A thunderstorm moved off of the Bighorn Mountains and became severe as it tracked southeast. Trained spotters reported quarter-sized hail east of Buffalo.
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The episode as NWS RIW wrote it up.
A thunderstorm moved off of the Bighorn Mountains and became severe as it tracked southeast. Trained spotters reported quarter-sized hail east of Buffalo.
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 9, 4:19 PM MST, 17 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 519 PM MDT Fri Jul 9 2021 .SHORT TERM...(This afternoon through Saturday night) Issued at 100 PM MDT Fri Jul 9 2021 Water vapor imagery early this afternoon shows an upper low over northwest Montana, and an upper ridge across the southern Rockies centered near the Four Corners area. A weak wave on the northern periphery of the upper ridge is allowing some weak convection across Sweetwater east northeast into extreme southeast Natrona County. This activity is expected to pull east of the area between 4 and 6 PM. The main hazard from this activity will be dry microbursts due to large dew point depressions. The main concern will be the aforementioned upper low/shortwave trough pulling east southeast this afternoon into tonight. By 12Z Saturday this system is progged to be in the High Plains of South Dakota and Nebraska. High resolution models show the potential for convection late this afternoon into the evening over the far north and northeast. Some guidance shows most if not all of the convection will grazing the forecast area. The best chance of strong to possibly severe convection will be across northern Johnson County between 5 PM and 9 PM. All of the convection should push east of the area by around midnight. The associated cold front with the northern system will sweep southward across the area tonight. The combination of decent pressure rises in wake of the cold front and thunderstorm outflow winds will result in strong northerly wind at least across the Bighorn Basin and Johnson County. These strong winds could also impact central areas. Ahead of this cold front today, critical fire weather conditions will be occurring this afternoon and early evening with low humidity and gusty westerly winds. Upper level ridging builds into the area Saturday which will result in dry conditions with cooler temperatures and much lighter winds. .LONG TERM...(Sunday through Friday) Issued at 100 PM MDT Fri Jul 9 2021 Critical fire weather conditions are expected today across much of the region. A strong, mostly dry cold front will move across the region from west to east between Noon and 1800 this afternoon. With the cold front, very gusty winds will occur. Winds 30 to 40 mph with gusts 55 to 60 mph are expected with gusts up 70 mph possible. Winds will shift from southwest to westerly with the frontal passage. Scattered showers and isolated thunderstorms are possible across the far west and northern areas during the late afternoon. Gusty and erratic winds will likely occur with these showers and thunderstorms. Winds, and any shower and thunderstorm activity, will diminish around midnight. RH recoveries will be good to excellent overnight with temperatures decreasing and RHs increasing behind the front. Friday will be cooler and dry. The heat returns for the weekend and record heat is expected by Monday and continuing through midweek with no chances of wetting rain. && .AVIATION...(For the 00Z TAFS through 00Z Saturday evening) Issued at 517 PM MDT Fri Jul 9 2021 VFR conditions and mostly clear skies are expected over the next 24 hours. Some patchy smoke is occurring across the area, which could briefly reduce visibility at times through the period. Wind gusts of 35 to 40 kts will continue for much of this evening. KCOD, KWRL, and KCPR terminals will have the additional concern of an abrupt wind shift this evening once a cold front blasts in from the north. The wind shift will occur right around 00Z. There also is a chance of isolated thunderstorms across the north this afternoon and evening with the cold frontal passage, but not expecting any terminal impacts at this time. Winds remain breezy during the early overnight, before finally decreasing during the early morning hours Saturday. Winds will remain mostly light through the day tomorrow with clear skies. && .FIRE WEATHER... Issued AT 100 PM MDT Fri Jul 9 2021 ...Multiple Red Flag Warnings through the Evening... Red flag conditions will occur through early evening mainly across central, western, and southern fire zones. A cold front will quickly sweep across the area tonight. There is the potential for isolated thunderstorms over the far north and northeast later this afternoon into the evening. All of the showers and thunderstorms will push away from the area by midnight. In wake of the cold front, strong northerly winds are expected along and east of the Divide. On Saturday, dry conditions will occur with cooler temperatures, and much lighter winds. Critical fire weather conditions could return to portions of the area by early to middle of next week. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning until 9 PM MDT this evening for WYZ277>280-283- 285>288-300-414>416. && SHORT TERM...Murrell LONG TERM...Fisher AVIATION...Fisher FIRE WEATHER...Murrell
Hazardous Weather Outlook National Weather Service Riverton WY 205 AM MDT Fri Jul 9 2021 WYZ001>020-022>030-101000- 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- 205 AM MDT Fri Jul 9 2021 ...Multiple Red Flag Warnings thru this evening mainly across central and southern areas... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Critical fire weather conditions over central, south and western areas during the afternoon and evening. Additionally, gusty west and northwest winds are expected during the afternoon and evening with gusts 40 to 50 mph. Winds and fire danger decreases after sunset. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. Tuesday and Wednesday...Isolated to widely scattered afternoon and evening thunderstorms possible across much area. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to standard operating procedures.
The Riverton/Lander radar (KRIW), 219 km away, was scanning. The volume nearest the first event began at 23:36 UTC.
1 hail. Local standard time, as filed.
Trained spotters reported quarter-sized hail across portions of northern Johnson County to the east of Buffalo.
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: 971099.