A slow-moving upper level low south of Wyoming sent waves of moisture northward over central and eastern Wyoming during the Memorial Day holiday weekend. Measured and estimated rainfall totals ranged from two to around five inches.
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.
A slow-moving upper level low south of Wyoming sent waves of moisture northward over central and eastern Wyoming during the Memorial Day holiday weekend. Measured and estimated rainfall totals ranged from two to around five inches. This resulted in flooding and flash flooding in several areas. Johnson County saw significant flooding along the tributaries and main stem of the Powder River and around Buffalo. The greatest impact was felt in Hot Springs County where heavy rains in the Wind River Canyon resulted in several mud and rock slides that closed State Highway 789 between Thermopolis and Shoshoni. The slides also damaged several sections of railroad track in the same area subsequently shutting down rail traffic. Minor flooding was also noted in Fremont County where water approached homes in low lying areas of Lyons Valley near Lander.
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 24, 8:01 AM MST, 4 hours 59 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 901 AM MDT SUN MAY 24 2015 .SHORT TERM...TODAY THROUGH TUESDAY NIGHT UPDATED TO EXTEND FLASH FLOOD WATCH UNTIL MIDNIGHT TONIGHT. ALSO ADDED ALL OF FREMONT AND HOT SPRINGS COUNTIES AND EASTERN SWEETWATER COUNTY TO THE FLASH FLOOD WATCH. RAINFALL OF AN INCH TO 2.5 INCHES HAS ALREADY OCCURRED OVER MUCH OF THE WATCH AREA OVER NIGHT AND THIS MORNING. WV IMAGERY SHOWING LARGE UPR TROF CONTINUES ACROSS THE WRN CONUS WITH CLOSED LOW OVR WRN CO/SRN WY...WITH NICE VORT LOBE ROTATING ANTI-CLOCKWISE AROUND CNTRL CLOSED PORTION OF THE LOW INTO SCNTRL WY. EMBEDDED SW ALSO ROTATING AROUND THE LOW...NOW STRETCHED DIAGONALLY ACROSS WY FROM THE SW TO NCNTRL. SFC HAS GENERAL HIGH P NORTH OVR MT...HIGH P SOUTH OVR CO/NM...WITH A WEAK LOW P TROF/DEVELOPING FRONT IN BETWEEN AND ACROSS CNTRL WY. EARLY MORNING FOG WILL CONTINUE TO BE POSSIBLE OVR MUCH OF THE FA THRU TUE MORNING...ESPECIALLY E OF THE DIVIDE. OTHERWISE...MAINLY HYDROLOGIC CONCERNS AS UPR TROF MIGRATES EWD WHILE THE CNTRL UPR LOW SLOWLY PULLS FROM CO/WY BORDER AREA INTO THE HIGH PLAINS BY THIS EVENING. WEAK SFC CYCLOGENESIS TO TAKE PLACE FROM ERN/NERN CO INTO SERN WY AND WRN NE. LOW TO MID LVL MOISTURE RETURN AROUND THE DEVELOPING SFC LOW...WHILE MID/UPR MOISTURE ACCOMPANYING THE UPR WAVE/CLOSED LOW DEEPENS MOISTURE E OF THE DIVIDE...WITH PWS RANGING FROM 0.70 TO 0.80...WILL GIVE ANOTHER DAY OF PLENTIFUL RAINFALL...AGAIN ESPECIALLY E OF THE DIVIDE. WIDESPREAD AREAS OF NEW PRECIPITATION THRU SUNDAY EVENING WILL RANGE FROM .75 TO 1.25 INCHES...WITH SOME PREFERRED LOCATIONS ALONG THE E SIDE OF THE BIGHORNS...NEAR/OVR THE SRN BIGHORNS...AND THE GREEN AND ERN WIND RIVER MOUNTAINS...WHERE PRECIP WILL BE ENHANCED DUE TO TERRAIN LIFT OR TERRAIN INDUCED CONVERGENCE. EVEN