A dry lower atmosphere allowed weak thunderstorms to produce gusty outflow wind gusts across the northern Big Horn Basin.
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 dry lower atmosphere allowed weak thunderstorms to produce gusty outflow wind gusts across the northern Big Horn Basin.
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 Aug 2, 2:06 PM MST, 3 hours 35 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 306 PM MDT SUN AUG 2 2009 .SHORT TERM...TONIGHT THROUGH WEDNESDAY A DISTURBANCE IN THE NORTHWEST FLOW...AND A DEFORMATION ZONE ALONG THE WYOMING/MONTANA BORDER WILL ALLOW FOR ISOLATED CONVECTION OVER THE NORTH THIS EVENING. RIDGE CONTINUES TO BUILD OVER THE ROCKIES MONDAY AND TUESDAY...AS UPPER LOW CONTINUES TO SPIN OFF THE NORTH CALIFORNIA COAST. THIS WILL RESULT IN SLIGHTLY ABOVE AVERAGE TEMPERATURES...WITH 700MB TEMPS RANGING BETWEEN 12 AND 18 DEG CELSIUS. WEAK DISTURBANCES IN THE FLOW...AND WEAK INSTABILITY WILL RESULT IN ISOLATED TO LOCALLY SCATTERED DIURNAL CONVECTION MONDAY AND TUESDAY. GUSTY WINDS WILL BE THE MAIN HAZARD WITH THESE STORMS DUE TO INVERTED-V SOUNDING PROFILES. MODELS HAVE BEEN KEYING ON A DISTURBANCE AFFECTING THE NORTHWEST MONDAY AFTERNOON...AND INCREASED POPS TO LOW CHANCE. UPPER LOW OFF THE CALIFORNIA COAST NOW...IS PROJECTED TO BEGIN PUSHING EAST NORTHEAST ON WEDNESDAY. THIS WILL HAVE LITTLE IMPACT INITIALLY...AS ISOLATED MAINLY AFTERNOON AND EVENING CONVECTION AND SLIGHTLY ABOVE AVERAGE TEMPERATURES WILL CONTINUE...AS THE RIDGE AXIS MOVES OVER THE FORECAST AREA...WITH WEAK DISTURBANCES ROTATING AROUND IT. HAVE KEPT MENTION OF CONVECTION OUT OF THE FORECAST AFTER MIDNIGHT...ALTHOUGH THE AREA COULD SEE SOME VERY ISOLATED ACTIVITY MONDAY NIGHT AND TUESDAY NIGHT. WILL WAIT TO SEE THE EVENTUAL TRACK/TIMING/STRENGTH OF THESE SMALL DISTURBANCES BEFORE INTRODUCING OVERNIGHT CONVECTION. .LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY AFTERNOON AND EVENING CONVECTION WILL OCCUR THURS AS MOISTURE FORECAST TO INCREASE AROUND THE EDGE OF THE UPPER HIGH IN THE SOUTHERN ROCKIES. ANY SHORTWAVE IN THE FLOW WILL ENHANCE THE PRECIP CHANCES. THE LONG RANGE MODELS HAVE THE UPPER LOW OFF THE CALIFORNIA COMING ONSHORE VARY ON EXACTLY WHEN. THE GFS AND ECMWF HAVE IT MOVING INTO NORTHERN CALIFORNIA BY 00Z FRI. THE CANADIAN MODEL HAS IT ONSHORE 00Z THU IN NORTHERN CALIF AND UKMET BY 12Z THU. ONCE ONSHORE THE LONG RANGE MODELS HAVE DIFFERENCES IN LIFTING IT NORTHEAST INTO THE ROCKIES. GFS/UKMET/CANADIAN MODELS ARE FASTER THAN THE ECMWF MODEL THROUGH 12Z SAT. THE SYSTEM WILL MOST LIKELY OPEN INTO A TROUGH AND LIFT TOWARD THE NORTHERN ROCKIES. HAVE GONE WITH THE ECMWF TIMING TONIGHT AS IT IS CONSISTENT WITH ITS 12Z AUG 1ST RUN. THE GFS SEEMS TOO FAST AND INCONSISTENT FROM RUN TO RUN. FOR OUR AREA...THE BEST CHANCES OF STORMS WILL BE IN THE WEST AND NORTH ON FRIDAY AND ALONG THE COLD FRONT SATURDAY AS IT MOVES INTO THE WEST AND NORTH. THE CENTRAL AND SOUTH WOULD SEE ISOLATED ACTIVITY AT BEST WITH THE ECMWF TRACK WITH A HOT AND WINDY DAY FRIDAY AHEAD OF THE FRONT. THEN ONLY ISOLATED STORMS IN THE CENTRAL AND SOUTH SAT WITH THE FRONTAL PASSAGE. NEXT SUNDAY LOOKS MOSTLY DRY EXCEPT FOR SOME SHOWERS OR STORMS IN THE NW MTS PER THE ECMWF. GFS SEEMS TOO FAST WITH ANOTHER TROUGH IMPACTING WESTERN WY BY 00Z MON. AS FOR TEMPS...SUMMERTIME HEAT THROUGH FRIDAY. COOLER FOR SURE ON SAT AND SOME MODEL GUIDANCE AS MUCH AS 15 TO 20 