Another day of slow-moving thunderstorms produced flooding in Rock Springs. The flooding was much less severe than two days previous and produced little additional damage.
Every event with a place to put it. Tap one for its entry below.
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.
Another day of slow-moving thunderstorms produced flooding in Rock Springs. The flooding was much less severe than two days previous and produced little additional damage.
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 Jul 30, 4:11 PM MST, 1 hour 49 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 511 PM MDT Fri Jul 30 2021 .SHORT TERM...(This afternoon through Saturday evening) Issued at 1150 AM MDT Fri Jul 30 2021 An elongated ridge stretches from the Southern Plains to the Pacific Northwest. This pattern continues to bring monsoonal moisture into western and central Wyoming as subtropical air moves northward around the western edge of a broad surface high. The morning sounding from RIW had precipitable water values near 1.2 inches, which is about twice the normal for this time of year. Other basins across the region have similar anomalies, and this means any afternoon and evening thunderstorms are likely to be heavy rain producers. Current high-resolution guidance is in good agreement showing a shortwave meandering northwestward over the Upper Green River Basin this afternoon and evening which places the greatest mesoscale lift over southwestern Wyoming. This lines up well with the Flash Flood Watch currently in place highlighting the most likely area to see localized flooding underneath thunderstorms cores and in drainages with thunderstorms upstream. Other areas across central Wyoming, including the Wind River and Powder River Basin also have ample opportunity for thunderstorms this evening, so even those outside the Flash Flood Watch may want to keep a close eye on radar this afternoon and evening. Daytime high temperatures will be slightly cooler than normal (80s) thanks to abundant cloud cover. A related minor change to the forecast is lower afternoon CAPE values, with 300 to 500 J/kg during peak heating (down from 500-700 J/kg, which was the consensus yesterday). This trend combined with dearth of shear means that large hail and damaging wind gusts look unlikely this afternoon, with flash flooding remaining as the primary concern. Given widespread cloud cover and low-level moisture, overnight temperatures remain mild with patchy fog possible in valleys where rain falls in the evening or overnight. Saturday's weather will be similar to Friday's; however, temperatures will be 3 to 5 degrees cooler, and weak push of drier air edges into northeast Wyoming as the Midwestern trough sags southward, causing slightly lower chances for thunderstorms east of the Divide. Southwest Wyoming remains under wet monsoonal flow, and another round of afternoon and evening thunderstorms could cause localized flash flooding there. .LONG TERM...(After midnight Saturday night through Thursday) Issued at 1150 AM MDT Fri Jul 30 2021 The monsoon moisture fetch will remain across areas west of the Continental Divide Sunday. Deeper moisture continues feeding northward into this part of the forecast area from the Desert Southwest. Precipitable water values remain high across the west Sunday, so scattered to widespread showers and thunderstorms will persist. In keeping with previous runs, models show drier air working southward across areas east of the Divide Sunday as surface high pressure slides into the Northern Plains from Canada. There is a noticeable downturn in precipitable water values across northern and central Wyoming Sunday. The result will be only isolated, mainly terrain-driven, convection anticipated Sunday east of the Divide. The additional sunshine along with slightly warmer mid-level temperatures causes eastern areas to return to around climatological normal daytime highs, while the west remains cool. The monsoon moisture begins to show an eastward movement on Monday as drier air in southwest flow aloft stretches across the Great Basin behind a shortwave set to reach northwest Wyoming late Monday. This shortwave and the eastward progression of the moisture sets the stage for a better chance of showers and storms spreading west to east Monday. The most widespread precipitation should be across the northwest quarter of the state as the shortwave swings through. Dynamics will be such that light showers are to linger across the north half of the forecast area overnight. Daytime highs continue below normal west of the Divide, while climbing to around normal in the east. Tuesday will be drier in the wake of the shortwave. Moisture is still available for convective development, so diurnally-driven showers and storms develop in the afternoon into the evening. However, coverage will be much less widespread. Increased steering wind aloft and lower precipitable water values will substantially diminish the threat for heavy rain. Wednesday may end up being the driest day of the period as a 500mb ridge axis sets-up along our western border. Medium-range models and cluster analysis show a deamplification to the ridge Thursday and Friday as a series of shortwaves rides through southwest flow aloft ahead of a Pacific Northwest trough. Isolated to scattered afternoon and evening showers and storms, typical of early August, along with near normal temperatures are expected as the upcoming work week draws to a close. