869
FXUS65 KRIW 050203
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
803 PM MDT SUN MAY 4 2025
KEY MESSAGES
- SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS CONTINUE THIS
EVENING, MAINLY ACROSS NORTHWEST WYOMING. STRONG OUTFLOW WINDS
GUSTING 40 TO 50 MPH ARE POSSIBLE WITH THIS ACTIVITY.
- WIDESPREAD RAIN AND MOUNTAIN SNOW MONDAY THROUGH TUESDAY,
ESPECIALLY ACROSS CENTRAL WYOMING AND THE STAR VALLEY AREA.
LIGHT SNOW IS POSSIBLE (60% CHANCE) ACROSS LOWER ELEVATIONS
TUESDAY MORNING.
- PRECIPITATION MAY LEAD TO ELEVATED RIVER LEVELS TUESDAY INTO
WEDNESDAY.
UPDATE
ISSUED AT 1245 PM MDT SUN MAY 4 2025
CUMULUS ARE DEVELOPING OVER THE HIGH TERRAIN TO START THIS
AFTERNOON. CLOUDS ARE ALSO INCREASING FROM THE SOUTH, OVER SOUTHERN
WYOMING. SOME OF THESE CLOUDS HAVE GOTTEN BIG ENOUGH TO PRODUCE
SHOWERS. ONLY SMALL ADJUSTMENTS WERE MADE TO THE FORECAST THIS
AFTERNOON, WHICH WAS INCORPORATING LATEST MODEL DATA. SHOWERS AND
THUNDERSTORMS ARE STILL EXPECTED ACROSS MUCH OF THE AREA. HOWEVER,
THEY WILL REMAIN ISOLATED TO SCATTERED. CHANCES ARE GENERALLY 15% TO
40%, THE BEST OF THOSE CHANCES OVER SOUTHERN AND WESTERN WYOMING, AS
WELL AS OVER MOUNTAINS. AN ISOLATED SHOWER OR STORM IS POSSIBLE
ALMOST ANYWHERE THOUGH. THE MAIN HAZARD WITH ANY CONVECTION REMAINS
WINDS; GUSTS UP TO AROUND 50 MPH ARE POSSIBLE.
LOOKING TO THE SPRING STORM FOR MONDAY AND TUESDAY, THE OVERALL
THOUGHTS REMAIN CONSISTENT WITH THE PREVIOUS FORECAST (SEE FULL
DISCUSSION BELOW). AMOUNTS HAVE INCREASED SLIGHTLY FOR THE HIGHER
TERRAIN OF THE EAST FACING MOUNTAINS; SUCH AS THE ABSAROKAS, WIND
RIVERS, OWL CREEKS, GREEN/RATTLESNAKES, AND CASPER MOUNTAIN.
CONTINUING TO SEE CONVERGENCE IN THE MODELS FOR THE STAR VALLEY AREA
MONDAY AFTERNOON AND EVENING, SO PRECIPITATION TOTALS THERE REMAIN
SIMILAR.
HAVE DROPPED TEMPERATURES SLIGHTLY FOR TUESDAY MORNING, AS MODEL
TRENDS WOULD SUPPORT. THIS BRINGS SNOW LEVELS DOWN TO ABOUT 5500 TO
6500 FEET, SUPPORTING SOME LOW ELEVATION SNOW. BEING MAY, WOULD NOT
EXPECT WINTER IMPACTS, BUT TUESDAY MORNING COULD SEE SOME SNOW
ACCUMULATE ON GRASS, WITH MAYBE EVEN MINOR ROAD/PAVEMENT
ACCUMULATIONS. ANY SNOW SHOULD MELT PRETTY QUICKLY.