DEGREES COOLER FROM FRI TO SAT. FOR NOW WENT 10 TO 15 DEGREES COOLER. A WARMER DAY NEXT SUNDAY. && .AVIATION... VFR CONDITIONS ARE EXPECTED THROUGH MONDAY...THOUGH ISOLATED THUNDERSTORMS ARE POSSIBLE THIS EVENING ACROSS THE NORTH AND THE ENTIRE AREA MONDAY AFTERNOON AND EVENING. A WEAK COLD FRONT WILL MOVE INTO THE NORTH THIS EVENING...SHIFTING THE WINDS TO THE NORTH AND NORTHEAST. WEST TO NORTHWEST WINDS ARE EXPECTED MOST OTHER AREAS WITH SOME GUSTS OF 20-25KTS. THESE WINDS WILL PICK UP AGAIN LATE MORNING/EARLY AFTERNOON MONDAY AS THE NORTHWEST FLOW ALOFT CONTINUES. && .FIRE WEATHER... SLIGHTLY ABOVE AVERAGE TEMPERATURES AND ISOLATED TO LOCALLY SCATTERED CONVECTION WILL OCCUR OVER THE NEXT COUPLE OF DAYS. MINIMUM RELATIVE HUMIDITY WILL GENERALLY BE 10 TO 20 PERCENT...WITH NIGHTLY RECOVERY POOR TO FAIR. SOME BREEZY CONDITIONS COULD OCCUR MONDAY FROM RKS TO CPR AREA. FIRE WEATHER CONCERNS INCREASE THURSDAY AND FRIDAY...AS THE NEXT STORM SYSTEM APPROACHES THE REGION. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...WM LONG TERM...AR AVIATION...TWB FIRE WEATHER...WM
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 500 AM MDT SUN AUG 2 2009 WYZ001>020-022>030-031200- 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- 500 AM MDT SUN AUG 2 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT ISOLATED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER THE NORTHERN WYOMING THIS AFTERNOON AND EVENING. SMALL HAIL AND GUSTY WINDS WILL BE POSSIBLE WITH THESE STORMS. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY ISOLATED AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS ARE IN THE FORECAST FOR THIS PERIOD. .SPOTTER INFORMATION STATEMENT... SPOTTERS ARE ASKED TO REPORT HAIL AND WIND GUSTS IN EXCESS OF 40 MPH FROM THUNDERSTORMS TODAY AND TONIGHT. THANK YOU. TEAM RIVERTON
SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RIVERTON WY
632 PM MDT SUN AUG 2 2009
WYZ004-008-030115-
BIGHORN MOUNTAINS WEST-NORTH BIG HORN BASIN-
632 PM MDT SUN AUG 2 2009
...SIGNIFICANT WEATHER ADVISORY FOR BIG HORN COUNTY UNTIL 715 PM
MDT...
AT 629 PM MDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED THAT
THUNDERSTORM OUTFLOW WINDS WERE LOCATED NEAR ALONG A LINE EXTENDING
FROM 12 MILES NORTH OF MEDICINE WHEEL TO STATE HWY 30 MILE MARKER
120...OR ALONG A LINE EXTENDING FROM 29 MILES NORTHEAST OF LOVELL TO
22 MILES WEST OF BASIN...MOVING SOUTHEAST AT 45 MPH.
WINDS AROUND 50 MPH AND DEADLY CLOUD TO GROUND LIGHTNING ARE
POSSIBLE WITH THESE STORMS.
RESIDENTS IN CENTRAL BIG HORN COUNTY...INCLUDING THE LOCATIONS OF
U.S. HWY 14A MILE MARKER 70 TO 80...HWY 310/789 MILE MARKER
215...MEDICINE WHEEL...OTTO...U.S. HWY JCT 310 AND 14...GREYBULL AND
BASIN...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...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 4492 10784 4488 10785 4483 10781 4480 10782
4477 10774 4473 10775 4472 10769 4469 10767
4469 10757 4461 10748 4462 10744 4460 10741
4419 10750 4434 10847 4496 10788
TIME...MOT...LOC 0030Z 305DEG 39KT 4499 10784 4436 10847
MEUNIERThe Billings radar (KBLX), 154 km away, was scanning. The volume nearest the first event began at 00:39 UTC.
1 thunderstorm wind. Local standard time, as filed.
The ASOS at the South Big Horn County Airport measured a thunderstorm wind gust of 62 mph.
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, August 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: 178005.