Strong to severe thunderstorms developed along outflow boundaries on either side of the Owl Creek Mountains. The storms produced wind gusts near 60 mph and hail up to penny size.
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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.
Strong to severe thunderstorms developed along outflow boundaries on either side of the Owl Creek Mountains. The storms produced wind gusts near 60 mph and hail up to penny size.
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 Jul 21, 1:38 PM MST, 4 hours 13 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 238 PM MDT TUE JUL 21 2009 .SHORT TERM...TONIGHT THROUGH FRIDAY LARGE UPPER LEVEL HIGH PRESSURE OVER THE WEST COAST AND CENTERED OVER SOUTHERN CA/SOUTHERN NV THIS EVENING WILL SLOWLY PROGRESS EASTWARD TO NEAR THE FOUR CORNERS BY THURSDAY EVENING. A WEAK SHORTWAVE MOVING ALONG THE PERIPHERY OF THE UPPER RIDGE WILL MOVE SOUTHEASTWARD ACROSS CENTRAL WY THIS EVENING. GFS AND NAM INDICATE THAT PRECIPITABLE WATER ACROSS CENTRAL WY WILL BE AROUND 0.75 OF AN INCH THIS EVENING...WHILE INSTABILITY WILL BE WEAK WITH LIFTED INDICES RANGING FROM 0 C TO MINUS 2 C. H5 WINDS THIS EVENING ARE PROJECTED TO BE 35 TO 40 KT...SO THUNDERSTORMS THIS EVENING WILL HAVE THE POTENTIAL TO PRODUCE WIND GUSTS IN EXCESS OF 50 MPH. A LIMITING FACTOR FOR CONVECTIVE DEVELOPMENT ACROSS CENTRAL WY THIS EVENING WILL BE SIGNIFICANT LAYER OF VERY DRY AIR BETWEEN H6 AND H4. SKIES WILL CLEAR OUT LATE TONIGHT WITH INCREASING SUBSIDENCE ALOFT AND DRY NORTHWEST FLOW ALOFT. AS THE RIDGE AMPLIFIES ACROSS THE NORTHERN ROCKIES WEDNESDAY AND THURSDAY...H7 THERMAL RIDGE WILL BUILD ACROSS THE REGION. HIGHS WEDNESDAY WILL BE NEAR NORMAL TO SLIGHTLY ABOVE NORMAL...AND HIGHS THURSDAY WILL BE 2-5 F ABOVE NORMAL. WEAK INSTABILITY WEDNESDAY AND THURSDAY IS PROJECTED WITH CAPS VALUES GENERALLY LESS THAN 200 J/KG. MOISTURE WILL ALSO BE LACKING WEDNESDAY AND THURSDAY...AS PRECIPITABLE WATER VALUES OVER MUCH OF THE REGION WILL BE BELOW 0.5 INCHES. TERRAIN CIRCULATIONS WILL CREATE DAYTIME LIFT OVER THE MOUNTAINS WEDNESDAY AND THURSDAY...AND INCLUDED SLIGHT CHANCES FOR AFTERNOON AND EVENING THUNDERSTORMS IN THE HIGHER ELEVATIONS BOTH DAYS. INTERESTING SITUATION MAY DEVELOP ON FRIDAY ACROSS WESTERN WY. GFS BRINGS A SHORTWAVE OVER THE TOP OF THE RIDGE FROM THE PAC NW INTO WESTERN MT FRIDAY AFTERNOON. THE 12Z ECMWF ALSO BRINGS THIS SHORTWAVE OVER THE RIDGE...BUT IS 6-12 HOURS SLOWER. IN CONTRAST...THE NAM MAINTAINS THE AMPLIFIED RIDGE OVER THE NORTHERN ROCKIES. THE NAM INDICATES THAT INCREASING MOISTURE WILL MOVE INTO WESTERN WY IN CIRCULATION AROUND THE FOUR CORNERS UPPER HIGH...WHILE THE GFS BRINGS