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) Issued at 510 PM MDT Fri Jul 30 2021 Widespread showers and thunderstorms are ongoing across the forecast area late this afternoon. The primary aviation hazard remains localized MVFR conditions due to the potential for heavy rain. With little steering flow aloft, the showers and storms will be slow to move. The best chance for MVFR will be at KJAC, KCPR, KRKS, and KLND until about 02Z/Saturday. Otherwise, all terminals will be VFR through 00Z/Sunday. It does appear a weak wave may bring another round of showers to central Wyoming overnight, but conditions should remain VFR. Surface wind speeds, other than brief outflow wind to around 20 kts, will generally be less than 12 kts through Saturday afternoon. The lone exception will be KRKS where the pressure gradient could allow for 10-20 kt winds Saturday afternoon. Occasional mountain top obscurations continue through 08Z/Saturday. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued AT 1150 AM MDT Fri Jul 30 2021 Monsoon moisture will continue to rotate northward into Wyoming through the weekend around a ridge of high pressure located in the central Rockies. The moisture content of the atmosphere is well- above normal, particularly across southwest Wyoming where a Flash Flood Watch is in effect through Saturday afternoon. A lack of steering winds aloft means that any showers or storms are likely to be slow-movers further increasing the chance for localized heavy rain. Widespread showers and thunderstorms will continue this evening and again Saturday afternoon and evening. With the amount of moisture in place, persistent light showers linger through the overnight hours. There is at least a hint that drier air may work into far northern Wyoming Saturday and Sunday causing a downturn in precipitation coverage in that part of the forecast area, especially Sunday. Meanwhile, areas west of the Continental Divide can expect the monsoon moisture to keep widespread showers and storms in the forecast through Sunday. It may be Tuesday or Wednesday before drier air begins to make its way back into the state, signaling a return to more typical isolated to scattered showers and storms each afternoon and evening. Temperatures rebound to around climatological normal daytime highs by Sunday across the east. To the west, the cooler temperatures will continue through Monday before warming up. && .RIW WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch through Saturday afternoon for WYZ012>015-019- 023>030. && SHORT TERM...VandenBoogart LONG TERM...Jones AVIATION...Jones/Murrell FIRE WEATHER...Jones
Hazardous Weather Outlook National Weather Service Riverton WY 258 PM MDT Fri Jul 30 2021 WYZ001>020-022>030-312100- 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- 258 PM MDT Fri Jul 30 2021 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...This Afternoon and Tonight. Numerous thunderstorms are likely this afternoon and evening, especially west of the Divide. Some thunderstorms may produce locally heavy rainfall, and a Flash Flood Watch covers the area with the highest potential to see localized flash flood. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. Showers and thunderstorms are likely Saturday through Monday. The main hazard will be locally heavy rainfall which could cause localized flash flooding. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to standard operating procedures.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE RIVERTON WY
227 PM MDT FRI JUL 30 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0220 PM HEAVY RAIN ATLANTIC CITY 42.50N 108.73W
07/30/2021 M0.51 INCH FREMONT WY CO-OP OBSERVER
ABOUT HALF AN INCH OF RAIN IN ROUGHLY 15
MINUTES. NO FLOOD ISSUES SEEN. CELL MOVED
THROUGH QUICKLY.
&&
LVSpecial Weather Statement
National Weather Service Riverton WY
301 PM MDT Fri Jul 30 2021
WYZ019-302130-
Green Mountains and Rattlesnake Range-
301 PM MDT Fri Jul 30 2021
...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN NATRONA COUNTY UNTIL
330 PM MDT...
At 300 PM MDT, Doppler radar was tracking a strong thunderstorm near
Split Rock, or 18 miles east of Jeffrey City, moving east at 10 mph.
Half inch hail and winds in excess of 30 mph will be possible with
this storm.
This storm will remain over mainly rural areas of southwestern
Natrona County.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.
Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.
