A brief tornado formed out of the thunderstorm to north of Ten Sleep.
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The episode as NWS RIW wrote it up.
A brief tornado formed out of the thunderstorm to north of Ten Sleep.
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 9, 10:43 AM MST, 28 minutes earlier. Its key messages:
Area Forecast Discussion National Weather Service Riverton WY 1143 AM MDT Sat May 9 2026 .KEY MESSAGES... - Showers and thunderstorms will continue until roughly 5PM to 8PM today east of the Continental Divide. A severe thunderstorm or two cannot be ruled out (20% chance) across southern Natrona County from noon to 4PM. Gusty winds and hail are possible in any strong to severe thunderstorm. - Warmer temperatures and dry weather is expected Sunday through at least mid next week. The warmest days will likely be Monday through Wednesday with widespread highs in the 80s, and potentially the low 90s. - Elevated to near critical fire weather is expected for the next several days, with Monday and Thursday looking like the most concerning days at this time due to low relative humidity and gusty winds. && .UPDATE... Issued at 1134 AM MDT Sat May 9 2026 As of 11:30AM MDT, showers and thunderstorms have been ongoing across the Bighorn Basin, northern Fremont County, the Bighorn Mountains, and Johnson and Natrona Counties. Showers and thunderstorms will increase in coverage early this afternoon, especially over Johnson and Natrona Counties, as a shortwave treks southeastward over Wyoming. Marginal instability is present across these areas with surface based CAPE values around 500 J/kg, effective bulk shear ranging from 25 to 40 kts from north to south, and saturated soundings. Thus, most will see showers and thunderstorms with periods of heavy rain, lightning, small hail, and gusty 30-40 mph outflows. In fact, we received a report of very small hail (smaller than pea sized) in a thunderstorm south of Worland around 10:50 AM. A few thunderstorms could be strong to severe early this afternoon (roughly Noon to 4PM) across southern Natrona County. Bulk shear is maximized (50 to 60kts) across this area early this afternoon because of northeasterly flow at the surface and westerly flow aloft. This shear combined with decent CAPE indicates that there is potential for a stronger to severe thunderstorm or two, most likely across southern Natrona County. The main threats in a severe storm would be gusty 60 mph winds and 1 inch hail. As mentioned in the morning discussion, this is an afternoon show due to the shortwave trough axis trekking out of central Wyoming between 4PM and 6PM MDT. However, a few lingering rain showers and rumbles of thunder are possible until sunset over Johnson and Natrona Counties. The rest of the forecast remains on track with a warm up to near record temperatures and elevated to near critical fire weather conditions this upcoming work week. && .DISCUSSION... Issued at 229 AM MDT Sat May 9 2026 In some of my discussions I talk about the signs of seasons changing. And today, I saw one of the signs that summer is approaching. As I was driving up for my usual graveyard shift around 9:45 pm, it was the first day when there was still some light in the western sky. It will continue to get slowly brighter as we move toward the summer solstice on June 20th, when time of total darkness shrinks to only around 5 hours. And as for the weather today, two of the main weather impacts through the forecast period are most common in the summertime, thunderstorms and fire weather concerns. As for today, the main concern will be thunderstorms. The trigger for this will be a shortwave dropping southeastward out of Montana. The main impacts from this should remain East of the Continental Divide, most model guidance and the general thunderstorm outlook shows this. The next question is, will any of the thunderstorms be of the stronger variety? There are some decent parameters for stronger storms, with around 35 knots of bulk shear to lead to longer lasting storms. We also have decent CAPE with up to 500 J/Kg across southern Natrona County. This lines up well with the marginal risk of severe thunderstorms issued by the Storm Prediction Center this morning. Model soundings are showing inverted V signatures initially, indicating that storms would be more of the high based variety. Wet bulb zero levels are also fairly low, meaning that small hail will be possible of any storm that develops. A strong wind gust will also be possible. This may also be an earlier show, with most guidance showing storms developing by noon and ending by around sunset this evening. Coverage looks scattered so it