An upper level low passing over Wyoming combined with a strong jet stream to bring an outbreak of severe thunderstorms. Four tornadoes were reported on this day, the most in one day since June 12, 2017.
Every event with a place to put it. Tap one for its entry below.
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.
An upper level low passing over Wyoming combined with a strong jet stream to bring an outbreak of severe thunderstorms. Four tornadoes were reported on this day, the most in one day since June 12, 2017. There was one each in Johnson and Sweetwater County and two in Natrona County. The largest tornado was south of Midwest in northern Natrona County where a horse trailer was flipped and fuel tanks were blown away. Very large hail also occurred with some of the storms. The largest hail occurred in southern Johnson County and northern Natrona County where baseball size hail was reported. Another severe thunderstorms moved from Lander to Riverton and dropped quarter to golf ball size hail, causing damage to many vehicles.
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 Jun 23, 10:02 AM MST, 13 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1102 AM MDT Fri Jun 23 2023 .DISCUSSION...(Today through Thursday) Issued at 218 AM MDT Fri Jun 23 2023 At this point, today is looking like maybe the most notable outbreak of severe weather we have seen so far this year in central Wyoming. We continue to have some showers moving across the area, with even a bit of embedded thunder. However, nothing strong at this time. This should remain the case through the morning hours. A few thunderstorms will develop in western Wyoming, but the main event should hold off until after noon. We have a good set up for a severe weather outbreak across central Wyoming. A trough / upper level low will be moving toward western Wyoming, with models showing good upper level divergence over the area this afternoon. Soundings are showing good directional shear over the area, with 0 to 6 kilometer bulk shear over 40 knots across the area East of the Divide, especially Johnson County and the Big Horn Basin, which is where the bulk of the severe weather is expected. We will also have a trigger moving in from the south, in the form of a push of drier air moving in from the south to make a sort of a pseudo dry line. For example, one guidance, dew points will range from the mid 20s in Rock Springs to the middle 50s in the Big Horn Basin and Johnson County. How far the boundary gets when storm development begins will really determine the southern boundary of the severe weather. The tricky areas are areas like the Wind River Basin and Natrona County, which will be close to the boundary when storm generation really gets fired up. Instability parameters are also fairly impressive with CAPE over 1000 J/Kg in the Big Horn Basin and approaching 2000 J/Kg in eastern Johnson County at 21Z. Lifted indices of minus 4 to as low as minus 7 are also progged across the same area. However, if there is one thing I'm concerned about, it is that sky cover may hold instability down just a bit. But that is a minor concern at this time. And finally, to enhance lift some more, we will be the left front quadrant of a jet streak this afternoon. Most convective models show thunderstorm genesis East of the Divide initiating around noon and then progressing eastward across the area through the mid afternoon hours. And, most looks to be north of a Riverton to Casper line and then moving northward as the drier surge of air pushes north. The best chance of severe storms still looks to be at the time of peak heating, between 2 pm and 8 pm. Some storms will continue late tonight, but the threat of severe storms should end after sunset. Now for the expected hazards. We continue to have a moist airmass in place over the area. Some of this will be pushed northward as the drier air moves in though. Values are still over 125 percent of normal across the north though. Storms will be moving with good steering flow though so there could be localized flooding but this is not the main threat. Next we tackle strong wind. This may be the most widespread threat, Strong low level winds will mix down in the dry air mass, with wind gusts possibly to 50 mph across southern Wyoming. This looks to be main threat in areas like Riverton, Lander and Casper as these winds may arrive with the storms and get mixed to the ground when the drier air arrives. All areas will have a chance though. Now we move to large hail. This may be the greatest threat for really impactful weather. We have a rare significant hail threat across portions of the area. No surprise, it is over areas roughly east of of Buffalo to Midwest line, where moisture will be the deepest and tilted updrafts given the ample shear should bring a good chance of large hail, possibly over golf ball size. However, some threat exists further west into the Big Horn Basin, with the best chance from east of the Lovell to Worland line, where storms usually get stronger as they move into higher dew point air in the Bighorn Basin. Many areas could get small hail though. Now on to the "T" word, tornadoes. Most models show the best low level shear in Johnson and Natrona Counties. Model soundings also show the lowest Lifted Condensation Levels (LCL) here, making the best chance of a funnel to touch the ground. The 2 percent area extends roughly from Casper to the Big Horn / Johnson County line. The more important 5 percent area goes, again, from the favored Interstate 25 corridor and east from Buffalo to Midwest. This is probably the lowest threat but this is probably the best chance of seeing a tornado that we have had so far this year. As for the rest of the forecast period, things haven't changed much previous days. Ridging will build into Wyoming on Saturday, bringing warmer and drier, but not dry weather. With the state near the top of the ridge, we will see shortwaves ride over the area and bring a daily chance of showers and thunderstorms. The best chance each day would be across northern Wyoming. The question each day will be how far south will coverage get and there has been poor model consistency in regards to this. So, given this and the active weather expected today, we have made few changes to continuity. This will be with higher chances of convection across the north and tapering them to low to none across southern Wyoming. && .AVIATION...