The combination of an unusually cool air mass, a slow moving area of low pressure and copious amounts of Pacific moisture moving into Wyoming combined to produce heavy late season snow in many areas. The heaviest fell in the mountains.
Every event with a place to put it; 6 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.
The combination of an unusually cool air mass, a slow moving area of low pressure and copious amounts of Pacific moisture moving into Wyoming combined to produce heavy late season snow in many areas. The heaviest fell in the mountains. The Wind River Range had to most with amounts of 12 to 18 inches commonplace with a maximum of 20 inches at the Hobbs Park SNOTEL. Over a foot of snow also fell in portions of the Absarokas, Bighorns and Casper Mountain. In the Lower Elevations, anywhere from 6 to 15 inches fell in the Lander Foothills depending on elevation with 11 inches reported at Jeffrey City.
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, 12:40 PM MST, 1 hour 20 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 140 PM MDT Wed May 17 2017 .SHORT TERM...(This evening through Saturday) Issued at 140 PM MDT Wed May 17 2017 A large upper level low pressure Pacific storm system over southern Idaho and northern Utah will dig south and east into western Colorado Thursday and Thursday night bringing significant Pacific moisture and cold air to the region. This is a very difficult storm to forecast snow accumulations as the snow level will fluctuate from the foothills to the lower valley elevations at night and rise again during the day. Mountain snowfall most certain while lower elevation amounts very questionable. Have issued winter weather advisories for eastern Johnson, Natrona, Fremont and eastern Sweetwater counties from later tonight, 3 am through 6 am Friday morning. Snow will likely change to rain at times but heavy snow may significantly impact visibility and driving conditions. Temperatures consistently colder over the past 24 hours. Worst case is colder and all snow which could produce a foot to a foot and a half snowfall around Casper, Riverton and Lander. Best case all rain but too cold for mountain snow melt which would reduce flooding potential. Models keep more precipitation through Friday now but air will be rather cold limiting amounts. Slow clearing and warming on Saturday. .LONG TERM...(Saturday night through Wednesday) Issued at 140 PM MDT Wed May 17 2017 Unsettled nw (becoming n) flow through the period with multiple sewd moving weather systems to deal with. The first one looks to move from ID across srn WY and nrn CO Saturday night into Sunday with considerable mountain rain/snow and a chance of lower elevation rain showers. Another quick moving system drops right behind this one Sunday night to keep in unsettled in advance of a stronger swd moving system moving out of Canada. This one looks to pack more of a punch for the north and east with a good frontal passage with widespread mountain snow (possibly heavy in the Big Horns and Beartooth Plateau) and a good chance of mostly rain showers across the lower elevations with falling snow levels into the foothills later Monday into Tuesday. One more system is expected to drop straight south on Wednesday to keep the area unsettled and cool with a chance of showers and higher elevation snow. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) Issued at 140 PM MDT Wed May 17 2017 WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS Terminals Widespread snow over northwest WY through 06Z Thursday, before drying moves into that location. The rest of the region will see isolated to scattered rain or snow showers through Thursday. Mountains will be obscured in NW WY through the period. Windy conditions will be common today in southwest WY through 00Z, then decreasing. Please see terminal forecasts for details. EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL Terminals Showers will become more numerous over northern WY this afternoon with widespread rain through 18Z Thursday. Then drying will move into this area after 18Z Thurs. Some areas above 5K feet will change to or mix with snow like KCOD after 12z Thu. Expect areas of MVFR/IFR conditions over the north through Thursday. Windy conditions will occur in the central part of WY today with the wind decreasing after 00Z. Widespread MVFR to IFR cigs/vsbys will move into the rest of central WY after 09-12z Thu and continue all day with widespread rain and snow. Please see terminal forecasts for details. && .FIRE WEATHER... Issued AT 140 PM MDT Wed May 17 2017 A large upper level low pressure Pacific storm system over southern Idaho and northern Utah will dig south and east into western Colorado Thursday and Thursday night bringing significant Pacific moisture and cold air to the region. This is a very difficult storm to forecast snow accumulations as the snow level will fluctuate from the foothills to the lower valley elevations at night and rise again during the day. Mountain snowfall most certain while lower elevation amounts very questionable. Have issued winter weather advisories for eastern Johnson, Natrona, Fremont and eastern Sweetwater counties from later tonight, 3 am through 6 am Friday morning. Snow will likely change to rain at times but heavy snow may significantly impact visibility and driving conditions. Temperatures consistently colder over the past 24 hours. Worst case is colder and all snow which could produce a foot to a foot and a half snowfall around Casper, Riverton and Lander. Best case all rain but too cold for mountain snow melt which would reduce flooding potential. Models keep more precipitation through Friday now but air will be rather cold limiting amounts. Slow clearing and warming on Saturday. With widespread cold clouds and precipitation, mixing heights will be low with poor smoke dispersal through Friday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 3 AM Thursday to 6 AM MDT Friday for WYZ007-017-018-030. Winter Storm Warning from midnight tonight to 6 AM MDT Friday for WYZ015-019-022. Winter Weather Advisory until 6 PM MDT this evening for WYZ012- 024. Flood Watch from 6 PM MDT this evening through late Thursday night for WYZ010-011. Winter Weather Advisory from midnight tonight to 6 AM MDT Friday for WYZ010-011-020. Winter Weather Advisory until 6 AM MDT Thursday for WYZ001. Winter Storm Warning until 6 PM MDT Thursday for WYZ008-009. Winter Weather Advisory until noon MDT Thursday for WYZ002. && SHORT TERM...Baker LONG TERM...Skrbac AVIATION...Skrbac FIRE WEATHER...Baker
