A thunderstorm developed in the Lee of the Bighorn Mountains and become strong as it moved east. A spotter to the southeast of Buffalo, Wyoming reported nickel size hail.
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The episode as NWS RIW wrote it up.
A thunderstorm developed in the Lee of the Bighorn Mountains and become strong as it moved east. A spotter to the southeast of Buffalo, Wyoming reported nickel 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 Riverton, WY wrote its last area forecast discussion before the first event at May 12, 2:02 PM MST, 2 hours 20 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 302 PM MDT Tue May 12 2020 .SHORT TERM...(This evening through Wednesday night) Issued at 216 PM MDT Tue May 12 2020 Initial vort max is sweeping across northern WY currently, with showers and thunderstorms starting to fire and increase in number. Warm front will continue to focus the chances for thunderstorms this afternoon, especially for Johnson and Natrona Counties. Have received a report of 1/2" hail in the past hour, so any storms will bring a hail threat, along with gusty winds at times. Behind this impulse, another will sweep northeastward mainly across the northwest corner of the state tonight. This will bring another round of showers from Yellowstone southward along the Teton and Wyoming Ranges, likely after midnight. For Wednesday, general showers will sweep across most of the state. Models are noting two separate waves, one moving across the northern portion of the state, one moving across Sweetwater County and points east. CAPE values remain low, so thunderstorms will be tougher to form. The best chance for thunder looks to be the southern Sweetwater County during the afternoon. Most shower activity should fade after sunset as the wave sweeps east and with the loss of day time heating. .LONG TERM...(Thursday through Tuesday) Issued at 216 PM MDT Tue May 12 2020 A weak shortwave may keep light showers over the Yellowstone area on Thursday morning. A stronger wave will arrive Thursday afternoon, bringing two shots of reinforcing moisture and showers across western Wyoming, spreading east of the Divide overnight. This will continue through Friday night, with the most QPF falling across northern areas. 700mb temperatures will generally be in the 0 to -2C range across the northern half of the state, thus snow is expected at higher elevations with only light accumulations. Precipitation will mainly end by early Saturday afternoon, with moderate ridging building across the area. Western Wyoming may see continued isolated showers through Sunday night, but most areas should remain dry after Saturday. With the ridging a warming trend will begin on Sunday with temperatures east of the Divide possibly peaking into the 80s on Sunday into early next week. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 216 PM MDT Tue May 12 2020 East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Shower and thunderstorm activity will continue east of the divide this afternoon, with some restrictions near heavier thunderstorms. Be alert for gusty winds in and around any storms. Activity will decrease overnight, with cigs remaining above minimums. Another round of showers will begin to impact eastern sites generally right around 18z Wednesday, with better chances tomorrow afternoon. West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals Showers and thunderstorms have been able to get into KPNA this afternoon, and are expected to continue in the vicinity through the rest of the afternoon. Be aware of localized vis reductions and gusty winds in the vicinity of any storms. Thunderstorms will end briefly this evening, before another round of showers moves into the western TAF sites after 06Z. Showers will continue into the day Wednesday, with some thunderstorms possible around KRKS starting around 18z. 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 216 PM MDT Tue May 12 2020 Gusty winds continue across zone 279 this afternoon, where humidity values remain below 20 percent. Winds will decrease around sunset, with humidity recovering to 70 to 75 percent overnight. Showers and thunderstorms continue across the remainder of the state. Temperatures remain cooler, with resulting humidity values well above criteria. Another round of showers moves across the state on Wednesday, with minimal lightning expected. This will also keep winds lower and humidity elevated for all areas. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Straub LONG TERM...Myers AVIATION...Straub FIRE WEATHER...Straub
Hazardous Weather Outlook National Weather Service Riverton WY 309 AM MDT Tue May 12 2020 WYZ001>020-022>030-131200- 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- 309 AM MDT Tue May 12 2020 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. A chance of afternoon and evening thunderstorms over the north. Some storms may be strong producing hail and wind gusts over 40 mph. There will be isolated thunderstorms in the central and south. Elevated fire weather conditions in Sweetwater county this afternoon and early evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. Wednesday through Friday...A chance of afternoon and evening thunderstorms each day. Some rain showers will linger through the late night and morning hours each day. Saturday and Sunday...Isolated afternoon and evening thunderstorms. .SPOTTER INFORMATION STATEMENT... Spotters should report their rainfall amounts over the coming days as well as any hail or strong wind gusts produced by thunderstorms.
