An upper level disturbance tracked northeast across Wyoming and initiated strong showers and a few thunderstorms during the afternoon. One of the strong showers aided in the development of a brief landspout tornado near Alcova Reservoir.
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The episode as NWS RIW wrote it up.
An upper level disturbance tracked northeast across Wyoming and initiated strong showers and a few thunderstorms during the afternoon. One of the strong showers aided in the development of a brief landspout tornado near Alcova Reservoir.
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 12, 10:35 AM MST, 1 hour 55 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RIVERTON WY 1135 AM MDT MON AUG 12 2013 .SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT ANOTHER SHORTWAVE NOW OVER UTAH WILL STREAM NORTH NORTHEASTWARD WITHIN THE FLOW ON THE BACK SIDE OF THE STATIONARY RIDGE OVER THE ROCKIES. THIS WILL INITIATE ANOTHER ROUND OF CONVECTION OVER THE CWA LASTING WELL INTO THE EVENING WHEN THE SHORTWAVE RIDES UP AND OVER THE RIDGE. THEN A MORE DISTINCT SHORTWAVE TROUGH WILL RIDE UP AND OVER THE RIDGE LATE TUESDAY WITH STRONGER CONVECTION EXPECTED TUESDAY AFTERNOON AND WELL INTO TUESDAY EVENING FROM THIS FEATURE AS IT WILL HAVE SIMILAR DIURNAL TIMING. THEN ON WEDNESDAY THE RIDGE WILL PROPAGATE WEST TO THE GREAT BASIN WITH THE UPPER FLOW TURNING MORE NORTHWEST. THIS WILL DRIVE A WEAK COOL FRONT ACROSS THE AREA WHICH WILL STABILIZE THE AIRMASS SOMEWHAT FOR LESS WIDESPREAD CONVECTIVE ACTIVITY. AS HAS BEEN THE CASE FOR THE LAST SEVERAL DAYS...THE BEST CAPES WILL BE IN JOHNSON COUNTY RESULTING IN THE MOST SIGNIFICANT THUNDERSTORM ACTIVITY THERE THROUGH WEDNESDAY COMPARED TO THE REST OF THE CWA. TEMPERATURES WILL BE FAIRLY CLOSE TO SEASONAL THROUGH WEDNESDAY.. .LONG TERM...THURSDAY THROUGH MONDAY MEDIUM RANGE PERIOD WILL GENERALLY BE DRY AND VERY WARM TO HOT. RIDGE AXIS CENTERED TO OUR WEST TO START THE PERIOD IS GRADUALLY FORECAST TO MOVE OVERHEAD OVER THE WEEKEND AND THEN SHIFT SLIGHTLY OFF TO OUR EAST BY EARLY NEXT WEEK. MOST AREAS WILL BE DRY THROUGH THE PERIOD WITH POSSIBLY A FEW MORE STORMS DEVELOPING SUNDAY OR MONDAY AS SOME MOISTURE AND A POSSIBLY A WEAKENING PIECE OF THE ERN PACIFIC TROUGH MOVES INTO THE AREA. WILL BE CONSERVATIVE WITH THIS IDEA FOR NOW AND KEEP JUST ISOLATED POPS MAINLY OVER AND NEAR THE MOUNTAINS SUNDAY AND MONDAY. OTHERWISE...DRY WITH WARMING TEMPERATURES THROUGH AT LEAST SATURDAY BEFORE WE POTENTIALLY COOL A LITTLE WITH INCREASED CLOUDS AND MAYBE CONVECTION SUNDAY OR MONDAY. && .AVIATION.../18Z ISSUANCE/ ISOLATED TO LOCALLY SCATTERED SHOWERS AND THUNDERSTORMS ARE CURRENTLY DEVELOPING UNDER A COUPLE OF UPPER LEVEL DISTURBANCES THIS MORNING. THE BEST SHOT FOR CONVECTION WILL BE OVER THE HIGHER TERRAIN. THESE SHOWERS AND THUNDERSTORMS WILL BE VERY SLOW MOVERS...HAVING A HARD TIME ACTUALLY MOVING AWAY FROM THE MOUNTAINS IN MANY CASES. THE MAIN HAZARD TODAY WILL BE GUSTY OUTFLOW WINDS BETWEEN 25 AND 40 KTS...SOME OF WHICH MAY IMPACT AREAS WELL AWAY FROM THE SHOWERS AND THUNDERSTORMS. OTHERWISE...AWAY FROM THUNDERSTORM INDUCED