A combination of an abnormally strong upper level low, strong jet dynamics and an approaching cold front produced one of the biggest severe weather outbreaks across western and central Wyoming in many years. A total of 4 tornadoes were confirmed, including an EF2 tornado which was the strongest tornado that occurred in the NWS Riverton county warning area since 2001 which completely destroyed a…
Every event with a place to put it; 7 filed by county or forecast zone are drawn at its center, in a lighter ring. 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.
A combination of an abnormally strong upper level low, strong jet dynamics and an approaching cold front produced one of the biggest severe weather outbreaks across western and central Wyoming in many years. A total of 4 tornadoes were confirmed, including an EF2 tornado which was the strongest tornado that occurred in the NWS Riverton county warning area since 2001 which completely destroyed a trailer and threw it's base over 200 yards. There were also multiple reports of large hail, especially in Johnson County where hail up to tennis ball size was reported. In the dry slot across central and southern Wyoming, strong mid level winds mixed to the surface. The strongest winds were in the Lander foothills where wind gusts up to 76 mph occurred. Many trees were uprooted with some damage to outbuildings. Wind gusts 58 mph or over were also reported in Sweetwater County.
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 12, 11:00 AM MST, 2 hours 11 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1200 PM MDT Mon Jun 12 2017 .SHORT TERM...Monday through Wednesday night ...A SEVERE WEATHER OUTBREAK LOOKS LIKELY ACROSS PORTIONS OF NORTHERN AND CENTRAL WYOMING... At this point, today may be shaping up to be one of the more active severe weather days we have seen in a while. An abnormally strong upper level low continues to move through the Great Basin with a strong cold front moving across the area. The models soundings are showing extremely steep lapse rates. CAPE values are extremely high with the NAM showing over 2500 J/KG across portions of Johnson and Natrona Counties with values close to 2000 J/KG even wrapping back into the Bighorn Basin with values over 1000 toward the Wind River Basin. Shear is also very impressive with 40 knots in many areas with enough to sustain storms and likely produce some supercell storms. Lifted indices on some models fall as low as minus 9 in portions of Natrona and Johnson Counties. Moisture also appears ample in many areas to sustain storms. And one additional thing, LCL levels are also rather low for this part of the country, averaging around 5000 feet. The Storm Prediction Center believes it as well. Approximately the northern half of the County Warning area with the exception of West of the Divide is under a slight risk. The enhanced risk is fairly similar to yesterday, running east of the line from Buffalo to Casper. And there is another unusual site, a moderate risk in Wyoming. It is east of the county warning area but too close for comfort, only over to around Douglas. At this point, with the large amounts of shear along with tons of instability and low wet bulb zero levels, the main threat looks to be large hail, especially in the east could be very large, possibly over golfball size. With the shear and lower LCL levels, tornadoes are also a possibility. The best shear is over northern areas so the best chance of one would be over Natrona or Johnson Counties. There is a 5 percent area over the Bighorn Basin as well but I am not quite confident to include those there at this time. We did add them to the enhanced risk area however. In addition, we will always have the threat of damaging wind with 50 to 55 knots of wind at 700 millibars at times. The greatest threat here would be in areas where the dry slot can move in, maybe areas like the Wind River Basin. However, the models can be shaky in pinpointing these. As a result, we went with severe wording in most areas covered by the slight risk except west of the Divide and added the "t" word, tornadoes, to portions of Johnson and Natrona Counties. The most likely time for the storms will be from around 2 pm to 9 pm with the worst centered around 4 to 7 pm. Please stay alert today to the weather this afternoon in these areas. However, we have more to talk about. Across Sweetwater County and the