Moisture and instability ahead of an upper level low allowed for afternoon thunderstorms to develop across portions of central and northern Wyoming. Many of the storms were moving south to north within the counter-clockwise rotation of the low.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS RIW wrote it up.
Moisture and instability ahead of an upper level low allowed for afternoon thunderstorms to develop across portions of central and northern Wyoming. Many of the storms were moving south to north within the counter-clockwise rotation of the low. Heavy rain, small hail, and funnel clouds were all observed in the Wind River and Big Horn basins.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Riverton, WY wrote its last area forecast discussion before the first event at May 15, 10:33 AM MST, 39 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 1133 AM MDT FRI MAY 15 2015 .SHORT TERM...FRIDAY THROUGH SUNDAY NIGHT QUITE AN ACTIVE DAY YESTERDAY AS THUNDERSTORMS DEVELOPED NEAR THE WIND RIVER RANGE AND TRACKED NORTHEASTWARD AND BLOSSOMED INTO A COMPLEX OF THUNDERSTORMS THAT PRODUCED HAIL AS WELL AS SOME HEAVY RAIN IN SPOTS. A FEW SPOTS IN THE BIG HORN RANGE PICKED UP AROUND 2 INCHES OF RAIN LAST EVENING. EVEN LATE AT NIGHT...SHOWERS ARE ONGOING WITH SOME THUNDER IN THE EAST AS A LOW LEVEL JET MOVES UP IN EASTERN WYOMING. ANOTHER WAVE OF MOISTURE IS CROSSING THE AREA AS I WRITE THIS BUT SHOULD BE ENDING BY MORNING. EXPECT ANOTHER WET DAY TODAY AS THE SLOW MOVING UPPER LEVEL LOW CROSSES THE GREAT BASIN. SOME LEE CYCLOGENESIS WILL DEVELOP ACROSS COLORADO TODAY AND THEN MOVE NORTH-NORTHEASTWARD INTO THE WESTERN HIGH PLAINS. YET ANOTHER SHORTWAVE WILL LIFT ACROSS THE AREA AND BRING ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS. THE DIFFERENCE THIS TIME IS IT MAY BE AN EARLIER SHOW AS BOTH THE NAM AND GFS SHOW A DRY SLOT WORKING ACROSS MUCH OF THE AREA THIS EVENING AND ACROSS MOST OF THE AREA BY LATE TONIGHT. THE EXCEPTION WILL BE NORTHWESTERN WYOMING WHERE SOME MOISTURE WILL WRAP AROUND INTO THAT AREA. A LITTLE TRICKY TODAY WITH SOME THINGS. ONE IS INSTABILITY AS A LOT OF THE AREA MAY START CLOUDY. MODELS DO SHOW DECENT CAPE VALUES HOWEVER...ESPECIALLY ACROSS NORTHERN WYOMING. ANOTHER IS FOR MAYBE LOCALIZED FLOODING. AREAS NEAR THE BIG HORN RANGE AS WELL AS JOHNSON COUNTY SAW A LOT OF RAIN YESTERDAY. PRECIPITABLE WATER VALUES ARE DECENT..APPROACHING AN INCH...SO HEAVY RAIN IS A POSSIBILITY. HOWEVER...THE MODELS HAVE NOT BEEN DOING A GOOD JOB PICKING OUT THE AREAS OF THE BEST THUNDERSTORMS. SO WE KEPT THINGS GENERIC ONCE AGAIN WITH THE HIGHEST POPS IN THE NORTH AND EAST. AS FOR SATURDAY...MUCH OF THE AREA WILL START IN THE DRY SLOT SO IT LOOKS LIKE A DRY START TO THE DAY. AGAIN...THE EXCEPTION WILL BE IN THE NORTHWEST. THE