Some mid and high level monsoonal moisture moving in from the south and weak shortwave approaching the area triggered some high based convection. With a very large dew point depression, some of the showers and thundertorms produced very strong winds as the thunderstorms collapsed.
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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.
Some mid and high level monsoonal moisture moving in from the south and weak shortwave approaching the area triggered some high based convection. With a very large dew point depression, some of the showers and thundertorms produced very strong winds as the thunderstorms collapsed. At the Lander Airport, a collapsing high based thunderstorm produced a wind gust of 63 mph. Another collapsing thunderstorm produced a 59 mph wind gust at the Worland 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, 12:14 PM MST, 1 hour 51 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 114 PM MDT Sun Aug 12 2018 .SHORT TERM...This evening through Wednesday Issued at 115 PM MDT Sun Aug 12 2018 A Pacific low pressure trough extending from southern Alberta, Southeast across the northern Rockies to central California will bring a cold front south and east through the area tonight through Tuesday. Increased cloud cover with some light rain showers and isolated thunderstorms will increase over the northern and western mountains this evening with showers lingering through the night and increasing through Monday afternoon and evening. Relative humidity will steadily increase with cooling temperatures. Winds will remain on the light side shifting from the southwest to north, northeast. Drift smoke from western wildfires will increase over the area through midweek. .LONG TERM...Wednesday night through Sunday Issued at 115 PM MDT Sun Aug 12 2018 Medium range models prog shortwave troughs/compact upper lows moving across Northern Rockies at times along the northern periphery of a subtropical high. Also a quasi-stationary front will generally be along and east of the Divide. This will result in isolated to locally scattered diurnal PoPs Thursday and Friday with an emphasis across the east. Temperatures are expected to be seasonal east, with seasonal to slightly above average temps west. GFS and ECMWF are indicating an upper low over MT on Saturday with a following front on Sunday so the weekend could end up pretty active. && .AVIATION...For the 00Z TAFS through 00Z Monday afternoon Issued at 115 PM MDT Sun Aug 12 2018 VFR conditions will prevail through 00z Monday. Southwest flow will bring more smoke into the region today, but visibility greater than 6 miles should continue to prevail. Isolated showers and thunderstorms will continue over/near the western mountains this evening with most of this activity moving NE along a vicinity KBPI- KDUB-KCOD line. Conditions will be favorable for microburst winds, with erratic wind gusts over 40 knots possible. A few showers, possibly a thunderstorm or two, are expected to linger from dusk to dawn tonight into tomorrow. && .FIRE WEATHER... Issued AT 115 PM MDT Sun Aug 12 2018 A Pacific low pressure trough extending from southern Alberta, Southeast across the northern Rockies to central California will bring a cold front south and east through the area tonight through Tuesday. Increased cloud cover with some light rain showers and isolated thunderstorms will increase over the northern and western mountains this evening with showers lingering through the night and increasing through Monday afternoon and evening. Relative humidity will steadily increase with cooling temperatures. Winds will remain on the light side shifting from the southwest to north, northeast. Drift smoke from western wildfires will increase over the area through midweek. Mixing heights will be favorable, but transport winds will be light through Monday. Most areas should still see very good to excellent smoke dispersal from local fires, but most of the area will remain covered in drift smoke with local visibility restrictions. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Baker LONG TERM...Hulme AVIATION...Hulme FIRE WEATHER...Baker
Hazardous Weather Outlook National Weather Service Riverton WY 1253 AM MDT Sun Aug 12 2018 WYZ001>020-022>030-131300- 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- 1253 AM MDT Sun Aug 12 2018 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Hot with isolated afternoon and evening showers and thunderstorms. Strong gusty winds are possible. .DAYS TWO THROUGH SEVEN...Monday through Saturday Monday...Isolated afternoon and evening thunderstorms, especially and east of the divide. Tuesday through Saturday...Isolated thunderstorms possible. .SPOTTER INFORMATION STATEMENT... Spotters are urged to report wind gusts past 45 mph. Thank you.
The Riverton/Lander radar (KRIW), 34 km away, was scanning. The volume nearest the first event began at 21:03 UTC.
2 thunderstorm wind. Local standard time, as filed.
The ASOS at the Lander Airport reported a wind gust of 63 mph from a collapsing thunderstorm.
The ASOS at the Worland airport reported a wind gust of 59 mph from a collapsing thunderstorm.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2018Every state in 2018This day in weather historyWarnings in Wyoming, August 2018
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: 779032, 779034.