&&
LAT...LON 4244 10746 4243 10753 4257 10754 4266 10724
4243 10719
TIME...MOT...LOC 2100Z 259DEG 11KT 4250 10746
21Special Weather Statement National Weather Service Riverton WY 332 PM MDT Fri Jul 30 2021 WYZ019-302200- Green Mountains and Rattlesnake Range- 332 PM MDT Fri Jul 30 2021 ...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN NATRONA COUNTY UNTIL 400 PM MDT... At 330 PM MDT, Doppler radar was tracking a strong thunderstorm near Devil's Gate, or 26 miles east of Jeffrey City, moving east at 20 mph. Half inch hail and winds in excess of 30 mph will be possible with this storm. This strong thunderstorm will be near... Morman Handcart Visitor Center around 345 PM MDT. Independence Rock around 355 PM MDT. PRECAUTIONARY/PREPAREDNESS ACTIONS... Torrential rainfall is also occurring with this storm, and may lead to localized flooding. Do not drive your vehicle through flooded roadways. && LAT...LON 4244 10740 4253 10740 4256 10706 4243 10705 TIME...MOT...LOC 2130Z 271DEG 18KT 4244 10730 21
Special Weather Statement National Weather Service Riverton WY 411 PM MDT Fri Jul 30 2021 WYZ020-302245- Natrona County Lower Elevations- 411 PM MDT Fri Jul 30 2021 ...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL NATRONA COUNTY UNTIL 445 PM MDT... At 409 PM MDT, Doppler radar was tracking a strong thunderstorm over Waltman Rest Area, or 42 miles east of Shoshoni. This storm was nearly stationary. Torrential Rainfall is occurring between Hiland and Waltman including the Waltman Rest area. Also half inch hail and winds in excess of 30 mph will be possible with this storm. Significant ponding of water is expected along Highway 26. Please slow down to reduce the risk of hydroplaning. PRECAUTIONARY/PREPAREDNESS ACTIONS... Torrential rainfall is also occurring with this storm, and may lead to localized flooding. Do not drive your vehicle through flooded roadways. && LAT...LON 4303 10740 4317 10739 4313 10720 4302 10714 TIME...MOT...LOC 2209Z 270DEG 4KT 4308 10729 21
Special Weather Statement National Weather Service Riverton WY 442 PM MDT Fri Jul 30 2021 WYZ020-302315- Natrona County Lower Elevations- 442 PM MDT Fri Jul 30 2021 ...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL NATRONA COUNTY UNTIL 515 PM MDT... At 440 PM MDT, Doppler radar was tracking a strong thunderstorm 7 miles northeast of Powder River, or 34 miles northwest of Casper, moving east at 10 mph. Penny size hail and winds in excess of 30 mph will be possible with this storm. This strong thunderstorm will be near... Goldeneye Reservoir around 515 PM MDT. Natrona around 525 PM MDT. Other locations impacted by this storm include Natrona. PRECAUTIONARY/PREPAREDNESS ACTIONS... Torrential rainfall is also occurring with this storm, and may lead to localized flooding. Do not drive your vehicle through flooded roadways. && LAT...LON 4306 10701 4321 10695 4315 10664 4293 10671 TIME...MOT...LOC 2240Z 284DEG 9KT 4311 10689 21
Special Weather Statement National Weather Service Riverton WY 456 PM MDT Fri Jul 30 2021 WYZ004-008-302330- North Big Horn Basin-Bighorn Mountains West- 456 PM MDT Fri Jul 30 2021 ...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN BIG HORN COUNTY UNTIL 530 PM MDT... At 456 PM MDT, Doppler radar was tracking a strong thunderstorm 8 miles northwest of Medicine Lodge Archaeological Site, or 17 miles east of Basin, moving south at 5 mph. Half inch hail and winds in excess of 30 mph will be possible with this storm. This storm will remain over mainly rural areas of southeastern Big Horn County. PRECAUTIONARY/PREPAREDNESS ACTIONS... Torrential rainfall is also occurring with this storm, and may lead to localized flooding. Do not drive your vehicle through flooded roadways. && LAT...LON 4446 10777 4441 10752 4427 10757 4431 10788 TIME...MOT...LOC 2256Z 019DEG 6KT 4437 10769 21
The Riverton/Lander radar (KRIW), 175 km away, was scanning. The volume nearest the first event began at 00:59 UTC.
1 flash flood. Local standard time, as filed.
Heavy rain and runoff from higher desert landscape produced flash flooding in sections of Rock Springs that had seen similar conditions on July 28.
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: 971340.