NAM EVEN HINTS THAT THE SOUTHEASTERN HALF OF THE AREA WILL BE LARGELY DRY. AS A RESULT...WE RAISED TEMPERATURES EAST OF THE DIVIDE A FEW MORE DEGREES. HOWEVER...COLDER AIR WILL BEGIN TO WRAP INTO NORTHWESTERN WYOMING AS A COLD FRONT MOVES IN LATER IN THE DAY. THE MODELS SHOW 700 MILLIBAR TEMPERATURES FALLING TO MINUS 2 TO MINUS 3 WHICH COULD LOWER SNOW LEVELS TO AROUND 7500 FEET BY THE END OF THE DAY. THE COLD FRONT THEN SWEEPS ACROSS THE AREA SATURDAY NIGHT AND USHERS IN COLDER TEMPERATURES FOR MANY AREAS. AS FOR THE DREADED S WORD...SNOW...THE NAM IS COOLER WITH 700 MILLIBAR TEMPERATURES FALLING TO MINUS 5 IN THE NORTHWEST AND AROUND MINUS 3 IN EAST OF THE DIVIDE. THIS WOULD PUT THE LEVELS AROUND 7000 FEET EAST OF THE DIVIDE SO LOOK FOR MAINLY RAIN FOR THE LOWER ELEVATIONS ALTHOUGH SEVERAL INCHES OF SNOW ARE POSSIBLE FOR THE HIGHER ELEVATIONS. THERE IS STILL SOME DISAGREEMENT ON THE PLACEMENT OF THE BEST QPF SO WE DID NOT MAKE A LOT OF CHANGES. EXPECT A GUSTY NORTH TO NORTHWEST WIND TO DEVELOP FOLLOWING THE FRONTAL PASSAGE WITH STRONG WINDS POSSIBLE IN AREAS FAVORED WITH STRONG NORTHWEST COLD ADVECTION...LIKE THE BUFFALO AREA. BY SUNDAY...IT LOOKS COOL AND UNSETTLED. AGAIN...SOME DIFFERENCES IN THE MODELS WITH THE GFS HANGING MORE MOISTURE BACK WHILE THE NAM DRIES THINGS OUT. AGAIN...WITH THE DETAILS STILL MUDDY...WE MADE FEW CHANGES. BUT IT DOES LOOK LIKE A 3C'S KIND OF DAY...CLOUDY...COOL AND CRUDDY. MOISTURE SHOULD SHIFT OUT OF THE AREA SUNDAY NIGHT AS SOME SHORT WAVE RIDGING BUILDS ACROSS THE STATE. .LONG TERM...MONDAY THROUGH FRIDAY AN UPPER LOW/TROUGH WILL PUSH INTO CALIFORNIA MONDAY. SOME DYNAMICS COULD IMPACT THE WEST/SOUTHWEST...BUT THE EAST SHOULD REMAIN DRY UNDER UPPER LEVEL RIDGING AND RELATIVELY STABLE CONDITIONS. THIS WILL BEGIN TO CHANGE MONDAY NIGHT...AS THE UPPER LEVEL SYSTEM PULLS INTO THE GREAT BASIN WITH CHANCES OF PRECIP GRADUALLY INCREASING ACROSS THE ENTIRE AREA AS THE DIFFLUENT FLOW ALOFT OVERSPREADS THE REGION. THIS UPPER LOW IS EXPECTED TO GRADUALLY PUSH ACROSS THE NORTHERN/CENTRAL ROCKIES TUESDAY AND WEDNESDAY. BY WEDNESDAY...MODELS ARE SHOWING THIS UPPER LOW/TROUGH FILLING IN...AND GETTING SHEARED OUT AS IT PUSHES INTO THE PLAINS. TUESDAY AND TUESDAY NIGHT...LOOK TO BE THE BEST TIME FOR WIDESPREAD PRECIPITATION...AS MODELS ARE SHOWING ABOVE AVERAGE PRECIPITABLE WATER VALUES IN A MOIST EASTERLY SURFACE FLOW OVER MUCH OF THE AREA. BOTH THE ECMWF AND GFS KEEP A RELATIVELY STABLE ATMOSPHERE OVER THE NORTH AND EAST...WHILE THE WEST AND SOUTH STILL SHOW SOME INSTABILITY. WILL KEEP MENTION OF THUNDER CONFINED MAINLY TO THE WEST AND/OR SOUTH BOTH DAYS...AS THE GFS AND ECMWF HAVE HINTED AT A STABLE...BUT RAINY ATMOSPHERE NORTH AND EAST. ON THURSDAY...DESPITE BEING IN BETWEEN SYSTEMS...WITH WEAK RIDGING OVER THE