAIN SPREADING ACROSS THE AREA
THIS AFTERNOON.
- COLD TONIGHT WITH RAIN CHANGING TO SNOW, WITH SNOW LEVELS
POSSIBLY FALLING AS LOW AS 5200 FEET AT TIMES.
- DRIER AND WARMER FROM THURSDAY INTO THE WEEKEND.
DISCUSSION
ISSUED AT 232 AM MDT MON MAY 5 2025
IF YOU HAVE BEEN IN A CAR ACCIDENT, ESPECIALLY IF YOU HAVE BEEN
REAR ENDED, YOU PROBABLY KNOW ABOUT THE NECK INJURY CALLED
WHIPLASH. IT IS CAUSED BY THE RAPID AND SUDDEN MOVEMENT OF YOUR
NECK. AND OVER THE NEXT COUPLE OF DAYS, WE WILL HAVE WHAT I CALL
WEATHER WHIPLASH. SOME LOCATIONS WILL SEE HIGH TEMPERATURES UP
TO 40 DEGREES COLDER TUESDAY WHEN COMPARED TO OUR VERY WARM
SUNDAY. AND, WITH THE COLDER TEMPERATURES WILL COME THE RETURN
OF SOME OF THE WHITE STUFF THAT STARTS WITH THE LETTER "S", THIS
INCLUDES IN THE LOWER ELEVATIONS WHERE OUT POPULATION CENTERS
ARE. NOT ALL OF THEM, BUT SOME. SO, LETS GET INTO IT.
WE HAVE THREE CONCERNS WITH THIS SYSTEM THROUGH TUESDAY: RAINFALL,
SNOW AND WIND. WE WILL START WITH RAINFALL, OR SHOULD I SAY
PRECIPITATION AMOUNTS. THE WAVE OF SHOWERS AND STORMS TODAY IS NOW
MOVING INTO MONTANA AND MOST AREAS SHOULD START OFF THE DAY LARGELY
DRY AND MUCH OF THE MORNING MAY END UP BEING DRY. THIS WILL CHANGE
THOUGH AS WE GET INCREASING MOISTURE AND THE AREA GETS INTO A
SQUEEZE PLAY BETWEEN TWO SYSTEMS. ONE AN UPPER LEVEL LOW MOVING
ACROSS THE DESERT SOUTHWEST AND THE OTHER A TROUGH DROPPING
SOUTHWARD OUT OF MONTANA. PRECIPITATION WILL FILL IN THIS AFTERNOON
AS THE MONTANA SYSTEM DROPS SOUTHWARD AND FLOW TURNS MORE EAST TO
NORTHEASTERLY, ENHANCING UPSLOPE FLOW. AS A RESULT, THE HEAVIEST QPF
AMOUNTS WILL FALL ACROSS THE EAST FACING SLOPES AND ADJACENT AREAS
OF THE BIGHORNS, ABSAROKAS, WIND RIVER RANGE AND POSSIBLY CASPER
MOUNTAIN. A SECONDARY AREA MAY OCCUR IN SOUTHWESTERN WYOMING,
ESPECIALLY IN LINCOLN COUNTY. ALL OF THESE AREAS HAVE AT LEAST A 1
OUT OF 2 CHANCE OF QPF AMOUNTS OVER A HALF AN INCH THROUGH WEDNESDAY
MORNING, WITH SOME AREAS HAVING AS MUCH AS A 1 IN 3 CHANCE OF AN
INCH OR MORE OF PRECIPITATION, LIKE THE LANDER FOOTHILLS. AS FOR
FLOODING CHANCES, THERE ARE SOME. HOWEVER, THIS LOOKS TO BE A COLDER
SYSTEM, ESPECIALLY AFTER SUNSET TONIGHT WHEN THE HEAVIEST
PRECIPITATION IS EXPECTED TO ARRIVE. ESPECIALLY IN AREAS EAST OF THE
DIVIDE WHERE SNOW LEVELS WOULD FALL TO AROUND 7500 FEET BY 6 PM THIS
EVENING, SHUTTING OF RAPID SNOWMELT. THE MAIN CONCERN MAY BE THE
STAR VALLEY, WHERE SNOW LEVELS REMAIN HIGHER, AROUND 9000 FEET. NOT
ENOUGH CONFIDENCE FOR A FLOOD WATCH, BUT THIS NEEDS TO BE WATCHED.
