Low pressure passed over Wyoming on April 25th. A deep east to southeasterly flow moved into Johnson County and produced substantial upslope flow that brought heavy snow to many areas.
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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.
Low pressure passed over Wyoming on April 25th. A deep east to southeasterly flow moved into Johnson County and produced substantial upslope flow that brought heavy snow to many areas. Across northern Johnson County, heavy rain changed to a heavy wet snow that continued for many hours. The snow combined with gusty winds that knocked the power out for several hours in Buffalo. Interstates 25 and 90 were also shut down during the height of the storm. Heavy snow fell in the Bighorns as well with up 18 inches of new snow. Powder River Pass was closed at the height of the storm. Around Buffalo, snowfall amounts generally ranged from 5 to 11 inches.
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 Apr 24, 12:34 PM MST, 4 hours 26 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 134 PM MDT Mon Apr 24 2017 .SHORT TERM...(Tonight through Thursday) Issued at 131 PM MDT Mon Apr 24 2017 Imagery shows a weak ridge over deep closed low across the ern CONUS with an evolving longwave trof taking over west of the Mississippi. embedded upstream sw/jet moving out of the EPAC and through the wcntrl CONUS and over the Great Basin headed toward wrn WY. SFC currently has weak low pressure over WY...with further deepening and development expected over the next 24 hours. Precipitation, both liquid and frozen, moving into and across the forecast area (FA) from the west with snow levels this afternoon above 7500 feet. ...Winter Weather Advisories for most of the FA's mountains, with northern Johnson County also included.... As the shortwave/jet moves into/across WY this afternoon and evening, a lee side weakly cold core cyclone will spin up across ern/nern WY. Convective instability surrounding the development of this low will also be increasing as the low spins up bringing a period of mixed stratiform/convective rain/snow-fall to central/northern/northeastern WY. Changeover to rain/snow or all wet snow with this portion of the system...mainly across Johnson and Natrona Counties...will occur generally between 3 AM and 6 AM. With periods of heavy snow possible during morning commute around Buffalo and the I 25 and I90 interchanges...with lighter snowfall generally to the south. 2 to 6 inches possible through Tuesday across northern Johnson County...1 to 4 inches over the southern portion of Johnson. Otherwise east of the Divide across the lower elevations, changeover to rain/snow mix or all wet snow will arrive near sun-up with best accumulating snow expected above 6000-6500 feet over srn/ern Fremont and wrn Natrona Counties (1 to 3 inches). The Bighorn Mountains will see Advisory level snowfall...6 to 12 inches...locally higher. West of the Divide, the mountains will also see Advisory level snowfall...with greatest amounts over the Teton, Gros Ventre, and northern Absaroka mountains (up to a foot of wet snow through the day Tuesday). Tuesday afternoon and night...the associated cold front will push south into Colorado with shortwave ridging aloft and precipitation ending. Wednesday afternoon/night through Thursday: Cool and/or wet persists with somewhat of a reprieve from precipitation until Wednesday afternoon (across the west). With the jet dropping south of the FA and overall weak or weakening forcing aloft across WY, the terrain, near surface fronts/boundaries (convergence zones) and weak/modest convective instability will provide most of the impetus for getting precip on the ground. As an embedded sw moves across/near/or just south of WY Thursday, a better chance for frontogenesis over/near the scntrl FA will occur...while weak cyclogenesis occurs from cntrl to sern WY and into nern CO. This will likely be the best period of precipitation over the FA with some return of moisture from the Plains making its way back into portions of the northern/northeastern forecast area. As the better forcing and evolution of this system will be driven within the boundary layer, it will largely be a waiting game to a certain extent to see just where the ll fronts/boundaries will actually set up and move to through the day Thursday...regarding highlight possibilities. Although it continues to look as though ecntrl to ern WY will have the best chances for heavier precipitation. Snow levels with/behind the prospective cold front will make it to valley floor levels west of the Divide by Thursday morning...with rain changing to or mixing with snow east of the Divide by Thursday afternoon. .LONG TERM...