THE FOOTHILLS OF THE WESTERN SIDE OF THE BIGHORNS WILL BE SUSCEPTABLE TO ENHANCED PRECIP AS DEEP FLOW MOVES AROUND FROM THE NORTH. FLASH FLOODING WILL CONTINUE TO BE A CONCERN FOR ANOTHER DAY E OF THE DIVIDE...AND HAVE ADDED THE WESTERN BIGHORNS AND WASHAKIE COUNTY TO THE CURRENT FLASH FLOOD WATCH AREA. AREAS SATURATED MOST OVR THE PAST COUPLE OF DAYS INCLUDE FREMONT COUNTY...PARTICULARLY WRN AND SRN PORTIONS...MOST OF NATRONA COUNTY...THE SRN AND ERN PORTIONS OF WASHAKIE COUNTY...AND AREAS IMMEDIATELY ADJACENT TO THESE COUNT AREAS. AGAIN...THIS SYSTEM CONTINUES TO LOOK COMPARATIVELY WARM...WITH H7 TEMPS REMAINING MOSTLY ABOVE -1 DEG C THRU THE ENTIRE FCST PERIOD...AND REALLY WELL ABOVE ZERO FOR MOST OF THAT TIME. EXPECT...A TRACE TO AN INCH OR TWO DURING THE COLDEST PORTION OF THE NIGHTS ABOVE 9500 FEET...WITH ONLY THE HIGHEST ELEVATIONS RECEIVING AN ISOLATED 3 OR 4 INCHES BY MONDAY MORNING. STORM MOVEMENT IMPROVES SLIGHTLY...EVENTUALLY BECOMING NRLY AT 15 TO 20 MPH MOST LOCATIONS BY THE END OF THE TODAY EXCEPT FOR JOHNSON COUNTY WHICH WILL KEEP MOVEMENT OUT OF THE E OT NE. SEVERE WX WILL REMAIN MOSTLY WELL EAST TO SOUTHEAST OF FA WITH CONTINUED RELATIVELY LOW SHEAR CONDITIONS. THE BEST CHANCES FOR SMALL HAIL UP TO ONE HALF INCH AND PERHAPS A BOUNDARY LOCKED FUNNEL CLOUD WILL LAY ACROSS THE FAR ERN FA...WITH A SMALLER CHANCE FOR SMALL HAIL NEAR THE FAR WRN BORDER AREA WHERE SOME POSSIBLE INCREASED SUNSHINE MAY ENHANCE A COUPLE OF STORMS. TONIGHT THRU TUESDAY...SYSTEM PUSHES EWD INTO THE PLAINS WITH TRANSITIONAL RIDGING TAKING PLACE ACROSS THE FAR WRN FA MONDAY WHILE THE REST OF THE FA WILL BE UNDER THE INFLUENCE OF A SW ROTATING INTO THE AREA FROM THE NE AROUND THE EXITING LOW...WITH MORE SHOWER/THUNDER ACTIVITY...ALBEIT ON A LESSER BASIS. OTHERWISE...A BIT WARMER AND MORE UNSTABLE WITH PERHAPS AN BETTER SPRINKLING OF STRONGER THUNDERSTORMS MIXED IN WITH SHOWERS ON TUESDAY. .LONG TERM...WEDNESDAY THROUGH SUNDAY GFS MODEL CONTINUES ITS TREND FROM PREVIOUS NIGHTS BY TAKING A WEATHER SYSTEM FROM THE PACIFIC NW WEDNESDAY AND MOVING IT INTO IDAHO THURSDAY. ON FRIDAY THE GFS TAKES A MORE SOUTHEAST TRACK TOWARD SW WY OR NORTHERN UTAH AND THEN TO NW COLORADO BY FRIDAY NIGHT. FROM THERE IT DRIFTS EAST THROUGH NORTHERN COLORADO SATURDAY AND OUT OF THE AREA SATURDAY NIGHT. WITH MOISTURE PRESENT AND ANY SUNSHINE THAT OCCURS...INSTABILITY WILL OCCUR AND THUS SHOWERS AND THUNDERSTORMS WILL FORM IN THE AFTERNOON AND EVENING. LINGERING SHOWERS DURING THE LATE NIGHT AND MORNING HOURS. NEXT SUNDAY LOOKS LIKE A QUIETER DAY WITH ONLY ISOLATED SHOWERS OR STORMS IN NW WY IN THE AFTERNOON. THE ECMWF MODEL IS SIMILAR WEDNESDAY THROUGH THURSDAY BUT THEN THE ECMWF SPEEDS UP THE WEATHER SYSTEM MOVEMENT ON FRIDAY AND TAKES IT OUT OF THE AREA BY SATURDAY MORNING. THEN IT HAS MOSTLY DRY WEATHER FOR SATURDAY AND NEXT SUNDAY WITH ONLY ISOLATED SHOWERS OR STORMS. WILL FOLLOW A MORE GFS SPEED FOR NOW AS MOST OF THESE SYSTEMS HAVE BEEN SLOWER MOVING OUT OVER THE PAST MONTH. WILL PLAY