Special Weather Statement National Weather Service Riverton WY 214 PM MDT Tue May 12 2020 WYZ001>011-015>020-022-130200- 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- Wind River Mountains East-Upper Wind River Basin-Wind River Basin- Lander Foothills-Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain- Including the cities of Lake, Mammoth, Old Faithful, Clark, Cody, Meeteetse, Basin, Greybull, Lovell, Powell, Shell, Thermopolis, Ten Sleep, Worland, Buffalo, Kaycee, Dubois, Riverton, Shoshoni, Lander, Jeffrey City, and Casper 214 PM MDT Tue May 12 2020 ...Scattered showers and isolated thunderstorms continuing into this evening... * WHAT...Showers and thunderstorms. Some stronger storms could contain gusty wind, brief heavy rain, and hail up to one half an inch in diameter. * WHERE...Much of Wyoming east of the Continental Divide. * WHEN...Through 8 pm. * ADDITIONAL DETAILS...Please seek shelter if threatening weather approaches.
Special Weather Statement
National Weather Service Riverton WY
219 PM MDT Tue May 12 2020
WYZ019-020-022-122045-
Green Mountains and Rattlesnake Range-Casper Mountain-Natrona County
Lower Elevations-
219 PM MDT Tue May 12 2020
...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN NATRONA COUNTY UNTIL
245 PM MDT...
At 217 PM MDT, Doppler radar was tracking a strong thunderstorm near
Alcova, which is 31 miles southwest of Casper, moving northeast at 40
mph.
Wind gusts up to 40 mph will be possible with this storm.
This strong thunderstorm will be near...
Bates Hole around 240 PM MDT.
Other locations impacted by this storm include Alcova Dam, Bessemer
Bend and Casper Mountain.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
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.
If on or near Pathfinder or Alcova Reservoirs, 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 4243 10664 4243 10700 4245 10701 4282 10644
4253 10620
TIME...MOT...LOC 2017Z 242DEG 33KT 4251 10675
HulmeSpecial Weather Statement National Weather Service Riverton WY 229 PM MDT Tue May 12 2020 WYZ001>011-014>020-022-025-026-130200- 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- 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- Upper Green River Basin Foothills-Upper Green River Basin- Including the cities of Lake, Mammoth, Old Faithful, Clark, Cody, Meeteetse, Basin, Greybull, Lovell, Powell, Shell, Thermopolis, Ten Sleep, Worland, Buffalo, Kaycee, Dubois, Riverton, Shoshoni, Lander, Jeffrey City, Casper, Pinedale, La Barge, Big Piney, and Farson 229 PM MDT Tue May 12 2020 ...Scattered showers and isolated thunderstorms continuing into this evening... * WHAT...Showers and thunderstorms. Some stronger storms could contain gusty wind, brief heavy rain, and hail up to one half an inch in diameter. * WHERE...Much of Wyoming east of the Continental Divide and portions of Sublette County. * WHEN...Through 8 pm. * ADDITIONAL DETAILS...Please seek shelter if threatening weather approaches.
Special Weather Statement National Weather Service Riverton WY 453 PM MDT Tue May 12 2020 WYZ028>030-130200- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- Including the cities of Rock Springs, Green River, and Wamsutter 453 PM MDT Tue May 12 2020 This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Isolated high-based showers. Wind gust between 45 and 55 mph could occur. Very little or no rain is expected with this activity. * WHERE...Sweetwater County * WHEN...Through 8 PM. 21
The Riverton/Lander radar (KRIW), 195 km away, was scanning. The volume nearest the first event began at 23:20 UTC.
1 hail. Local standard time, as filed.
A trained spotter reported nickel sized hail to the southeast of Buffalo, Wyoming.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2020Every state in 2020This day in weather historyWarnings in Wyoming, May 2020
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: 886387.