Rattlesnake Range, the dry slot should work it's way in. With 700 millibar winds of 50 to 55 knots and model guidance increasing surface winds to 36 knots, we will upgrade the high wind watch to a warning across Sweetwater County and the Green and Rattlesnake Range. We will cancel the watch elsewhere, since there is still much uncertainty on how far north the strong southerly flow and drier air will mix out. But, we have one more thing to talk about. We have had the "t" word, tornadoes, and the "w" word, wind. And now for late tonight and into Tuesday, we have to talk about the "s" word. Yes, there is a good chance for snow in the higher elevations of the western mountains above around 8000 feet. And there could even be some snow down to the valley floors at times. However, with most of the snow falling during the day and the high June sun angle, impacts on roads may be minimal. There could be advisory level amounts in the mountains however. For now, we will continue to SPS but an advisory may have to be considered later today. Quiet weather will return after that. Tuesday looks to be a cooler day. However, with low heights hanging over the area there will still be some showers and thunderstorms around. At this point, these look to be much less severe. It will be a rather windy day however with a tight pressure gradient remaining over the area 700 millibar winds still rather strong. Wednesday at this point, looks mainly dry with only isolated thunderstorms across the north with a shortwave brushing by to the north. .LONG TERM...Thursday through Monday A Pacific jet stream will bring a fast zonal flow across the northern Rockies through the weekend with upper ridge beginning to amplify over western U.S. Sunday and Monday. Overall, medium-range models have trended slower of the this amplification of the ridge over the west. The zonal flow will bring several fast moving shortwaves brushing across northern Wyoming along with chances of showers and afternoon- evening thunderstorms. These shortwaves are spaced about 24-30 hours apart, with low confidence in the specifics of timing. The main takeaway will a quasi-stationary frontal boundary across the north that will make southward advances with each passing shortwave, or almost on a daily basis. The strongest push appears to be Friday night and Saturday at this time where we bring slight chances of showers and thunderstorms to near a Casper-Riverton line by Saturday afternoon. Otherwise, precipitation chances are mostly confined to the northern mountains and far north. Breezy and warm conditions will prevail across the south through the extended period. Model trends in the 6-10 day timeframe are showing a further westward position of mean ridge along the west coast with NW flow along and east of the Divide. So temperatures look to stay closer to normal values into the start of Summer on Tuesday, June 20th. && .AVIATION...18Z Issuance As previously mentioned, the next 24 hours looks to be very busy weather-wise across western and central WY. First impact this afternoon will be strong southerly wind across south central WY moving northward into Upper Green River Basin and central WY as the day progresses. Second, thunderstorms are expected this afternoon and evening over both northwest WY and north central WY. The storms over NW WY will combine with strong southerly wind to create strong outflow gusts as well as small to moderate hail. Several severe thunderstorms are already moving through eastern ID. North central WY will see a potential for strong thunderstorms as well, combined with a tornado threat due to low level veering and increased surface moisture. After 21-22Z, a westerly push will the shift the winds starting in western WY at KBPI and move northeastward through the area. This westerly flow should transition the thunderstorms to showers. MVFR conditions in and around storms are highly likely. After 06Z Tue, additional shower activity is expected to develop over the west with snow levels dipping down to around 7000-7500 feet. Have indicated rain mixed with snow mainly at KJAC and possibly at KPNA. Another westerly push is expected across the area Tuesday between 12-18Z as the main cold front moves through the area and enhances precipitation across western WY. Expect conditions to gradually improve then Wednesday evening as the low pressure system lifts north. Also expect higher elevations