A severe thunderstorm developed over northwestern Natrona County and became severe as it moved southeastward.
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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 RIW wrote it up.
A severe thunderstorm developed over northwestern Natrona County and became severe as it moved southeastward.
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 17, 10:09 AM MST, 2 hours 6 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1109 AM MDT Wed May 17 2023 .SYNOPSIS... Issued at 108 AM MDT Wed May 17 2023 The flow pattern remains light, with several pesky weak disturbances through the period. A couple of shortwaves bring weak cold fronts into the area Wednesday and Thursday. Moisture values remain above normal, so moderate rainfall is possible along the cold front Wednesday and Thursday afternoon/evening. By this weekend temps will begin to rise in earnest as more amplified/organized ridging develops over the western US. There will be chances for afternoon convection each day somewhere in the CWA. Early next week an upper- low looks to clip western WY, which would bring more widespread rain and thunderstorm chances to the CWA, but temperatures should remain above normal. && .DISCUSSION...Today through Tuesday Issued at 108 AM MDT Wed May 17 2023 A shortwave and associated cold front passes through this afternoon/evening, with widespread rain and thunderstorms likely along the front. The front is likely to stall along the Continental Divide, so areas west of the Continental Divide shouldn't see much change temperature-wise today. For areas east of the Continental Divide this will drop temps slightly through Thursday, but temps will be about average for this time of year. A second shortwave will pass through southwest WY and northern UT Thursday into Friday, which will lead to rain showers and thunderstorms for southwest WY through Friday morning. Afternoon thunderstorm chances hang around through the end of the week into this weekend for most locations. The flooding threat continues for western valleys, particularly Star Valley. Flooding is already occurring along the Salt River near Etna, with drone footage of the flooding sent to us from Lincoln County EM. With warm temps and thunderstorm chances each afternoon the flood threat will not diminish anytime soon. By Saturday the ridge strengthens and temps warm substantially. Widespread highs in the 70s and 80s are likely from Saturday through Monday. Some locations in the Bighorn Basin may even approach 90 degrees by Monday. There is better agreement with the 00Z suite of model guidance tonight on the pattern early next week, but much remains uncertain. A strong Pacific low will move onshore into the PACNW region on Monday and will struggle to push east due to the strong ridge over the region. The exact evolution is yet to be determined, but would expect the strong ridge to stave off the upper- low. It could very well clip western portions of the area and have enough influence for some more vigorous convective activity from Monday through Wednesday and beyond. Overall expect temps to stay above normal into next week and possibly through the rest of the month of May. If this pattern holds true areas west of the Divide would likely have to deal with more flooding issues thanks to rain from thunderstorms and possible high mountain snowmelt. && .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) Issued at 1108 AM MDT Wed May 17 2023 A cold front continues passing through the area, with locations east of the Divide experiencing winds shifting northerly. Along with the front, scattered showers, with some thunderstorms, are likely to develop through the afternoon hours. This activity will likely last after sunset, diminishing from 06Z to 12Z. Gusty winds will likely accompany the front and any showers/thunderstorms, with gusts between 20 and 30 knots. Generally VFR conditions are expected through the TAF period, with exception being any heavier rain, potentially bringing conditions down to brief MVFR. KCPR could see MVFR ceilings after sunset, with lasting overcast clouds. Winds will become lighter overnight and as rain chances decrease across the area. Thursday, another chances of afternoon showers and thunderstorms for the afternoon. 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 108 AM MDT Wed May 17 2023 A cold front will bring widespread rain showers and thunderstorms to the area today. There is a chance of wetting rain for zones east of the Continental Divide today. East of the Continental Divide zones will also have a wind shift from north to south in the morning and afternoon. Winds will shift northerly behind the front and gusts of 25 to 35 mph will occur. West of the Continental Divide winds will be gusty from the west/northwest. Zones 277, 278, and 279 will have wind gusts of 25 to 35 mph. Smoke dispersal will be good to excellent this afternoon and Thursday afternoon. Another cold front drops down during the day on Thursday with northerly wind gusts of 20 to 30 mph. More rain showers and thunderstorms will occur Thursday across the area, ending from north to south during the day. The best chance of wetting rain will be west of the Continental Divide Thursday. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SYNOPSIS...Rowe DISCUSSION...Rowe AVIATION...Wittmann FIRE WEATHER...Rowe
Hazardous Weather Outlook National Weather Service Riverton WY 154 AM MDT Wed May 17 2023 WYZ001>020-022>030-181200- 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- 154 AM MDT Wed May 17 2023 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. A Cold front passes through the area today, which will bring rain showers and thunderstorms, plus gusty northerly winds east of the Continental Divide. Minor flooding continues across western valleys. .DAYS TWO THROUGH SEVEN...Thursday through Wednesday. Thursday...Another cold front sweeps through areas east of the Continental Divide. Showers and thunderstorms become more widespread. Friday...Isolated thunderstorms are possible in western mountains. Saturday through Wednesday...Above normal temperatures. Mountain snowmelt increases, resulting in rising water levels. Rivers and creeks across the far west and near foothill locations are most likely to be impacted. Isolated thunderstorms are possible, especially west of the Continental Divide. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed this afternoon or tonight.
Special Weather Statement
National Weather Service Riverton WY
1256 PM MDT Wed May 17 2023
WYZ020-171945-
Natrona County Lower Elevations-
1256 PM MDT Wed May 17 2023
...A strong thunderstorm will impact portions of central Natrona
County through 145 PM MDT...
At 1255 PM MDT, Doppler radar was tracking a strong thunderstorm near
Powder River, or 34 miles northwest of Casper, moving southeast at 20
mph.
HAZARD...Winds in excess of 30 mph and half inch hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
Goldeneye Reservoir.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
&&
LAT...LON 4314 10702 4319 10692 4301 10657 4285 10685
TIME...MOT...LOC 1855Z 318DEG 18KT 4310 10692
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH
WittmannThe Riverton/Lander radar (KRIW), 128 km away, was scanning. The volume nearest the first event began at 19:16 UTC.
2 hail. Local standard time, as filed.
The Wyoming Highway Patrol reported nickel size hail along US 20 26 near Natrona.
A trained spotter reported quarter size hail to the west of Casper.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2023Every state in 2023This day in weather historyWarnings in Wyoming, May 2023
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: 1100662, 1100663.