Heavy rainfall over portions of the Mullen Burn Scar with estimated rainfall of two to three inches. Subsequent mudslide and debris flow led to a road washout near Highway 230.
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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 CYS wrote it up.
Heavy rainfall over portions of the Mullen Burn Scar with estimated rainfall of two to three inches. Subsequent mudslide and debris flow led to a road washout near Highway 230. The Woods Landing Resort also had nearby stream rise out of its bank with water on area campgrounds and a few inches of water were reported in the resort kitchen. Strongest storm over the Mullen Burn Scar produced reported penny size hail.
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 Cheyenne, WY wrote its last area forecast discussion before the first event at Jul 29, 9:53 AM MST, 4 hours 15 minutes earlier.
Area Forecast Discussion National Weather Service Cheyenne WY 1053 AM MDT Thu Jul 29 2021 .SHORT TERM...(Today and tonight) Issued at 330 AM MDT Thu Jul 29 2021 Latest WV imagery/upper air analysis showed the center of the upper ridge from eastern CO through KS. Monsoonal moisture was circulating northward through western CO into all but southeast WY. A front extended from south central through southeast WY into northern NE. Winds behind the front were northeasterly and gusty, with dew points in the mid 50s to lower 60s. This front is progged to push south of I-80 early this morning. High temperatures behind the front will be 5-10 degrees cooler than Wednesday. Moderate easterly winds will provide the setup for orographic forcing as well as increasing low level moisture. Scattered convection will develop this afternoon. Thermodynamics will be marginal for severe storms, with more of a strong wind threat and less of a hail threat due to long skinny CAPE forecast soundings. Locally heavy rain will accompany some of showers and storms. Storm motion will be north to northeast ~25 kt. Precipitable water values will range from 0.8" to 1.3". The Day 1 Excessive Rainfall Outlook (ERO) has a Marginal risk west of a Cheyenne to Dawes-Box Butte County line. Scattered showers and thunderstorms will linger into the late evening south of a Rawlins to Alliance line, and past midnight to the north. .LONG TERM...(Friday through Wednesday) Issued at 330 AM MDT Thu Jul 29 2021 Model consensus has the NW-SE oriented upper ridge extending from the Northern Rockies to the Northern Gulf Coast through the weekend. A digging upper trough over the Eastern CONUS will amplify the ridge over the Rockies and High Plains early next week. Precipitable water (PW) values will remain between 1" and 1.3" Friday through Saturday. Locally heavy rainfall will be the primary concern, with the Day 2 (Friday/Friday night) ERO placing the Marginal risk along and west of a line from Douglas to Cheyenne. A Slight risk for excessive rainfall was introduced over much of Albany, southeast Carbon and far western Laramie counties. This includes the Mullen Burn Scar where hourly rainfall rates could exceed an inch. Later shifts may need to consider a Flash Flood Watch within the Slight risk area. Most models show PW values decreasing to 0.6" to 1" Sunday through Monday as some drier air aloft tries to spread south into the CWA. This should lower the flash flood risk over the burn scars. There will be a decrease in areal coverage of showers and thunderstorms Tuesday through Wednesday as the upper ridge builds over the region accompanied by drier air aloft (PW's lower to 0.4" to 0.75"). High temperatures will cool into the 70s and 80s this weekend, followed by slight warming early next week with 80s to lower 90s. && .AVIATION...(For the 18Z TAFS through 18Z Friday afternoon) Issued at 1050 AM MDT Thu Jul 29 2021 Latest current radar imagery indicates some light convection pulsing across the north and eastern portions of the CWA. Expecting mostly VFR to MVFR conditions through the early morning hours, outside of KCYS currently at IFR. Should see KCYS ceilings lift to around MVFR conditions over the next few hours. Otherwise, main concerns still exist with thunderstorm potential through the late night hours. A fair amount of uncertainty with these systems as the HiRes models have been fairly inconsistent. Early runs indicated a decent amount of widespread convection developing across southeastern Wyoming, and pushing eastward into the Nebraska Panhandle. However, the most recent runs have backed off some of the thunderstorm activity west of I-25, and primarily southward into Colorado, likely due to the cloud deck over the region. As a result, went ahead and did not update the VCTS in the TAFS. Will likely need to evaluate later model runs to determine the exact timing and location of any impacts. && .FIRE WEATHER... Issued at 330 AM MDT Thu Jul 29 2021 A pattern change toward cooler temperatures and increasing chances for showers and thunderstorms will commence today and persist through the weekend as the ridge of high pressure shifts to the southeast. Minimum humidities the next few days will increase to 25 to 35 percent. Periods of heavy rain will raise the potential for localized flash flooding, particularly over the burn scars. && .CYS WATCHES/WARNINGS/ADVISORIES... WY...None. NE...None. && SHORT TERM...MAJ LONG TERM...MAJ AVIATION...MD FIRE WEATHER...MAJ
Hazardous Weather Outlook National Weather Service Cheyenne WY 340 AM MDT Thu Jul 29 2021 NEZ002-003-019>021-054-055-095-096-WYZ101>119-300945- Dawes County-Box Butte County-Scotts Bluff County-Banner County- Morrill County-Kimball County-Cheyenne County- Northern Sioux County-Southern Sioux County- Converse County Lower Elevations-Niobrara County- North Laramie Range-Ferris/Seminoe/Shirley Mountains- Shirley Basin-Central Laramie Range and Southwest Platte County- East Platte County-Goshen County-Central Carbon County- North Snowy Range Foothills-Southwest Carbon County- Sierra Madre Range-Upper North Platte River Basin-Snowy Range- Laramie Valley-South Laramie Range-South Laramie Range Foothills- Central Laramie County-East Laramie County- 340 AM MDT Thu Jul 29 2021 This hazardous weather outlook is for the western Nebraska Panhandle...east central Wyoming...south central Wyoming and southeast Wyoming. .DAY ONE...TODAY AND TONIGHT Scattered thunderstorms will develop this afternoon and evening. There is a marginal risk for isolated strong to severe thunderstorms producing wind gusts up to 60 mph and hail to the size of pennies. Locally heavy rain will accompany some of the showers and storms, which may lead to minor flooding. .DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY Scattered to numerous showers and thunderstorms are forecast Friday and Saturday. Periods of heavy rain, in excess of an inch per hour, may lead to flash flooding, particularly over the burn scars. Shower and thunderstorm coverage will decrease Sunday through early next week, with less of a threat for locally heavy rainfall. .Spotter information statement... Spotter activation will not be required.
The Cheyenne radar (KCYS), 123 km away, was scanning. The volume nearest the first event began at 21:05 UTC.
3 flash flood, 1 hail. Local standard time, as filed.
Spotter reported one inch of rainfall in thirty minutes along with pea size hail.
Heavy rainfall over portions of the Mullen Burn Scar with estimated rainfall of two to three inches. Subsequent mudslide and debris flow led to a road washout near Highway 230. The Woods Landing Resort also had nearby stream rise out of its bank with water on area campgrounds and a few inches of water were reported in the resort kitchen.
Received video from Wyoming Game and Fish Biologist of flash flooding along Lake Creek in the Mullen Burn Scar.
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, July 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: 965848, 965845, 965846, 974008.