NEXT SATURDAY WITH A SLIGHT CHANCE OF SHOWERS AND ISOLATED STORMS AND THEN ISOLATED MOUNTAIN ACTIVITY NEXT SUNDAY. AS FOR TEMPS...WED HIGH TEMPS IN THE 60S TO LOWER 70S...THEN HIGHS THURSDAY ABOUT THE SAME AS WED TO SLIGHTLY COOLER IN SOME PLACES. FRIDAY LOOKS A LITTLE COOLER WITH MORE CLOUDS EXPECTED AS THE SYSTEM MOVES THROUGH THE AREA. THEN A LITTLE WARMER SATURDAY AND EVEN WARMER NEXT SUNDAY TO EXIT THE MONTH OF MAY. .AVIATION.../18Z ISSUANCE/ WIDESPREAD MVFR TO IFR CONDITIONS AND MTN OBSCURATIONS...LCL IFR CONDITIONS WITH NMRS -SHRA WILL CONTINUE ALONG AND EAST OF THE DIVIDE THROUGH 18Z MONDAY. THESE CONDITIONS WILL NOT BE AS WIDESPREAD WEST OF THE DIVIDE WHICH MAY SEE MORE FOG DEVELOPMENT THROUGH THE MORNING HOURS. THE UPPER LOW OVER WESTERN COLORADO WILL SLOWLY LIFT NE INTO EASTERN WYOMING ON SUNDAY AND SUNDAY NIGHT...AND THERE WILL LIKELY BE SOME LIFTING OF CIGS LATE IN THE AFTERNOON FOLLOWED BY RAPID DEVELOPMENT OF SHOWERS AND THUNDERSTORMS THROUGH THE AFTERNOON AND EARLY EVENING. WIDESPREAD MVFR -RA/AREAS OF IFR RA BR...MTN OBSCURATIONS ON THE BACKSIDE OF THE UPPER LOW WILL FOLLOW AFTER 03Z SUNDAY ACROSS MOST OF THE AREA. .FIRE WEATHER... FIRE DANGER LOW AND WILL REMAIN LOW THROUGH TUESDAY AS RELATIVELY COOL AND WET WEATHER REMAINS IN PLACE AS YET ANOTHER VIGOROUS UPPER LEVEL SYSTEM ROTATES ACROSS THE REGION. WIDESPREAD PRECIPITATION...WITH BETWEEN A HALF AN INCH AND AN INCH AND A QUARTER EAST OF THE DIVIDE...AND LIGHT MOUNTAIN SNOW IS EXPECTED TODAY AND TONIGHT...WITH EMBEDDED THUNDERSTORMS POSSIBLE EACH AFTERNOON AND EARLY EVENING THROUGH TUESDAY. && .RIW WATCHES/WARNINGS/ADVISORIES... FLASH FLOOD WATCH THROUGH THIS EVENING WYZ005>011-015>020-022-030. && SHORT TERM...BRAUN LONG TERM...AR AVIATION...C.BAKER FIRE WEATHER...BRAUN
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 400 AM MDT SUN MAY 24 2015 WYZ001>020-022>030-251200- 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 SUN MAY 24 2015 ...FLASH FLOOD WATCH FOR NATRONA...JOHNSON...FREMONT...AND WASHAKIE COUNTIES UNTIL 6 PM MDT TODAY... THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT FLASH FLOOD THREAT CONTINUES UNTIL 6 PM MDT WITH PROLONGED HEAVY RAIN OVER NATRONA...JOHNSON...FREMONT...AND WASHAKIE COUNTIES. COOL AND WET CONDITIONS CONTINUE...WITH WITH A COUPLE OF INCHES OF NEW SNOW POSSIBLE ABOVE 9500 FEET THIS MORNING. SHOWERS WITH ISOLATED THUNDERSTORMS AGAIN POSSIBLE BY AFTERNOON. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY MONDAY THROUGH FRIDAY...ISOLATED TO SCATTERED THUNDERSTORMS EACH AFTERNOON AND EVENING. STRONGER STORMS WILL PRODUCE LOCALLY HEAVY RAIN AND HAIL. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY AND TONIGHT. TEAM RIVERTON
The Riverton/Lander radar (KRIW), 182 km away, was scanning. The volume nearest the first event began at 19:58 UTC.
6 flash flood, 3 flood. Local standard time, as filed.
Several storm spotters reported flash flooding on Salt Creek and Castle Creek in the vicinity of Midwest. Some county and Bureau of Land Management roads were washed out. At least one fracking trailer was destroyed south of town.