AREA...EXPECT ISOLATED TO LOCALLY SCATTERED CONVECTION MAINLY OVER THE WEST. DRIER CONDITIONS ARE EXPECTED OVER THE EAST...ESPECIALLY IN BASINS AWAY FROM THE MOUNTAINS. MEDIUM RANGE MODELS THEN SHOW ANOTHER UPPER LOW OVER THE DESERT SOUTHWEST/SOUTHERN ROCKIES BY FRIDAY. ALTHOUGH THE TRACK IS A BIT FAR SOUTH...A RELATIVELY MOIST AND UNSTABLE ATMOSPHERE AND THE FACT THE TRACK COULD CHANGE IS ENOUGH TO HAVE SLIGHT VALLEY/CHANCE MOUNTAIN POPS FOR NOW. TEMPERATURES WILL BE BELOW TO MUCH BELOW AVERAGE...ESPECIALLY FOR HIGHS THROUGH AT LEAST WEDNESDAY...WITH TEMPS BECOMING MORE SEASONAL THURSDAY AND FRIDAY. && .AVIATION.../18Z ISSUANCE/ ISOLATED SHOWERS OVER THE REGION WILL SEE A GENERAL INCREASE IN COVERAGE WITH A TRANSITION TO ISOLATED EMBEDDED THUNDERSTORMS THIS AFTERNOON. THIS CONDITION WILL CONTINUE UNTIL 00Z-03Z...WHEN SHOWERS AND ISOLATED THUNDER WILL DECREASE ACROSS MOST OF THE FORECAST AREA EXCEPT FOR NORTHERN TERMINALS SUCH AS KCOD AND KWRL WHERE -SHRA/VCSH/VCFG WILL PERSIST THROUGH THE FORECAST PERIOD. EXPECT AREAS TEETERING BETWEEN VFR/MVFR THROUGH EARLY EVENING...WITH BRIEF IFR CONDITIONS IN LOW CIGS DUE TO LOCALLY HEAVY RAIN. ALSO...EXPECT GUSTY WIND AT TIMES AT MOST TERMINAL SITES TODAY IN HEAVIER SHOWERS. THEN MOST TERMINAL SITES EAST OF THE DIVIDE SHOULD SEE IMPROVEMENT TO VFR CONDITIONS...EXCEPT AGAIN FOR KCOD AND KWRL WHICH WILL CONTINUE TO FLIRT WITH MVFR. ADDITIONALLY...MOUNTAINS WILL BE OCCASIONALLY OBSCURED THROUGH THE FORECAST PERIOD. && .FIRE WEATHER... FIRE DANGER RELATIVELY LOW THROUGH THE WEEKEND AS SEASONAL TO COOL AND RELATIVELY MOIST CONDITIONS PERSIST. HOWEVER...IT WILL BE UNSTABLE AGAIN FRIDAY AND SATURDAY...OFFERING UP CHANCES FOR MORE THUNDERSTORMS AND ASSOCIATED LIGHTNING. LATE SPRING SNOWS LIKELY TO RETURN TO THE HIGHER ELEVATIONS AGAIN THIS WEEKEND...WHILE SNOW LEVELS MAY EVEN FALL TO THE VALLEY AND BASIN FLOORS OF THE AREA...MIXING WITH RAINFALL AT TIMES SATURDAY NIGHT AND EARLY SUNDAY. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...HATTINGS LONG TERM...WM AVIATION...BRAUN FIRE WEATHER...HATTINGS
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 400 AM MDT FRI MAY 15 2015 WYZ001>020-022>030-161200- ABSAROKA MOUNTAINS-BIGHORN MOUNTAINS SOUTHEAST- BIGHORN MOUNTAINS WEST-CASPER MOUNTAIN-CODY FOOTHILLS- EAST SWEETWATER COUNTY-FLAMING GORGE- GREEN MOUNTAINS AND RATTLESNAKE RANGE-JACKSON HOLE-LANDER FOOTHILLS- NATRONA COUNTY LOWER ELEVATIONS-NORTH BIG HORN BASIN- NORTHEAST JOHNSON COUNTY-OWL CREEK AND BRIDGER MOUNTAINS- ROCK SPRINGS AND GREEN RIVER-SALT RIVER AND WYOMING RANGES- SOUTH LINCOLN COUNTY-SOUTHEAST BIG HORN BASIN- SOUTHEAST JOHNSON COUNTY-SOUTHWEST BIG HORN BASIN-STAR VALLEY- TETON AND GROS VENTRE MOUNTAINS-UPPER GREEN RIVER BASIN- UPPER GREEN RIVER BASIN FOOTHILLS-UPPER WIND RIVER