DISCUSSION
ISSUED AT 432 AM MDT SUN MAY 4 2025
TODAY WILL SEE THE RIDGE THAT HAS BEEN OVER THE AREA FOR THE PAST
TWO DAYS BEGIN TO BREAK DOWN. THIS IS IN RESPONSE TO A LONGWAVE
TROUGH APPROACHING FROM THE WEST, AS WELL AS AN ASSOCIATED 500MB LOW
NOW DEEPENING ACROSS SOUTHERN CALIFORNIA. THIS WILL BRING THE
LEADING IMPACTS FROM THIS SYSTEM AS MOISTURE ADVECTS INTO WYOMING
FROM JUST ABOUT EVERY DIRECTION. WHILE DEWPOINTS WILL REMAIN LOW,
STEEP LAPSE RATES AND INCREASING DIVERGENCE ALOFT WILL HELP TO KICK
OFF SHOWERS AND THUNDERSTORMS BY EARLY AFTERNOON ACROSS WESTERN AND
SOUTHERN AREAS. THIS ACTIVITY WILL MOVE FROM SOUTHEAST TO NORTHWEST,
WITH ADDITIONAL CONVECTION DEVELOPING FARTHER NORTH AND EAST BY LATE
AFTERNOON. GIVEN THE DRY BOUNDARY LAYER AND LARGE TEMPERATURE-
DEWPOINT SPREADS, THE PRIMARY CONCERN WILL BE STRONG OUTFLOW WIND
GUSTS WITH THIS ACTIVITY; GUSTS OF 40 TO 50 MPH ARE POSSIBLE WITH
ANY SHOWER OR STORM THROUGH THE EVENING. ACCORDINGLY, MUCH OF
THE PRECIPITATION WILL NOT REACH THE GROUND TODAY, ESPECIALLY
EAST OF THE DIVIDE WHERE TEMPERATURES WILL PEAK 15 TO 20 DEGREES
ABOVE NORMAL. THE OVERALL DRY CONDITIONS AND STRONG CONVECTIVE
OUTFLOW POTENTIAL WILL LEAD TO ELEVATED FIRE WEATHER CONDITIONS
TODAY.
PRECIPITATION WILL END AROUND MIDNIGHT TONIGHT. THE LOW WILL THEN
MOVE EAST OVER THE ARIZONA/NEW MEXICO BORDER OVERNIGHT, BEFORE
DRIFTING AND EXPANDING OVER THE FOUR CORNERS REGION THROUGH TUESDAY.
THIS WILL BE THE IMPETUS FOR A RESPECTABLE PRECIPITATION EVENT
OVER WYOMING THROUGH THIS TIME. RAIN AND MOUNTAIN SNOW WILL
BEGIN TO DEVELOP BY EARLY MONDAY AFTERNOON, BECOMING WIDESPREAD
MONDAY EVENING, AND THEN CONTINUING THROUGH TUESDAY. WITH A
FAVORABLE NORTHEASTERLY UPSLOPE FLOW, THE HIGHEST AMOUNTS ARE
EXPECTED EAST OF THE DIVIDE, ESPECIALLY IN THE LANDER FOOTHILLS
AND CASPER AREAS. FORECAST PRECIPITABLE WATER VALUES ARE STILL
OVER 0.60", WHICH WOULD PUT IT NEAR THE CLIMATOLOGICAL MAXIMUM
FOR THIS TIME OF YEAR AT KRIW (SPC SOUNDING CLIMATOLOGY).