MAINLY PAC MOISTURE WITH THE SHORTWAVE. INCLUDED ISOLATED TO SCATTERED CONVECTION ACROSS THE WEST FRIDAY AFTERNOON/EVENING WHICH COULD RESULT IN EITHER SCENARIO. HIGHS FRIDAY COULD BE ABOVE NORMAL AGAIN PER THE NAM SOLUTION...BUT LOWERED HIGHS TO NEAR NORMAL TO ACCOUNT FOR THE COLDER GFS/ECMWF SOLUTIONS. .LONG TERM...FRIDAY NIGHT THROUGH TUESDAY MODELS CONTINUE TO CHANGE THE DETAILS EACH NIGHT WITH THE ECMWF BEING THE MOST CONSISTENT. TONIGHT THE ECMWF BRINGS THE PACNW UPPER LOW INTO NW WYOMING ON SATURDAY AND THEN SLOWLY FILLS IT OVER SWRN WYOMING ON SUNDAY. THE GFS KEEPS THIS SYSTEM IN THE PACNW UNTIL MONDAY WHEN IT WEAKENS AND MOVES ACROSS OUR AREA IN THE NW FLOW ALOFT. BY EARLY NEXT WEEK...BOTH MODELS BUILD THE RIDGE NWD ALLOWING THE NLY FLOW TO BUILD SLIGHTLY FURTHER WWD. BOTH MODELS PUSH A FAIRLY SIGNIFICANT FRONT THROUGH ON MONDAY WITH A SECONDARY SHORTWAVE POSSIBLE ON TUESDAY. THE CHANCE OF SHOWERS AND THUNDERSTORMS SHOULD INCREASE ACROSS THE NERN HALF IF THIS SCENARIO DEVELOPS EARLY NEXT WEEK. AT THE BEGINNING OF THE PERIOD...A BRIEF SHOT OF MONSOONAL MOISTURE WILL LIKELY MOVE UP FROM THE SOUTH ON FRIDAY AND BRING AT LEAST ISOLATED COVERAGE IF NOT MORE. THE WEEKEND WILL ALSO SEE AT LEAST ISOLATED COVERAGE WITH EITHER THE UPPER LOW AFFECTING THE WEATHER PER THE ECMWF OR THE SOMEWHAT NW FLOW WITH HIGHER LOW LEVEL MOISTURE ACROSS THE NORTH AND ERN SECTIONS. TEMPERATURES WILL LIKELY TREND DOWNWARD FROM THE WEEKEND INTO EARLY NEXT WEEK AND THE FORECAST WILL LEAN THIS WAY. DEPENDING ON HOW STRONG THIS NLY FLOW IS AND HOW FAR WEST IT DEVELOPS WILL DETERMINE HOW MUCH COOLER IT IS EARLY NEXT WEEK. && .AVIATION... ISOLATED TO WIDELY SCATTERED SHOWERS AND THUNDERSTORMS WILL BE FOUND ACROSS MUCH OF THE FORECAST AREA THROUGH 06Z. THESE SHOWERS AND THUNDERSTORMS MAY OBSCURE MOUNTAIN TOPS AT TIMES. SHOWER ACTIVITY THIS EVENING MAY BRING PERIODS OF MVFR CONDITIONS...WITH OTHERWISE VFR CONDITIONS EXPECTED. ISOLATED SHOWERS AND THUNDERSTORMS MAY DEVELOP AFTER 18Z WEDNESDAY ACROSS THE MOUNTAINS OF CENTRAL AND WESTERN WYOMING. THESE SHOWERS MAY AGAIN OBSCURE MOUNTAIN TOPS. THE LOWER ELEVATIONS...INCLUDING AREA AIRPORTS ARE EXPECTED TO SEE GENERALLY VFR CONDITIONS THROUGH 00Z THURSDAY. DAYTIME WINDS WILL RANGE FROM 10 TO 20 MPH WEDNESDAY...AND WILL BECOME LIGHTER...10 TO 15 MPH...THURSDAY UNDER BUILDING HIGH PRESSURE. && .FIRE WEATHER... A WEAK UPPER LEVEL DISTURBANCE WILL MOVE ACROSS CENTRAL WYOMING THIS EVENING...AND GENERATE ISOLATED SHOWERS AND THUNDERSTORMS EAST OF THE DIVIDE. CHANCES OF WETTING RAIN WITH THIS ACTIVITY WILL BE LOW...AND GUSTY OUTFLOW WINDS WILL BE MORE LIKELY. UPPER LEVEL HIGH PRESSURE WILL MOVE BUILD OVER THE NORTHERN ROCKIES TONIGHT THROUGH THURSDAY. AS DRIER AIR MOVES INTO THE REGION...DAYTIME MINIMUM RELATIVE HUMIDITIES WILL FALL INTO THE TEENS IN THE LOWER ELEVATIONS AND RECOVERIES AT NIGHT WILL BE WEAKER THAN THE PAST FEW NIGHTS. ISOLATED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE IN THE MOUNTAINS WEDNESDAY AND THURSDAY AFTERNOON AND EVENING. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...SMITH LONG TERM...SKRBAC AVIATION...BRANHAM FIRE WEATHER...SMITH
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 245 PM MDT TUE JUL 21 2009 WYZ001>020-022>030-221300- 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- 245 PM MDT TUE JUL 21 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT ISOLATED THUNDERSTORMS WITH LOCALLY HEAVY RAINFALL AND SMALL HAIL ARE EXPECTED ACROSS THE MOUNTAINS AND FOOTHILLS EAST OF THE CONTINENTAL DIVIDE THIS AFTERNOON AND EVENING. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY AT LEAST ISOLATED LATE DAY THUNDERSTORMS ARE EXPECTED EACH DAY. MOISTURE LEVELS MAY INCREASE FRIDAY AND SATURDAY LEADING TO A BETTER CHANCE OF MOUNTAIN THUNDERSTORMS. A COLD FRONT AFFECTING AREAS EAST OF THE DIVIDE EARLY NEXT WEEK MAY ALSO BRING INCREASED CHANCES OF THUNDERSTORMS. .SPOTTER INFORMATION STATEMENT... SPOTTERS ARE ASKED TO REPORT ANY HAIL AND WIND GUSTS IN EXCESS OF 40 MPH. THANK YOU. TEAM RIVERTON
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE RIVERTON WY 548 PM MDT TUE JUL 21 2009 WYZ015>017-220045- UPPER WIND RIVER BASIN-WIND RIVER BASIN-WIND RIVER MOUNTAINS EAST- 548 PM MDT TUE JUL 21 2009 ...SIGNIFICANT WEATHER ADVISORY FOR FREMONT COUNTY UNTIL 645 PM MDT... AT 544 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A STRONG THUNDERSTORM 9 MILES NORTHEAST OF U.S. HWY 287/26 MILE MARKER 90...OR 26 MILES NORTH OF FORT WASHAKIE...MOVING SOUTH AT 10 MPH. NICKEL SIZE HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THIS STORM. RAINFALL AMOUNTS AROUND ONE-HALF INCH...AND POSSIBLE ACCUMULATIONS OF HAIL ARE ALSO POSSIBLE WITH THIS STORM. RESIDENTS IN CENTRAL FREMONT COUNTY...INCLUDING THE LOCATIONS OF DIVERSION DAM REST AREA...BULL LAKE AND HWY JCT 287 AND 26...SHOULD MONITOR THE WEATHER SITUATION CLOSELY. LISTEN FOR POSSIBLE WARNINGS AND BE PREPARED TO TAKE APPROPRIATE ACTION SHOULD SEVERE WEATHER THREATEN. PRECAUTIONARY/PREPAREDNESS ACTIONS... IF ON OR NEAR BULL LAKE...GET OUT OF THE WATER AND MOVE INDOORS OR INSIDE A VEHICLE. REMEMBER...LIGHTNING CAN STRIKE OUT TO 15 MILES FROM THE PARENT THUNDERSTORM. IF YOU CAN HEAR THUNDER... YOU ARE CLOSE ENOUGH TO BE STRUCK BY LIGHTNING. MOVE TO SAFE SHELTER NOW. DONT BE CAUGHT ON THE WATER IN A THUNDERSTORM && REPORT HAZARDOUS WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN RIVERTON AT 1-800-211-1448. LAT...LON 4309 10874 4304 10910 4344 10915 4348 10891 TIME...MOT...LOC 2346Z 348DEG 11KT 4337 10901 MCNEEL