Hazardous Weather Outlook National Weather Service Riverton WY 1255 PM MDT Wed May 17 2017 WYZ001>020-022>030-181900- 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- 1255 PM MDT Wed May 17 2017 This Hazardous Weather Outlook is for Western and Central Wyoming. ...Multiple Winter Storm Warnings and Advisories in effect Today through Thursday night... ...Flood Watch Tonight through Thursday Night for Johnson County... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Widespread light to moderate rain and mountain snow expected across portions of western and northern Wyoming. Areas of heavy snow possible over the Absaroka and Bighorn mountains. Additionally, significant rainfall amounts could begin to elevate flooding concerns where many streams and some main stem rivers will already be at bankfull levels. Isolated afternoon and evening thunderstorms with small hail and strong gusty winds are expected. Snowfall will lower into the lower elevations of the Bighorn Basin, Wind River Basin, Sweetwater, Natrona and Johnson Counties. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Thursday and Friday...Widespread moderate to locally heavy rain and mountain snow begins to spread east of the Divide. Moderate to heavy snow continues over the Absaroka and Bighorn mountains. Snow becomes possible across the lower elevations east of the Divide Thursday and Thursday night. Rainfall and snowfall continue to elevate flooding concerns. Sunday and Monday...Isolated to locally scattered thunderstorms. .SPOTTER INFORMATION STATEMENT... Spotters are urged to report wind gusts past 45 mph, hail, heavy rain or snowfall and any flooding. Thank you.
Special Weather Statement National Weather Service Riverton WY 133 PM MDT WED MAY 17 2017 WYZ009>011-172000- Southeast Johnson County-Bighorn Mountains Southeast-Northeast Johnson County- 133 PM MDT WED MAY 17 2017 ...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL JOHNSON COUNTY UNTIL 200 PM MDT... At 133 PM MDT, Doppler radar was tracking a strong thunderstorm 14 miles north of Mayoworth, which is 21 miles south of Buffalo, moving north at 20 mph. Half inch hail and winds in excess of 40 mph will be possible with this storm. This storm will remain over mainly rural areas of central Johnson County, including the following locations: Ta Guest Ranch and Canyon Campground. This includes Interstate 25 between mile markers 271 and 292. 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 4397 10654 4393 10678 4422 10692 4427 10656 TIME...MOT...LOC 1933Z 168DEG 15KT 4403 10668 13
Special Weather Statement National Weather Service Riverton WY 225 PM MDT WED MAY 17 2017 WYZ009-010-172100- Bighorn Mountains Southeast-Northeast Johnson County- 225 PM MDT WED MAY 17 2017 ...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN JOHNSON COUNTY UNTIL 300 PM MDT... At 225 PM MDT, Doppler radar was tracking strong thunderstorms along a line extending from 12 miles north of Powder River Rest Area to near Buffalo. Movement was north at 15 mph. Half inch hail and winds in excess of 40 mph will be possible with these storms. Strong thunderstorms will be near... Saddlestring around 255 PM MDT. Lake De Smet and Fort Phil Kearny Historical Site around 300 PM MDT. This includes the following highways... Interstate 25 between mile markers 297 and 299. Interstate 90 between mile markers 43 and 62. LAT...LON 4435 10609 4429 10692 4456 10700 4456 10601 TIME...MOT...LOC 2025Z 182DEG 12KT 4440 10620 4435 10683 13
Special Weather Statement National Weather Service Riverton WY 227 PM MDT WED MAY 17 2017 WYZ010-011-172100- Southeast Johnson County-Northeast Johnson County- 227 PM MDT WED MAY 17 2017 ...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN JOHNSON COUNTY UNTIL 300 PM MDT... At 226 PM MDT, Doppler radar was tracking a strong thunderstorm over Powder River Rest Area, which is 27 miles east of Buffalo, moving north at 20 mph. Half inch hail and winds in excess of 40 mph will be possible with this storm. This storm will remain over mainly rural areas of northeastern Johnson County. This includes Interstate 90 between mile markers 76 and 94. LAT...LON 4413 10605 4412 10630 4443 10653 4446 10601 TIME...MOT...LOC 2026Z 176DEG 19KT 4420 10619 13
The national radar mosaic for the moment the first event began.
6 winter storm. Local standard time, as filed.
Heavy snow fell in portions of the Absaroka mountains. The highest amount recorded was 16 inches at the Marquette SNOTEL.
The Cloud Peak SNOTEL site measured 12 inches of new snow.
Heavy snow fell along the eastern slopes of the Wind River Range. Most areas received over a foot of new snow. The highest amount recorded was 20 inches at the Hobbs Park SNOTEL site.
The official spotter at the Lander airport measured 6.3 inches of snow. Amounts were higher closer to the Wind River Range where some areas received over a foot. However, with the high May sun angle, most roads were just mainly wet to slushy.
At the Casper Mountain SNOTEL 14 inches of new snow fell.
Heavy snow fell in portions of the Green and Rattlesnake Range. The highest amount measured was 11 inches at Jeffrey City.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2017Every state in 2017This day in weather historyWarnings in Wyoming, May 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: 690997, 690992, 691001, 690999, 690998, 691000.