(For the 18Z TAFS through 18Z Saturday) Issued at 1102 AM MDT Fri Jun 23 2023 Showers and thunderstorms continue to develop and spread across the area this afternoon. Most terminals begin the period with VCTS with the remainder seeing VCTS by 21Z. Showers and thunderstorms continue into the evening with stronger storms expected east of the Divide. Winds may become gusty at times this afternoon with gusts between 25 to 35 knots. Depending on the location of stronger storms gusts, over 50 knots can not be ruled out. Some storms may create periods of MVFR/IFR due to heavy rain. Frequent lightning along with small hail is also a threat with any storm. Thunderstorms begin to dissipate during the evening with light to moderate rain showers lingering over northern Wyoming terminals overnight. By Friday morning VFR conditions are expected to prevail through the end of the period. A few terminals may see some lingering MVFR ceilings but ceilings will improve by the late morning. Winds begin to increase during the morning with gusts between 20 to 30 knots possible at some terminals come the end of the period. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued at 218 AM MDT Fri Jun 23 2023 Low pressure will move across Wyoming and bring showers and thunderstorms, most numerous in the afternoon and evening and across northern Wyoming. Some storms could become strong to severe with large hail, strong winds and possibly a tornado. Heavy rain is also possible with any storm with a wetting possible. In southern Wyoming, strong wind will develop this afternoon with wind gusts past 40 mph at times. Relative humidity will fall below 15 percent across portions of Sweetwater County, otherwise it will remain above critical levels. Mixing and smoke dispersal will range from fair to good across the north to excellent across southern Wyoming. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && DISCUSSION...Hattings AVIATION...Dziewaltowski FIRE WEATHER...Hattings
Hazardous Weather Outlook National Weather Service Riverton WY 228 AM MDT Fri Jun 23 2023 WYZ001>020-022>030-241300- 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- 228 AM MDT Fri Jun 23 2023 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and tonight. Showers and thunderstorms developing later this morning and continuing into tonight. Some storms may become severe with large hail, damaging wind and even an isolated tornado. The best chance of severe thunderstorms will be in Big Horn, Washakie, Natrona and especially Johnson County. All areas East of the Divide will have a chance of stronger storms though. Strong wind developing this afternoon across Sweetwater County. .DAYS TWO THROUGH SEVEN...Saturday through Thursday. Saturday through Thursday...Daily chances of mainly afternoon and evening thunderstorms, especially in northern Wyoming. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to Standard Operating Procedures.
Special Weather Statement
National Weather Service Riverton WY
1050 AM MDT Fri Jun 23 2023
WYZ015-017-018-231715-
Wind River Basin-Wind River Mountains East-Lander Foothills-
1050 AM MDT Fri Jun 23 2023
...A strong thunderstorm will impact portions of south central
Fremont County through 1115 AM MDT...
At 1050 AM MDT, Doppler radar was tracking a strong thunderstorm over
Sinks Canyon State Park, which is also 8 miles southwest of Lander,
moving northeast at 20 mph.
HAZARD...Winds in excess of 30 mph and up to 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.
This strong thunderstorm will be near...Lander and Milford around
1110 AM MDT.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.
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.
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4309 10881 4278 10863 4265 10888 4293 10905
TIME...MOT...LOC 1650Z 232DEG 19KT 4275 10884
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH
DziewaltowskiThe Riverton/Lander radar (KRIW), 34 km away, was scanning. The volume nearest the first event began at 17:13 UTC.
5 hail, 4 tornado. Local standard time, as filed.
A trained spotter reported half dollar size hail north of Lander. Video from a local media outlet showed quarter size hail falling in Lander.
There were numerous reports of quarter to golf ball size hail in around around Riverton, with the largest hail stones reported on the east side of Riverton. Many vehicles sustained windshield and body damage from the hail. Dozens of homes on the east side of Riverton had roof shingle damage, with roof replacements occurring regularly over the next 4 months.
The public reported hail larger than golf balls to the north of Lysite.
Video from the public showed a narrow rope-shaped tornado northeast of Granger. The tornado touched down over open country, and caused no known damage. A rating of EFU was assigned because the brief tornado traversed land lacking damage indicators such as trees structures.
There were numerous reports, pictures, and video of a cone tornado that touched down to the south of Midwest, WY. The tornado remained over open country for the most part. However, the tornado damaged one home where a large section of the roof was removed. Exterior walls remained standing. A trailer was overturned, fuel tanks were blown away, and trees near the residence were damaged.
Based on the FR12 damage indicator category, this tornado was rated EF1 using a low to middle end DOD estimate for the type of construction. Winds are estimated to have been around 110 mph. Stronger winds could have occurred elsewhere along the track, but open country prevented any further evidence of wind damage. Damage survey completed by county emergency management officials.
A supercell thunderstorm developed in the lee of the Bighorn range and brought large hail from Barnum to Kaycee. Hail the size of baseballs fell near Barnum with quarter size hail reported in Kaycee.
The public sent a picture of a rope tornado to the north of Natrona. The tornado remained over open country and caused no known damage. A rating of EFU was assigned because the tornado traversed remote land lacking damage indicators such as trees structures.
A rancher reported a brief touchdown of a tornado north of Kaycee to the Johnson County Sheriffs office.
A supercell thunderstorm moved across northern Natrona County and brought very large hail to the south of Midwest. Hail almost baseball size fell in a farm to south of Midwest as well as along Interstate 25.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2023Every state in 2023This day in weather historyThe tornado archive for Wyoming, 2023Warnings in Wyoming, June 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: 1105502, 1105503, 1105504, 1105505, 1105509, 1105510, 1105507, 1105508, 1105511.