An upper level disturbance passing across Wyoming brought a couple of severe thunderstorms to the state. The first episode was near Thermopolis, where a funnel cloud was sighted by several spotters and lowered about three quarters of the way to the ground.
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The episode as NWS RIW wrote it up.
An upper level disturbance passing across Wyoming brought a couple of severe thunderstorms to the state. The first episode was near Thermopolis, where a funnel cloud was sighted by several spotters and lowered about three quarters of the way to the ground. Another thunderstorm formed in the Lee of the Absarokas and produced quarter size hail to the east of Cody. Some rotation was also observed with this storm.
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 21, 2:04 AM MST, 9 hours 26 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 304 AM MDT Mon May 21 2018 .SHORT TERM...Today through Wednesday night Issued at 255 AM MDT Mon May 21 2018 Overview...Weak steering flow will continue over the region today, with a closed low forming over central CA. Winds aloft will remain south-southwesterly, with minor shortwaves moving northward over the state. These troughs, along with warming temperatures and abundant moisture, will aid in developing isolated thunderstorms this afternoon and evening. The closed low will become cut off by tonight, as it gradually moves over S NV. The nose of the channel jet is expected to move over southern and central portions Tuesday afternoon, with areas of U/L divergence over Sweetwater and Natrona counties. At the same time, the upper low will begin to fill and slowly lift toward the NE. Minor shortwaves/perturbations will continue to move over the state Wednesday, as the remnants of the upper low move over northern portions of the Great Basin. Discussion...Convective activity will once again be isolated today, mainly over the northern and western mountains. PW's will stay near 0.50" and dewpoints in the mid 30s to mid 40s. Locally heavy rain will be the main threat from these storms. Drier air will make its way over Sweetwater County, where dewpoints drop into the low to mid 20s. This will be temporary, as southerly winds return Monday night into Tuesday. Dewpoints will range between the 40s and low 50s across much of the forecast area Tuesday, with isolated convection starting over the western mountains by early afternoon. Storms will increase in coverage through the rest of the afternoon, with portions of the far north staying dry. Some of these storms could initially be strong, but the main threat will continue to heavy rain. The focus will shift mainly east of the Divide Tuesday evening, as these storms move northward through the overnight hours. Heavy rain with localized flooding is likely with these storms through midnight. By Wednesday morning, light showers could be ongoing across the far north. The area will quickly reset, as another shortwave moves over the forecast area during the day. Scattered thunderstorms will occur over western portions, with isolated thunderstorms east of the Divide. Stronger storms will be possible over Johnson and Natrona counties. Most of the convection should end/exit the area by midnight Thursday morning. .LONG TERM...Thursday through Monday Issued at 255 AM MDT Mon May 21 2018 Thursday will see scattered showers and thunderstorms over the west. Some of this activity moves east later in the day into Thursday evening impacting the north and central basins. Friday will see only isolated shower and thunderstorm activity, mainly over and near the mountains. Saturday looks like a dry day with very isolated showers or storms over the mountains late in the day or evening. The long range models continue to struggle with the track of a weather system that moves into California Saturday. One would think the weather system will weaken as it encounters the warm ridge of high pressure over the Rockies on Sunday into Sunday night. The 00Z model runs tonight are more aggressive with the system smashing into the ridge with a track that takes the system across NW WY Sunday night/Monday morning and then into Montana Monday night. This would result in showers and storms over the west and north Sunday afternoon and night with additional showers and storms next Monday. Have added this to the forecast and can refine it more in the coming days. High temperatures will be warming up for Thurs with 70s for most places. Friday through Sunday will see upper 70s to mid 80s across the basins with 70s in the west and south. Next Monday will be cooler. High elevation snow melt will occur with rising stream and river levels over the Memorial Day weekend. && .AVIATION...For the 12Z TAFS through 12Z Tuesday Issued at 255 AM MDT Mon May 21 2018 West of the Continental Divide...KJAC/KBPI/KPNA/KRKS VFR conditions to prevail through the forecast period. Scattered showers and thunderstorms will develop over the west after 17Z and continue through 06Z. Then isolated showers to linger through 12Z Tuesday. Local MVFR conditions will occur with some thunderstorms. Mountains will be obscured at times. Wind gusts to 30 knots and small hail will occur with stronger storms. East of the Continental Divide...KCPR/KCOD/KRIW/KLND/KWRL VFR conditions to prevail through the forecast period. Scattered showers and thunderstorms will develop over and near the mountains after 17Z and continue through 06Z. Then isolated showers to linger through 12Z Tuesday. Local MVFR conditions will occur with some thunderstorms. Mountains will be obscured at times. KCOD airport looks to have the best chance of seeing thunderstorm activity this afternoon and evening. 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 255 AM MDT Mon May 21 2018 Isolated showers and thunderstorms possible across western and northern portions this afternoon and evening. Relative humidity will return to lower values across the lower elevations, with eastern portions of Zone 279 reaching critical values. Mixing and smoke dispersal will range from good to excellent. Showers and thunderstorms will develop across southern portions and the higher elevations by mid afternoon Tuesday, becoming widespread late in the afternoon into the evening. Heavy rain with localized flooding will be possible, as these storms move northward Tuesday night. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...LaVoie LONG TERM...AR AVIATION...AR FIRE WEATHER...LaVoie
Hazardous Weather Outlook National Weather Service Riverton WY 210 AM MDT Mon May 21 2018 WYZ001>020-022>030-221100- 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- 210 AM MDT Mon May 21 2018 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Isolated afternoon and evening thunderstorms mainly over the northern and western portions. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Tuesday...Widespread showers and thunderstorms by late afternoon, continuing overnight. Heavy rain with localized flooding with the stronger storms, especially east of the Divide. Some stronger storms will have hail and strong wind gusts. Wednesday...A chance of afternoon and evening thunderstorms, especially over western and northern portions. Locally heavy rain possible over the northwest. Thursday...A chance of afternoon and evening thunderstorms over and near the mountains. Isolated thunderstorms elsewhere. Sunday...A chance of thunderstorms in the west. Isolated thunderstorms elsewhere. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed today or tonight.
The national radar mosaic for the moment the first event began.
1 funnel cloud, 1 hail. Local standard time, as filed.
Several trained spotters reported a funnel cloud to the east-southeast of Thermopolis over the Owl Creek Range.
A television station forwarded a report of quarter size hail to the east of Cody.
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, May 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: 749591, 749592.