A shortwave moved across the state and tapped into some mid level moisture to produce scattered thunderstorms. With large dew point depressions, some showers and storms produced strong wind gusts.
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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.
A shortwave moved across the state and tapped into some mid level moisture to produce scattered thunderstorms. With large dew point depressions, some showers and storms produced strong wind gusts. The strongest was at the Sweetwater County airport near Rock Springs where a maximum wind gust of 78 mph was reported. Other notable gusgts included 60 mph at the Big Piney airport and 59 mph at the Lander airport.
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, 11:22 AM MST, 48 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1222 PM MDT Wed Aug 12 2020 .SHORT TERM...Today through Friday night The weather has been like a broken record for the past week or so, skipping along with same pattern. Again, the two concerns for today are convection and fire weather. As for convection, storms may be more widespread today as a shortwave moves across the state. The most numerous storms still look to be across the northwest. However, the models, including the convective models, are all over the place in nailing down a certain area. So for now, we have kept the POPS in the isolated to widely scattered variety but made them fairly widespread. And again, the main threat will be strong winds given the still large dew point depressions. And some of the storms could be of the dry variety. And this leads into the second concern... Which is fire weather. In some ways, it will be somewhat improved as humidity will likely be higher than yesterday in western Wyoming but not in eastern areas. However, there will still be a gusty breeze in most areas. And, we have the threat for dry thunderstorms. Like was stated above, the best coverage is likely to be in the west but nailing down a smaller area is still highly uncertain. But with the chance of dry thunderstorms, we felt that the warning looked good as-is and we continued it. Any storms should end shortly after midnight. Things begin to change on Thursday. Another shortwave will pass across Montana tomorrow but almost all convection should remain there. We allowed for some isolated POPs in the northwest but the models have trended drier over the past couple of runs so I would not be surprised if there was no convection. However, it will be close enough to tighten the pressure gradient. In addition, a weak cold front will start to drop into the area. The result will be a gusty wind to probably windy conditions for many areas. With warm to hot temperatures continuing and humidity excepted to remain rather low, we extended the Red Flag Warning for all zones through Thursday evening. We did have some concerns about Johnson County, especially the north not being critical, but it is close and we did not want to try to split hairs at this point. The front will drop southward into the area on Thursday night and into Friday, but the extent is still up for question. It will likely make it through the Big Horn Basin and possible Natrona County and this would likely bring an improvement of fire weather conditions. However, with a gusty wind persisting in the south, there may need to be more fire weather highlights needed for Friday in southern Wyoming. Otherwise, expect another warm dry with mainly dry conditions across the entire state. .LONG TERM... A quasi-stationary weak cold front should be along and east of the Continental Divide Saturday. South of this boundary across southwest Wyoming there could be enough wind for elevated to possibly critical fire weather conditions. Otherwise dry and warm. On Sunday and Monday, a subtropical ridge will build northward into the Great Basin/Rockies. As this occurs the wind fields will relax which in turn will give the forecast area a break in the continuous elevated to critical fire weather conditions. Otherwise dry and very warm temperatures. High temperatures both days will be about 10 degrees above average west, and about 5 above east. Global models continue to show the subtropical ridge flattening out Tuesday and Wednesday in response to a trough pulling into SW Canada. Although there are a few solutions on the intensity of this trough, models are showing increased wind fields and the possibility of diurnal showers and thunderstorms. && .AVIATION...18Z Issuance VFR conditions will occur through the period. Winds are already starting to pick up over most of the area and will continue to increase in strength through the afternoon. Isolated to widely scattered showers and storms will develop in most areas starting around 20Z and will continue through 03Z Thursday. Wind gusts of 30- 40 knots will be common, but gusts near 50 knots are not out of the question with some storms. && .FIRE WEATHER... Relative humidity will be somewhat higher than yesterday. However, with a gusty wind continuing and a chance of dry thunderstorms, much of the area will again see critical fire weather in the same locations as yesterday. The chance of dry thunder will be highest in the west with the stronger winds in the southern zones. Mixing and smoke dispersal will be generally good to excellent in the afternoon. Expect mainly dry weather Thursday. However, almost all areas will again see a very good chance of critical fire weather Thursday afternoon. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning from noon today to 8 PM MDT Thursday for WYZ275>283-285>289-300-414>416. && SHORT TERM...Hattings LONG TERM...Murrell AVIATION...Hulme FIRE WEATHER...Hattings
Hazardous Weather Outlook National Weather Service Riverton WY 317 AM MDT Wed Aug 12 2020 WYZ001>020-022>030-131200- Yellowstone National Park-Absaroka Mountains-Cody Foothills- North Big Horn Basin-Southwest Big Horn Basin- Southeast Big Horn Basin-Owl Creek and Bridger Mountains- Bighorn Mountains West-Bighorn Mountains Southeast- Northeast Johnson County-Southeast Johnson County- Teton and Gros Ventre Mountains-Jackson Hole- Wind River Mountains West-Wind River Mountains East- Upper Wind River Basin-Wind River Basin-Lander Foothills- Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain-Star Valley- Salt River and Wyoming Ranges-Upper Green River Basin Foothills- Upper Green River Basin-South Lincoln County- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- 317 AM MDT Wed Aug 12 2020 ...Red Flag Warning in effect this afternoon through Thursday evening... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Critical fire weather conditions across the area. Please see Red Flag Warning for details. Isolated to scattered afternoon thunderstorms possible. Strong winds and lightning caused wildfires will be the main hazards. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday. Thursday...Critical fire weather conditions likely across much of the area. Please see Red Flag Warning for details. Friday and Saturday...Elevated to critical fire weather possible for central and southern Wyoming. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions.
Special Weather Statement National Weather Service Riverton WY 1257 PM MDT Wed Aug 12 2020 WYZ012>015-019-023>030-130200- Teton and Gros Ventre Mountains-Jackson Hole- Wind River Mountains West-Wind River Mountains East- Green Mountains and Rattlesnake Range-Star Valley- Salt River and Wyoming Ranges-Upper Green River Basin Foothills- Upper Green River Basin-South Lincoln County- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- Including the cities of Jackson, Jeffrey City, Afton, Alpine, Star Valley Ranch, Thayne, Pinedale, La Barge, Big Piney, Farson, Kemmerer, Cokeville, Rock Springs, Green River, and Wamsutter 1257 PM MDT Wed Aug 12 2020 This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Widely scattered showers and thunderstorms producing wind gusts between 45 and 55 mph. Isolated gusts around 60 mph may occur. * WHERE...Northern and Central Wyoming. This also includes Interstate 25 in Natrona and Johnson Counties, and Interstate 90 in Johnson County. * WHEN...Through 800 PM MDT. * ADDITIONAL DETAILS...The strong wind gusts can occur with little or no rainfall, and even without any lightning. WYZ002>011-016>018-020-022-130200- Absaroka Mountains-Cody Foothills-North Big Horn Basin- Southwest Big Horn Basin-Southeast Big Horn Basin- Owl Creek and Bridger Mountains-Bighorn Mountains West- Bighorn Mountains Southeast-Northeast Johnson County- Southeast Johnson County-Upper Wind River Basin-Wind River Basin- Lander Foothills-Natrona County Lower Elevations-Casper Mountain- Including the cities of Clark, Cody, Meeteetse, Basin, Greybull, Lovell, Powell, Shell, Thermopolis, Ten Sleep, Worland, Buffalo, Kaycee, Dubois, Riverton, Shoshoni, Lander, and Casper 1257 PM MDT Wed Aug 12 2020 This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Widely scattered showers and isolated thunderstorms producing wind gusts between 45 and 55 mph. Isolated gusts around 60 mph may occur. * WHERE...Western and Southern Wyoming. This also includes Interstate 80 in Sweetwater county. * WHEN...Through 800 PM MDT. * ADDITIONAL DETAILS...The strong wind gusts can occur with little or no rainfall, and even without any lightning.
The Riverton/Lander radar (KRIW), 170 km away, was scanning. The volume nearest the first event began at 19:09 UTC.
3 thunderstorm wind. Local standard time, as filed.
Very strong winds occurred as thunderstorms moved east of Rock Springs. The strongest wind gust recorded was 78 mph at the Sweetwater County Airport.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2020Every state in 2020This day in weather historyWarnings in Wyoming, August 2020
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: 913124, 913125, 913126.