WINDS...THE AMBIENT WINDS WILL BE ON THE LIGHT AND VARIABLE SIDE. LOCAL MVFR/IFR CONDITIONS WILL OCCUR IN STRONGER STORMS. SHOWERS OR THUNDER WILL BE AROUND THROUGH 03Z IN MUCH OF THE FORECAST AREA...WITH A FEW SHOWERS LINGERING AROUND THE MOUNTAINS TIL AFTER 06Z. && .FIRE WEATHER... AFTERNOON AND EVENING THUNDERSTORMS WILL BE PREVALENT THROUGH WEDNESDAY WITH THE STRONGEST ACTIVITY EXPECTED IN ZONES 281 AND 285. AN UPPER LEVEL WEATHER DISTURBANCE OVER UTAH WILL TRACK NORTH NORTHEAST TO OUR CWA WHICH WILL BE RESPONSIBLE FOR OUR CONVECTION TODAY. TUESDAY SHOULD FEATURE A BIT MORE WIDESPREAD CONVECTIVE ACTIVITY AS A MORE PROMINENT UPPER LEVEL WEATHER DISTURBANCE WILL TRACK UP AND OVER THE RIDGE OF HIGH PRESSURE STILL CENTERED OVER THE ROCKIES. ON WEDNESDAY A COOL FRONT WILL SWING THROUGH THE AREA WITH SLIGHTLY MORE STABLE CONDITIONS BEHIND THE FRONT FOR LESS THUNDERSTORM ACTIVITY. TEMPERATURES WILL BE RATHER SEASONAL THROUGH WEDNESDAY. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...LIPSON LONG TERM...SKRBAC AVIATION...WM FIRE WEATHER...LIPSON
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE RIVERTON WY 507 AM MDT MON AUG 12 2013 WYZ001>020-022>030-131200- 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- 507 AM MDT MON AUG 12 2013 THIS HAZARDOUS WEATHER OUTLOOK IS FOR WESTERN AND CENTRAL WYOMING. .DAY ONE...TODAY AND TONIGHT ISOLATED THUNDERSTORMS MAINLY ACROSS THE NORTH AND EAST CENTRAL FORECAST AREA. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SATURDAY TUESDAY...ISOLATED STRONG TO SEVERE STORMS POSSIBLE ACROSS JOHNSON AND NORTHEAST NATRONA COUNTIES. .SPOTTER INFORMATION STATEMENT... PLEASE REPORT WINDS OVER 45 MPH AND HAIL OF ANY SIZE. THANK YOU. TEAM RIVERTON
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE RIVERTON WY 1232 PM MDT MON AUG 12 2013 WYZ001>003-005-007>020-022>026-028>030-122000- 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-NORTHEAST JOHNSON COUNTY- OWL CREEK AND BRIDGER MOUNTAINS-ROCK SPRINGS AND GREEN RIVER- SALT RIVER AND WYOMING RANGES-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- 1232 PM MDT MON AUG 12 2013 SHOWERS AND THUNDERSTORMS HAVE BEGUN TO DEVELOP ACROSS MUCH OF WESTERN AND CENTRAL WYOMING AGAIN THIS AFTERNOON. STORMS ARE MOVING SLOWLY...AND HAVE BEGUN TO PRODUCE LIGHTNING. AS STORMS MATURE OVER THE NEXT COUPLE OF HOURS...GUSTY WIND TO 40 MPH WILL BE POSSIBLE... ALONG WITH SMALL HAIL AND MODERATE TO HEAVY RAINFALL. STORMS ARE EXPECTED TO REMAIN OVER THE AREA FOR MUCH OF THE AFTERNOON. TL
The Riverton/Lander radar (KRIW), 151 km away, was scanning. The volume nearest the first event began at 19:29 UTC.
1 tornado. Local standard time, as filed.
A landspout tornado developed near the west shoreline of Alcova Reservoir in southern Natrona County. The brief tornado was photographed from at least three different angles. The tornado remained over open land and caused no damage.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2013Every state in 2013This day in weather historyThe tornado archive for Wyoming, 2013Warnings in Wyoming, August 2013
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: 460386.