ALL GUIDANCE SHOWS THE HEAVIEST QPF MONDAY NIGHT, WITH LIGHTER
PRECIPITATION LINGERING INTO TUESDAY.
NOW ON TO THE SNOWY PORTION OF THE SYSTEM. SNOW LEVELS WILL REMAIN
HIGH THROUGH THE MORNING, BUT BEGIN TO FALL THIS AFTERNOON AS COLDER
AIR MOVES IN BEHIND THE MONTANA SYSTEM. THE QUESTION THIS MORNING IS
HOW LOW WILL THEY GO. THIS DEPENDS ON LOCATION. MOST LOCATIONS WEST
OF THE DIVIDE ONLY HAVE 700 MILLIBAR TEMPERATURES FALL TO AS LOW AS
MINUS 3 CELSIUS, KEEPING THE POPULATIONS CENTERS MAINLY RAIN OR ONLY
A SMALL, SLUSHY ACCUMULATION. THINGS GET MORE COMPLICATED EAST OF
THE DIVIDE. WE DID ISSUE SOME HIGHLIGHTS THIS MORNING FOR THE
BIGHORN RANGE AND EASTERN SLOPES OF THE WIND RIVER RANGE. AMOUNTS
ARE BORDERLINE, BUT BOTH LOCATIONS HAVE A DECENT CHANCE (GREATER
THAN A 3 IN 4 CHANCE) OF SEEING 6 INCHES OR MORE OVER THE HIGHER
PEAKS. IN ADDITION, WITH THE BULK OF THE SNOW FALLING AT NIGHT,
THERE IS A GREATER CHANCE OF SEEING ACCUMULATION ON THE PASSES. WE
ALSO THOUGHT ABOUT ONE FOR THE ABSAROKAS. HOWEVER, IMPACTS ARE LESS
HERE GIVEN THE ONLY IMPACTED MAIN ROAD IS STILL HAS A SEASONAL
CLOSURE. THE HIGHEST ELEVATIONS WILL LIKELY SEE OVER 6 INCHES
THOUGH. WITH THE HEAVIEST NOW EXPECTED UNTIL TONIGHT, THE DAY
SHIFT CAN TAKE A FINAL LOOK AT IT.
NOW FOR THE LOWER ELEVATIONS. WE HAVE A TWO MAIN AREAS OF
CONCERN, THE WIND RIVER BASIN AND WESTERN BIGHORN BASIN AND THE
ADJACENT FOOTHILLS. ELSEWHERE, SNOW LEVELS SHOULD REMAIN HIGH
ENOUGH FOR ALL RAIN OR JUST A RAIN / SNOW MIX. IN THE ABOVE
MENTIONED LOCATIONS, 700 MILLIBAR TEMPERATURES FALL TO AS LOW AS
MINUS 5 TO MINUS 6.5 CELSIUS, WHICH WOULD PUT SNOW LEVELS
BETWEEN 5200 TO 6000 FEET. ONE AREA OF CONCERN IS BETWEEN CODY
AND THERMOPOLIS, WHERE HIGHWAY 120 RISES TO AROUND 6000 FEET.
THE OTHER IS THE WIND RIVER BASIN, ESPECIALLY WEST OF RIVERTON
AND THE LANDER FOOTHILLS. THIS IS WHERE 1 OR 2 DEGREES CAN MAKE
A BIG DIFFERENCE. PRECIPITATION RATES WILL ALSO MATTER, WHERE
HEAVY RATES WILL LEAD TO HEAVY SNOW AND LIGHTER RATES WILL SEE
MAINLY RAIN. IN ADDITION, THE GROUND IS WARM WITH THE RECENT
WARM DAYS AND SNOW MAY HAVE TROUBLE STICKING INITIALLY,
ESPECIALLY ON PAVED SURFACES. ENSEMBLE GUIDANCE IS ALSO SPLIT
WITH THE HREF ENSEMBLE MUCH SNOWIER THAN THE NBM VERSION. AT
THIS TIME, WITH MOST LOCATIONS HAVING LESS THAN 1 IN 2 CHANCE OF
3 INCHES OR MORE, WE WILL HOLD OFF ON ANY ADVISORIES FOR NOW.
HOWEVER, IN THESE LOCATIONS, DON'T BE SURPRISED IF YOU WAKE UP
TO A FEW INCHES OF SNOW TUESDAY MORNING, ALTHOUGH MAINLY ON NON
PAVED SURFACES. ANY IMPACTS SHOULD END FAIRLY QUICKLY TUESDAY
MORNING TEMPERATURES RISE AND THE STRONG MAY SUN MELTS THE SNOW.