BASIN- WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST-WIND RIVER MOUNTAINS WEST- YELLOWSTONE NATIONAL PARK- 400 AM MDT FRI MAY 15 2015 THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT ISOLATED TO SCATTERED AFTERNOON THUNDERSTORMS...SOME WITH HAIL... GUSTY WINDS AND HEAVY RAIN. .DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY SATURDAY AND SUNDAY...SIGNIFICANT MOUNTAIN SNOWFALL POSSIBLE OVER THE NORTHERN MOUNTAINS. MONDAY...ISOLATED TO LOCALLY SCATTERED THUNDERSTORMS...ESPECIALLY OVER THE WEST AND SOUTHWEST. TUESDAY AND WEDNESDAY...ANOTHER STORM SYSTEM HAS THE POTENTIAL TO BRING WIDESPREAD MOUNTAIN SNOW...AND VALLEY RAIN OVER THE REGION. .SPOTTER INFORMATION STATEMENT... SPOTTERS ARE URGED TO REPORT STRONG WIND GUSTS...HAIL...AS WELL AS HEAVY RAINFALL OVER A HALF AN INCH. THANK YOU. TEAM RIVERTON
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE RIVERTON WY 1209 PM MDT FRI MAY 15 2015 WYZ017-019-020-022-030-151930- CASPER MOUNTAIN-EAST SWEETWATER COUNTY- GREEN MOUNTAINS AND RATTLESNAKE RANGE- NATRONA COUNTY LOWER ELEVATIONS-WIND RIVER BASIN- 1209 PM MDT FRI MAY 15 2015 SHOWERS AND THUNDERSTORMS CONTINUE TO STREAM THROUGH SOUTHEASTERN WYOMING THIS AFTERNOON...MAINLY IMPACTING EASTERN SWEETWATER... SOUTHEASTERN FREMONT...AND NATRONA COUNTIES. STORMS MAY CONTAIN GUSTY WIND TO 30 MPH...PERIODS OF HEAVY RAIN...AND SMALL HAIL. THOSE ENJOYING OUTDOOR ACTIVITIES SHOULD KEEP AN EYE ON THE SKY AND SEEK SHELTER INDOORS WHEN STORMS APPROACH YOUR AREA. BRANHAM
The Riverton/Lander radar (KRIW), 54 km away, was scanning. The volume nearest the first event began at 18:10 UTC.
2 flash flood, 1 funnel cloud. Local standard time, as filed.
The public witnessed four funnel clouds south of US Highway 20/26 between Shoshoni and Moneta.
Heavy rain of one-half to one inch inundated dry washes and arroyos in and around Medicine Lodge State Archaeological Site. Water and mud flowing out of one small canyon caused damage to at least six campsites in the middle portion of the site.
A thunderstorm rapidly developed to the south of Cody and tracked north over the city producing heavy rain and copious amounts of small hail. Hail drifts were measured in feet in some areas with a few low spots accumulating close to six feet of hail. The hail prevented the heavy rain from draining easily and water accumulated on area roads. Several business owners in downtown Cody reported minor flooding as the water piled up in the streets, especially along Sheridan Avenue. A bakery had water on the floors and in the basement and a gift shop flooded.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2015Every state in 2015This day in weather historyWarnings in Wyoming, May 2015
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 570217, 583105, 583106.