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE RIVERTON WY 559 PM MDT TUE JUL 21 2009 WYZ005-007-220100- OWL CREEK AND BRIDGER MOUNTAINS-SOUTHWEST BIG HORN BASIN- 559 PM MDT TUE JUL 21 2009 ...SIGNIFICANT WEATHER ADVISORY FOR HOT SPRINGS COUNTY UNTIL 700 PM MDT... AT 557 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A STRONG THUNDERSTORM NEAR HWY JCT 431 AND 120...OR 19 MILES SOUTHEAST OF MEETEETSE...MOVING SOUTHEAST AT 25 MPH. ACCUMULATION OF A FEW INCHES OF SMALL HAIL...AND WINDS AROUND 40 MPH ARE POSSIBLE WITH THIS STORM. RAINFALL AMOUNTS AROUND ONE-HALF INCH ARE POSSIBLE WITH THIS STORM. RESIDENTS IN CENTRAL HOT SPRINGS COUNTY...INCLUDING THE LOCATIONS OF HWY JCT 431 AND 120...STATE HWY 120 MILE MARKER 30...HWY JCT 171 AND 120...STATE HWY 120 MILE MARKER 20 AND HWY JCT 170 AND 120...SHOULD MONITOR THE WEATHER SITUATION CLOSELY. LISTEN FOR POSSIBLE WARNINGS AND BE PREPARED TO TAKE APPROPRIATE ACTION SHOULD SEVERE WEATHER THREATEN. PRECAUTIONARY/PREPAREDNESS ACTIONS... MOVE INDOORS IMMEDIATELY AS THIS STORM APPROACHES YOUR LOCATION. THIS IS A POTENTIALLY DANGEROUS STORM. IF YOU ARE IN ITS PATH...YOU SHOULD BE PREPARED FOR GUSTY WINDS...HAIL...AND DEADLY CLOUD TO GROUND LIGHTNING. IF A STURDY STRUCTURE IS NOT AVAILABLE...SEEK SHELTER IN A VEHICLE WITH THE WINDOWS UP. && REPORT HAZARDOUS WEATHER TO THE NATIONAL WEATHER SERVICE OFFICE IN RIVERTON AT 1-800-211-1448. LAT...LON 4367 10817 4348 10828 4394 10878 4407 10862 TIME...MOT...LOC 2359Z 310DEG 20KT 4394 10860 MEUNIER
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE RIVERTON WY 644 PM MDT TUE JUL 21 2009 WYZ017-220115- WIND RIVER BASIN- 644 PM MDT TUE JUL 21 2009 A RAPIDLY DEVELOPING THUNDERSTORM WILL BE TRACKING SOUTHEAST ALONG THE WIND RIVER TOWARD RIVERTON THROUGH 715 PM. EXPECT BRIEF HEAVY RAINFALL...POSSIBLE SMALL HAIL...30 TO 40 MPH WINDS AND DANGEROUS CLOUD TO GROUND LIGHTNING WITH THIS STORM. LIPSON
The Riverton/Lander radar (KRIW), 2 km away, was scanning. The volume nearest the first event began at 00:49 UTC.
1 thunderstorm wind, 1 hail. Local standard time, as filed.
A microburst developed close to the Riverton Regional Airport ASOS and NWS Riverton office producing outflow wind measured at 58 mph.
Penny sized hail was reported west of Thermopolis near Missouri Flat Road.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2009Every state in 2009This day in weather historyWarnings in Wyoming, July 2009
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: 175611, 175616.