to be obscured after 03Z/Tue. && .FIRE WEATHER... A boundary across the area will keep the chance of a few showers and thunderstorms in the forecast late today and this evening, with gusty and erratic winds possible with any storm. Otherwise, expect higher relative humidity today with temperatures fairly close to what we saw yesterday. Winds should be lighter then the previous couple of days outside of any shower or thunderstorm. Mixing and smoke dispersal will range from fair to good in the basins to very good to excellent in southern Wyoming and the normally windier areas. && .RIW WATCHES/WARNINGS/ADVISORIES... High Wind Warning from noon today to 9 PM MDT this evening for WYZ019-028>030. && SHORT TERM...Hattings LONG TERM...Meunier AVIATION...McDonald FIRE WEATHER...Hattings
Hazardous Weather Outlook National Weather Service Riverton WY 218 AM MDT Mon Jun 12 2017 WYZ001>020-022>030-131100- 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- 218 AM MDT Mon Jun 12 2017 ...Flood Warnings for the Green and Wind River Basins Continue Through Mid-Week... ...High Wind Warning Until 9 PM This Evening for Southern Wyoming... ...Severe thunderstorms are likely for much of northern and central Wyoming... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Scattered thunderstorms will develop this afternoon. Some storms will likely become severe this afternoon and evening with large hail, damaging winds and even an isolated tornado possible. The best chance of severe weather will be in Natrona and Johnson Counties as well as the Big Horn Basin. High winds are likely across portions of Sweetwater and southeastern Fremont Counties. Snow will move into the higher elevations late tonight. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Tuesday...Heavy wet snowfall likely in the the northern and western mountains, including Grand Teton and Yellowstone National Parks. Snowfall, slick roads and limited visibility likely over mountain passes. A few inches of snow possible in far west valleys Tuesday morning. Very windy across southern and central areas. .SPOTTER INFORMATION STATEMENT... Spotter activation will be required today and tonight across central Wyoming. TEAM RIVERTON
Special Weather Statement National Weather Service Riverton WY 129 PM MDT MON JUN 12 2017 WYZ011-020-122000- Southeast Johnson County-Natrona County Lower Elevations- 129 PM MDT MON JUN 12 2017 ...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN JOHNSON AND NORTHEASTERN NATRONA COUNTIES UNTIL 200 PM MDT... At 127 PM MDT, Doppler radar was tracking a strong thunderstorm near Teapot Rock, which is 27 miles north of Casper, moving north at 40 mph. Half inch hail and winds in excess of 40 mph will be possible with this storm. This strong thunderstorm will be near... Midwest and Edgerton around 150 PM MDT. This includes Interstate 25 between mile markers 206 and 242. 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 10631 4313 10664 4359 10652 4353 10609 TIME...MOT...LOC 1927Z 194DEG 35KT 4323 10642 McDonald
Special Weather Statement National Weather Service Riverton WY 146 PM MDT MON JUN 12 2017 WYZ019-020-022-122015- Green Mountains and Rattlesnake Range-Casper Mountain-Natrona County Lower Elevations- 146 PM MDT MON JUN 12 2017 ...SIGNIFICANT WEATHER ADVISORY FOR SOUTHEASTERN NATRONA COUNTY UNTIL 215 PM MDT... At 146 PM MDT, Doppler radar was tracking a strong thunderstorm near Bates Hole, which is 26 miles south of Casper, moving north at 45 mph. Dime size hail and wind gusts up to 50 mph will be possible with this storm. This strong thunderstorm will be near... Bates Hole around 155 PM MDT. Mills, Red Butte, Paradise Valley, Emigrant Gap and Hogadon Ski Area around 215 PM MDT. LAT...LON 4243 10635 4243 10667 4286 10666 4283 10623 TIME...MOT...LOC 1946Z 188DEG 38KT 4248 10652 McDonald
Special Weather Statement National Weather Service Riverton WY 158 PM MDT Mon Jun 12 2017 WYZ026>029-122100- Upper Green River Basin-South Lincoln County- Rock Springs and Green River-Flaming Gorge- Including the cities of La Barge, Big Piney, Farson, Kemmerer, Cokeville, Rock Springs, and Green River 158 PM MDT Mon Jun 12 2017 Showers with isolated embedded thunderstorms will continue moving to the north-northwest this afternoon. Wind gusts up to 50 mph will be possible with these showers...as they quickly move across the area. Brief heavy rain and small hail will also be possible.