NOW FOR THE WIND PORTION. THE MAIN AREAS OF CONCERN WILL BE THE COLD
ADVECTION / NORTHWEST FLOW LOCATIONS, MAINLY THE NORTHERN BIGHORN
BASIN AND NORTHERN JOHNSON COUNTY. THERE IS ALSO A CHANCE IN
SWEETWATER COUNTY AS WELL. ONE THING WE ARE LACKING IN NORTHERN
WYOMING IS LACK OF GOOD JET FORCING. PROBABILISTIC GUIDANCE ONLY
GIVES AROUND 1 IN 3 CHANCE OF GUSTS PAST 50 MPH AROUND BUFFALO WITH
SIMILAR CHANCES IN THE BIGHORN BASIN. SO, NO HIGHLIGHTS HERE. THE
STRONGEST WIND WOULD BE THIS EVENING THOUGH, SO THERE IS STILL TIME
TO ISSUE HIGHLIGHTS DURING THE DAY. THE OTHER AREA IS SWEETWATER
COUNTY. THERE WILL BE A BIT MORE JET ENERGY HERE AND WE DO HAVE A
FEW 50 KNOT WIND BARBS. BUT AGAIN, MOST AREAS HAVE LESS THAN 1 IN 2
CHANCE OF WIND GUSTS PAST 50 MPH AND THE AREAS THAT DO HAVE FEW
PEOPLE, ROADS OR IMPACTS. SO AGAIN, NO HIGHLIGHTS WILL BE ISSUED. IT
WILL BE RATHER WINDY ACROSS MUCH OF THE AREA TONIGHT THROUGH, WITH
STRONG WINDS LINGERING IN SOUTHERN WYOMING THROUGH TUESDAY BEFORE
THINGS WIND DOWN.
THINGS LOOK QUIETER AND WARMER ON WEDNESDAY. ALTHOUGH SOME LINGERING
MOISTURE AND CYCLONIC CURVATURE WILL KEEP A FEW SHOWERS AROUND
THROUGH THE DAY. WARMER WEATHER THEN RETURNS FOR THURSDAY THROUGH
THE END OF THE WEEK AS FLOW TURNS SOUTHWESTERLY.
AVIATION /12Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 445 AM MDT MON MAY 5 2025
VFR CONDITIONS TO START THE PERIOD BUT WILL QUICKLY DETERIORATE
BEHIND A COLD FRONT THIS MORNING. MVFR CEILINGS EXPECTED AT ALL
LOCATIONS BY 15-19Z, WITH RAIN MORE EMPHASIZED ON STATION UNTIL
AFTER 19-21Z. WINDS WILL CONTINUE TO BE GUSTY UP TO 25-35KTS,
HIGHEST AT RKS/CPR AS PER USUAL. A SWITCHOVER TO SNOW IS MORE
LIKELY (40-60%) FOR ALL LOCATIONS OVERNIGHT AFTER 06Z INTO
TUESDAY MORNING. MINOR ACCUMULATIONS BUT AN AVIATION IMPACTS
NONE THE LESS. WITH THE SNOW, WILL COME THE MORE LIKELY LIFR TO
IFR CONDITIONS. CONFIDENCE IS HIGH ON BAD FLYING CONDITIONS
AFTER THIS POINT, BUT LOWER CONFIDENCE ON HOW BAD IT WILL BE
DOWN TO LESS THAN A MILE AND LESS THAN 300 FEET AT SOME
LOCATIONS. FOR NOW, QUIET TO START WITH INCREASING WINDS AND
LOWERING CLOUDS THIS MONDAY AFTERNOON AND THROUGHOUT THE
REMAINDER OF THE PERIOD INTO MUCH OF TUESDAY AS WELL FOR THE
NEXT COUPLE OF TAF CYCLES.
PLEASE SEE THE AVIATION WEATHER CENTER AND/OR CWSU ZDV AND ZLC FOR
THE LATEST INFORMATION ON ICING AND TURBULENCE FORECASTS.
RIW WATCHES/WARNINGS/ADVISORIES
WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO NOON MDT
TUESDAY FOR WYZ008-009.
WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON MDT
TUESDAY FOR WYZ015.
DISCUSSION...HATTINGS
AVIATION...LOWE
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