Very heavy rain fell in southern Hot Springs County. This triggered numerous mud and rock slides, some tumbling several hundred feet or more to the bottom of the Wind River Canyon. State Highway 789 through the canyon was covered with mud, rock, and debris in several locations between mileposts 120 and 123. One Wyoming Department of Transportation employee was briefly trapped in his front-end loader as he tried to clear away small slides at the onset of the event. The massive amount of debris on the highway resulted in the closure of the road from Sunday afternoon, May 24, until Wednesday morning, May 27. Additional slides on the west side of the Wind River Canyon destroyed several sections of railroad track that resulted in a halt of train traffic for several days.
Heavy rain fell across southern Johnson County and brought flooding to the tributaries of the North Fork and Middle Fork of the Powder River. Flooding was observed between Mayoworth and Kaycee and also between Barnum and Kaycee.
Trained spotters reported flood waters up to buildings on the south side of Bar Nunn.
Heavy rainfall led to flooding in Lyons Valley near Lander. The Little Popo Agie River overflowed its banks and flooded low lying areas along Lyons Valley Road. The water approached some homes along the river but did not enter them.
Heavy rain caused flooding near the town of Thermopolis. South of Thermopolis, Buffalo Creek overflowed its banks and flooded nearby low lying areas. A mobile home community experienced flooding in East Thermopolis as small creeks and streams were inundated with water.
Heavy rainfall of two to three inches fell across northern Johnson County and caused flooding along Clear Creek. The flooding was mainly over low lying fields and caused little to no damage.
Heavy rain fell in the tributaries of the Powder River in southern Johnson and northern Natrona counties. The South Fork of the Powder River along with Salt Creek were two of the major contributors to a rapid rise in the water level of the Powder River near Sussex. High water on both the North and Middle forks of the Powder River also served to flood the main stem. The river climbed more than 10 feet in less than 24 hours at Sussex and crested at 15.63 feet around 0500MST on Monday, May 25. This crest topped the previous record crest of 15.16 feet set in May 1978. Agricultural land downstream of Kaycee near Sussex was completely inundated and several ranches and vehicles were surrounded by water.
A flood wave moved north down the Powder River as a result of heavy rain over the Memorial Day Weekend. For the most part, impacts were confined to low lying land nearest the river. However, there was flooding of some infrastructure near the Interstate 90 Powder River bridge in eastern Johnson County.
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: 570907, 571080, 583485, 570908, 583491, 571081, 571079, 581880, 583497.