Special Weather Statement National Weather Service Riverton WY 209 PM MDT MON JUN 12 2017 WYZ020-122045- Natrona County Lower Elevations- 209 PM MDT MON JUN 12 2017 ...SIGNIFICANT WEATHER ADVISORY FOR EAST CENTRAL NATRONA COUNTY UNTIL 245 PM MDT... At 208 PM MDT, Doppler radar was tracking a strong thunderstorm near Emigrant Gap, which is 12 miles west of Casper, moving north at 40 mph. Half inch hail and winds in excess of 40 mph will be possible with this storm. This strong thunderstorm will be near... Goldeneye Reservoir around 220 PM MDT. LAT...LON 4279 10641 4281 10671 4329 10675 4327 10632 TIME...MOT...LOC 2008Z 183DEG 37KT 4289 10656 McDonald
The Riverton/Lander radar (KRIW), 110 km away, was scanning. The volume nearest the first event began at 22:00 UTC.
7 high wind, 6 hail, 4 tornado, 1 funnel cloud, 1 flash flood. Local standard time, as filed.
High winds occurred in portions of the Upper Green River Basin. The highest wind gust recorded was 67 mph at Farson. A 58 mph gust was recorded at the Big Piney airport.
High winds occurred across much of the Green and Rattlesnake Ranges. The highest winds occurred at Camp Creek where several gusts of hurricane force occurred with a maximum gusts of 86 mph. Some other notable gusts included 82 mph at Fales Rock and 66 mph at Beaver Rim.
Strong winds occurred in portions of Eastern Sweetwater County. The strongest gust was 63 mph at Bitter Creek.
High winds occurred in portions of the Rock Springs and Green River areas. The highest gust reported was 62 mph at the site on the west side of Rock Springs.
The Big Horn County sheriff reported hail from the size of nickels up to the size of Ping Pong balls.
A trained spotter reported nickel size hail 10 miles west of Wapati.
A thunderstorm developed along the east side of the Bighorn Range and slowly strengthened. The storm became a supercell and dropped very large hail from Barnum to Kaycee. There were widespread reports of quarter size or bigger hail along the path. The largest report of hail was tennis ball size 5 miles southwest of Kaycee, not far from where a tornado developed. Hail up to the size of ping pong balls was reported in Barnum.
A storm survey was conducted near Kaycee and confirmed a tornado. The tornado touched down on a hill just west of Interstate 25 and just north of the Middle Fork of the Powder River. The tornado downed many trees moving down the hill. The tornado then strengthened as it crossed a field. An abandoned trailer was destroyed with the base thrown over 200 yards. Trees in the field were snapped off above the bases with some ground scouring noted. The tornado then crossed Barnum Road and moved northward toward Pine Ridge. The tornado narrowly missed a house and a barn as it moved up the ridge. Numerous trees were downed along both sides of the ridgeline before the funnel lifted. Winds were estimated at 110 to 120 mph, a minimal EF2 tornado.
Very strong winds occurred across the Lander Foothills. Some of the strongest winds included 76 mph at the Lander Airport and 75 mph at the North Fork of the Popo Agie River. The wind downed many trees in and around Lander and ripped the roof off of a garage.
The public reported a tornado that briefly touched down several times in the Oregon Basin southeast of Cody. The tornado remained over open country and caused no damage.
The public reported hail up to the size of half dollars around Evansville.
A supercell thunderstorm dropped a weak rope tornado that moved slowly northward in southern Big Horn County and skipped along around a 3 mile path. The tornado remained over open country and caused no damage.
The Natrona County emergency manager reported a brief tornado touchdown to the west of Bar Nunn. The tornado remained over open country and did little damage.
An off-duty NWS meteorologist reported a funnel cloud near the Trabing Road interchange of Interstate 25.
The public reported quarter size hail to the east of Ten Sleep.
A CocoRaHS reporter reported flash flooding along Crazy Woman Creek near the south end of Espinoza Road south of Interstate 90.
The public reported hail up to the size of nickels in Midwest and Edgerton.
The RAWS site at Sharpnose near Hudson reported a wind gust of 58 mph.
A pair of weather stations in Thermopolis reported wind gusts of 62 mph.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2017Every state in 2017This day in weather historyThe tornado archive for Wyoming, 2017Warnings in Wyoming, June 2017
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: 697912, 697909, 697910, 697911, 698408, 698409, 698411, 698415, 697917, 698418, 698420, 698417, 698419, 698421, 698422, 698424, 698423, 697928, 697940.