is still difficult to pinpoint them, but the best chance will be across Johnson and Natrona Counties with around a 1 in 2 chance at any location, with decreasing chances further west where moisture is not as deep. Temperatures look to remain below normal East of the Divide, around 5 degrees on average. West of the Divide, expect temperatures close to seasonal normals. It will begin to more summer like for the Sunday and much of next week. Sunday looks nice a nice Spring day with highs in the 60s and 70s, light to moderate wind and a good deal of sunshine. We then turn to the other concern, fire weather, for Monday. The ridge will get flattened by an approaching shortwave. This wave has little moisture to work with, but will bring some gusty wind, with a greater than 2 out of 3 chance of wind gusts past 30 mph for much of the area. Combined with temperatures 15 to 20 degrees above normal and relative humidity falling under 15 percent across much of the area, elevated to critical fire weather looks fairly likely for much of the area Monday afternoon. Ridging then builds back over the area and brings lighter wind for Tuesday before another wave approaches for Wednesday and may bring a few showers and storms West of the Divide. Southwest, downsloping flow will bring even warmer temperatures. And with the theme of summer, we may see the first appearance of a summer phenomenon, highs in the 90s. Ensemble guidance is showing locations mainly below 4200 feet in the Big Horn Basin with anywhere from a 1 in 3 to a 1 out of 2 chance of the first 90 degree high on Wednesday, largely in any area following the Big Horn River from Thermopolis though Worland, Greybull and up toward Lovell. Many other areas will see highs well into the 80s. A gusty breeze will also bring elevated fire weather, although wind does not look as strong on Wednesday as on Monday. Conditions look similar on Thursday as well, although maybe a couple of degrees cooler. The next more potent system in the form of an upper level low may approach for the end of the next week. However, there is low model agreement in timing and placement for any precipitation, so details are not possible to hash out this far out. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1130 AM MDT Sat May 9 2026 West of the Divide...KJAC/KRKS/KPNA/KBPI Terminals Shortwave energy along the northwest Wyoming border tracks southeast Saturday afternoon and early evening. A few showers could pop over the mountains to the east, and potentially west, of KJAC early Saturday afternoon. These would be short-lived with no impacts. All other terminals to see little more than cumulus clouds through sunset Saturday. Gusty west-northwest wind 15-30kts prevails until 01Z-03Z/Sunday. Wind speeds diminish at sunset, although KRKS will see a surge of east-northeast wind 10-15kts for the late evening. East of the Divide...KCPR/KCOD/KRIW/KLND/KWRL Terminals Shortwave energy along the northwest Wyoming border tracks southeast Saturday afternoon and early evening. Convection has already begun and coverage increases from 19Z-23Z/Saturday. Expect the primary impacts to be embedded thunderstorms, small hail, and wind gusts around 30kts. Scattered showers linger around KCPR until around sunset Saturday, while most of the convection quickly tapers of by 01Z/Sunday as the shortwave gradually clears the area. VFR and dry conditions prevail at all terminals the rest of the night and Sunday morning. Please see the Aviation Weather Center and/or CWSU ZLC and ZDV for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued at 229 AM MDT Sat May 9 2026 Locally elevated fire weather is possible across southern Wyoming today. After a lull on Sunday, and approaching weather system on Monday will bring a gusty wind. Combined with low humidity and above normal temperatures, elevated to near critical fire weather looks likely for Monday. The middle of next week could also see elevated fire weather. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && UPDATE...Gerhardt DISCUSSION...Hattings AVIATION...CNJ FIRE WEATHER...Hattings
The Riverton/Lander radar (KRIW), 141 km away, was scanning. The volume nearest the first event began at 18:09 UTC.
1 tornado. Local standard time, as filed.
A non-supercell (a.k.a. landspout) tornado formed out of a thunderstorm to north of Ten Sleep and was filmed by a trained spotter. The tornado remained over open country and caused no known damage.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2026Every state in 2026This day in weather historyThe tornado archive for Wyoming, 2026Warnings in Wyoming, May 2026
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: 1340296.