HOWEVER, RECENT TRENDS WITH THE LOW STAYING A LITTLE FARTHER SOUTH
HAVE RESULTED IN QPF DECREASING A BIT. PRECIPITATION AMOUNTS ACROSS
CENTRAL WYOMING ARE GENERALLY IN THE 0.4" TO 0.9" RANGE, WITH
THE HIGHER VALUES TOWARD THE NORTHEASTERLY FACING MOUNTAIN
SLOPES. THIS INCLUDES THE LANDER AND CASPER AREAS, WHICH
CURRENTLY HAVE ABOUT A 50% CHANCE OF SEEING AT LEAST 1" OF
PRECIPITATION DURING THE PERIOD FROM MONDAY THROUGH TUESDAY. ON
THE FLIP SIDE, ONE LOCATION THAT HAS TRENDED UP WITH QPF
RECENTLY IS STAR VALLEY. THIS APPEARS TO BE DUE TO AN EMBEDDED
SHORTWAVE AT THE TOP OF THE TROUGH DROPPING IN FROM MONTANA
MONDAY AFTERNOON, LEADING TO A PERIOD OF CONVERGENCE OVER
EASTERN IDAHO AND WESTERN WYOMING. CURRENTLY, THE STAR VALLEY
AREA HAS AROUND A 60% CHANCE OF SEEING AT LEAST 1" OF
PRECIPITATION, MAINLY FROM MONDAY AFTERNOON THROUGH MONDAY
NIGHT.
ELSEWHERE, ACCUMULATIONS ARE GENERALLY EXPECTED TO BE UNDER HALF AN
INCH. 700MB TEMPERATURES APPROACHING NEGATIVE 7C TUESDAY MORNING
CERTAINLY MEAN MOUNTAIN SNOW, AND LIKELY EVEN A CHANGE TO ALL
SNOW FOR A TIME IN THE LOWER ELEVATIONS AS WELL. IF A CHANGEOVER
TO SNOW DOES OCCUR TUESDAY MORNING, IT VERY WELL MAY LAST
THROUGH MOST OF THE MORNING AS TEMPERATURES STRUGGLE TO WARM.
HOWEVER, ACCUMULATIONS WOULD LIKELY BE LIMITED TO ELEVATED OR
VEGETATED SURFACES, AND WIDESPREAD WINTER WEATHER IMPACTS ARE
NOT EXPECTED. THE PRIMARY CONCERN WITH THIS EVENT IS THE
POTENTIAL FOR RIVER AND SMALL STREAM FLOODING. ELEVATED RIVER
LEVELS CAN BE EXPECTED, AIDED IN PART BY THE WARM WEEKEND
PRECEDING THE RAIN.
PRECIPITATION WILL COME TO AN END TUESDAY EVENING. FURTHER AHEAD,
ENSEMBLE CLUSTER GUIDANCE CONTINUES TO INDICATE A QUICK RETURN TO
RISING 500MB HEIGHTS BY WEDNESDAY. THIS FAVORS WARMER AND DRIER
CONDITIONS THROUGH THE END OF NEXT WEEK.
AVIATION /00Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 338 PM MDT SUN MAY 4 2025
SHOWERS CONTINUE TO BUBBLE ACROSS THE STATE THIS AFTERNOON, WITH
ISOLATED LIGHTNING STRIKES NOTED. SHOWER ACTIVITY REMAINS
ISOLATED IN NATURE, BUT HAS ENOUGH COVERAGE TO WARRANT TEMPO
GROUPS FOR THE NEXT COUPLE HOURS. ONCE DAYTIME HEATING ENDS,
MOST SHOWER ACTIVITY WILL BE ENDING, WITH ONLY A FEW SHOWERS
STILL IN THE VICINITY OF KCOD LATER INTO THE EVENING. VFR
CONDITIONS WILL THEN HOLD INTO THE OVERNIGHT, BUT THEN THE COLD
FRONT WILL REINTRODUCE LOW CLOUDS IN THE PRE-DAWN HOURS. CIGS
WILL DROP QUICKLY, STARTING IN THE NORTH BEFORE SPREADING SOUTH
AND WEST OF THE DIVIDE NO LATER THAN 18Z. LOW CLOUDS WILL ALSO
GRADUALLY BRING LIGHT RAIN SPREADING SOUTHWARD THROUGH THE DAY.
LOW CIGS AND VIS WILL CONTINUE THROUGH THE REST OF THE DAY AND
INTO MONDAY NIGHT.
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
NONE.
UPDATE...WITTMANN
DISCUSSION...MYERS
AVIATION...STRAUB
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