An upper-level low passed over Wyoming and spawned thunderstorms. Some of the storms combined with the gradient wind wrapping around the low to generate strong wind.
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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.
An upper-level low passed over Wyoming and spawned thunderstorms. Some of the storms combined with the gradient wind wrapping around the low to generate strong wind. At Lander, the wind accelerated downhill and produced hurricane force wind gusts at the airport in Lander. The wind downed numerous tree branches and limbs around the city. Thunderstorm outflow wind produced a peak gust of 64 mph at the Big Piney-Marbleton 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 15, 10:31 AM MST, 6 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1131 AM MDT Sun Aug 15 2021 .SHORT TERM...(Today through Monday) Issued at 238 AM MDT Sun Aug 15 2021 An upper high will retrograde westward into the Great Basin/California area today with light to modest northwest flow aloft. A disturbance pulling across the area in combination with daytime heating and weak instability will result in isolated to locally scattered convection. The convection should begin to develop around 11 AM and should end from northwest to southeast in the early evening. The best chance of convection will be across central and southern areas with the least chance over the far northwest and north. The main hazard from convection will once again be strong outflow winds between 40 to 55 mph. Yesterday, a few gusts near 55 mph were observed. Due to the increased coverage and earlier start of convection, most locations will be cooler than yesterday, but still above average. On Monday, the upper ridge begins to flatten out as a trough pulls into southwest British Columbia. Overall, flow will be generally light for much of the day, but should increase from the west late Monday mainly over the northwest. The combination of wind and low RH could result in critical fire weather conditions over the far northwest Monday afternoon and evening. A drier air mass will be over the area, and any isolated convection is expected to be only over the northwest. 700 millibar temperatures warm on Monday to between 17 and 20 Celsius. This will result in near record/record highs across the forecast area. .LONG TERM...(Monday night through Saturday) Issued at 238 AM MDT Sun Aug 15 2021 Northwest flow slowly turns westerly then southwesterly, through Monday evening into Tuesday morning, as a Pacific-Northwest trough digs west of the region. Models have been showing this trough the last several days and confidence is growing with the impacts associated with it. There has been some variability with the most recent runs as the GFS has the trough being further north and east compared the ECMWF which has been the most consistent. The GFS solution has a quicker ejection of the trough with minimal impacts to the CWA. The ECMWF and NBM solution give the best chances for PoP through the CWA Wednesday into Thursday evening. Ahead of the front, elevated to critical fire weather conditions will exist as a tight gradient and low RH values will be present until the front passes Wednesday. As the front passes, widespread showers and storms will be possible. Presently leaning toward the ECMWF and NBM solution keeping PoP chances in the area through the day Thursday before the trough departs late Friday afternoon. Zonal, westerly flow sets up behind the departing trough keeping PoP chances across the CWA but mainly across the northern zones through Saturday. Models want to drag another trough through the region toward the end of the weekend keeping PoP and below average temperatures in the forecast through Sunday. && .AVIATION...(For the 18Z TAFS through 18Z Monday) Issued at 1131 AM MDT Sun Aug 15 2021 VFR will be the prevailing condition with the exception of MVFR visibility due to wildfire smoke at KJAC and the Bighorn Basin terminals of KCOD and KWRL. Shortwave moving through central Wyoming late this morning with an area of convection centered around the Wind River Range. Outflow wind gusts around 45 kts have been the primary hazards east of the range. This convection and cloud cover may delay additional shower and thunderstorm development until a tad later this afternoon. Have added TEMPO groups at KCPR and KLND, and looks like some late afternoon outflow and convection could also reach KRKS. Impacts will generally be due to the outflow wind of 35- 45 kts. Conditions quickly improve at all terminals between 00Z- 03Z/Monday as the shortwave leaves the region. The exception again could be MVFR visibility in some basin terminals because of smoke. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued AT 238 AM MDT Sun Aug 15 2021 Elevated fire weather conditions are expected mainly across fire zones 277 and 414 this afternoon with marginal low relative humidity and wind. Isolated to locally scattered thunderstorms are expected this afternoon and early evening with the best chance over central and southern areas. The main hazard from thunderstorms will be localized very strong and erratic winds to near 50 mph. On Monday expect drier and warmer conditions with only very isolated thunderstorms possible mainly over the northwest. There is a chance for critical fire weather thresholds to be met late Monday afternoon and evening across the far northwest mainly for zones 140 and 286. Will hold from issuing a fire weather watch, as confidence in strong enough winds is low at the moment. A more widespread elevated to possibly critical fire weather conditions looks to occur Tuesday. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Murrell LONG TERM...Vorst AVIATION...CNJ FIRE WEATHER...Murrell
Hazardous Weather Outlook National Weather Service Riverton WY 255 AM MDT Sun Aug 15 2021 WYZ001>020-022>030-161200- 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- 255 AM MDT Sun Aug 15 2021 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Isolated to scattered afternoon and early evening thunderstorms with the best chance over central and southern areas. The main hazard will be very strong winds between 45 and 55 mph. Elevated fire weather conditions over the the far southwest this afternoon and evening. .DAYS TWO THROUGH SEVEN...Monday through Saturday. Monday...Elevated to possibly critical fire weather conditions late Monday across the far northwest. Tuesday...Elevated to possibly critical fire weather conditions especially in the lower elevations east of the Divide, as well as over the southwest. Some thunderstorms over the west and northwest. Wednesday...Scattered thunderstorms across the entire area. Thursday...Isolated to locally scattered thunderstorms over much of the area. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to standard operating procedures. TEAM RIVERTON
Special Weather Statement
National Weather Service Riverton WY
951 AM MDT Sun Aug 15 2021
WYZ015>018-151615-
Wind River Basin-Upper Wind River Basin-Wind River Mountains
East-Lander Foothills-
951 AM MDT Sun Aug 15 2021
...Strong thunderstorms will impact portions of central Fremont
County through 1015 AM MDT...
At 949 AM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Crowheart to Fort Washakie. Movement was
east at 15 mph.
HAZARD...Outflow wind gusts in excess of 40 mph.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects.
Strong thunderstorms will be near...
Fort Washakie around 955 AM MDT.
Other locations in the path of these storms include Ethete.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
If on or near Morton Lake, get out of the water and move indoors or
inside a vehicle. Remember, lightning can strike out to 10 miles
from the parent thunderstorm. If you can hear thunder, you are close
enough to be struck by lightning. Move to safe shelter now! Do not
be caught on the water in a thunderstorm.
&&
LAT...LON 4342 10900 4294 10837 4298 10891 4336 10943
TIME...MOT...LOC 1549Z 279DEG 12KT 4331 10927 4302 10889
MAX HAIL SIZE...0.00 IN
MAX WIND GUST...40 MPH
FisherSpecial Weather Statement National Weather Service Riverton WY 1030 AM MDT Sun Aug 15 2021 WYZ016-017-151900- Upper Wind River Basin-Wind River Basin- Including the cities of Dubois, Riverton, and Shoshoni 1030 AM MDT Sun Aug 15 2021 ...Showers and thunderstorms producing strong, gusty winds... This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Scattered showers and thunderstorms are producing gusty outflow wind gusts of 40 to 50 mph. * WHERE...Wind River Basin. * WHEN...Through 1 PM. * ADDITIONAL DETAILS...Wind gusts of up to 55 mph associated with the showers and thunderstorms have been observed, but most storms are producing wind gusts between 35 and 45 mph. Occasional lightning and brief periods of heavy rain are also possible with some of the stronger storms.
The Riverton/Lander radar (KRIW), 34 km away, was scanning. The volume nearest the first event began at 17:38 UTC.
2 thunderstorm wind. Local standard time, as filed.
A maximum wind gust of 79 mph was measured by the ASOS at Hunt Field in Lander. Numerous limbs were downed in the city of Lander. There was damage report to one large metal-sided structure on the west side of town.
The ASOS at Big Piney-Marbleton Airport recorded a wind gust of 64 mph.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2021Every state in 2021This day in weather historyWarnings in Wyoming, August 2021
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: 971534, 971535.