(Thursday night through Monday) Issued at 131 PM MDT Mon Apr 24 2017 Thursday Night through Monday Long wave trough to be in place across the northern and central Rockies at the start of the forecast period. Pattern will be very convoluted with much to be decided based upon where upper low develops Thursday night and Friday. Models in fairly good agreement in producing widespread precipitation Thursday night and Friday with cyclonic flow enveloping the state. Plenty of cold air aloft with H7 temperatures around -8C to -10C during this time. Therefore, precipitation would be in the form of snow across the lower elevations. Northerly flow within this trough would enhance upslope potential across the central basins, mainly Fremont and Natrona counties. Secondary closed low develops Friday night and Saturday over the Four Corners, which would likely slow the precipitation as dynamics and moisture shift south. However, with trough lingering, it looks to remain unsettled through Saturday before the Four Corners low emerges east of the Divide in the central Plains Saturday night and Sunday. Sunday appears dry with northwest flow aloft and a shortwave ridge in place. Next system begins to take shape to our northwest Sunday night and Monday. ECMWF shows this system beginning to carve another trough Monday night and Tuesday, while the GFS solution shows a more progressive northwest flow aloft with embedded clipper-like systems. Inclination is to trend toward the ECMWF solution as long wave trough may very well re-emerge Tuesday and Wednesday. Overall, areas east of the Divide may generally be dry Monday as the west and northwest get another round of precipitation ahead of the main system. Temperatures will be below normal, in some cases 15-20F below normal, during the forecast period. Friday will be the coldest day and it may be difficult for the central basins to get to 40F for a daytime high. Depending upon precipitation intensity, the snow on Thursday night and Friday morning may remain as such into the afternoon hours. Main impact would be to the Friday morning commute as roads will likely become wet by midday. Early indications are that 1-3 inch snowfall amounts could be widespread with higher amounts certainly within the realm of possibility. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 131 PM MDT Mon Apr 24 2017 WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES Mid-level circulation and associated moisture working across far western Wyoming this afternoon. Best chance for rain and snow showers will be through the evening at KJAC and through the afternoon at KBPI, KPNA, and KRKS. Precipitation will be widespread over the mountains through about 06Z/Tue. Basins and valleys can expect the best chance into the 00Z-03Z/Tue timeframe. Any of the heavier rain or snow showers will produce MVFR and possibly IFR conditions through about 03Z/Tue. KJAC the most likely terminal to see prevailing IFR/MVFR through this evening and overnight. Again, confidence is decreasing that these conditions will persist through sunrise Tuesday. Instead, the scenario could play out with conditions improving between 08Z and 12Z/Tue. Mid-level circulation will wind-up over northeast Wyoming late tonight with much of the precipitation shifting east of the Divide. Conditions should primarily be VFR from late Monday night through 00Z/Wed. Cold pool aloft will probably allow for continue cold air cumulus through Tuesday afternoon. After 00Z/Tue, trough passage will bring gusty west-northwest wind at KBPI, KPNA, and KRKS which will downslope and dry things out overnight. Mountain obscurations will be widespread throughout the period. Expect another period of gusty northwest wind 15-25kts at KBPI, KPNA, and KRKS after 16Z/Tue with these winds persisting up to 00Z/Wed. EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES Westerly flow aloft will largely allow for VFR conditions to prevail at most terminals through 12Z/Tue. The exception will be KCOD as a cold front nudges south late this evening as a mid-level circulation develops over northeast Wyoming. First, expect gusty south to southwest wind at KCPR and in the Wind River Basin this afternoon ahead of the incoming system. Trough passage will allow winds to go northwest this evening favoring KRIW. Weak convection this afternoon across the central terminals from KLND to KCPR will be diurnal in nature and will decrease after 02Z/Tue. Mid-level circulation will intensify and wrap-up over northeast Wyoming between 06Z and 12Z-Tue. Northerly flow on the backside of this system will serve to drive the cold front farther south during this timeframe. Behind the cold front, IFR/MVFR conditions will develop at KCOD late this evening as lift enhances along the foothills and atmosphere moistens. This trend will continue as the front pushes south overnight reaching a KLND-KCPR line by 12Z-15Z/Tue. MVFR, and perhaps IFR, conditions anticipated Tuesday morning in the central basins, particularly at KCPR and KLND where terrain will enhance orographic lift. Mountain tops will be obscured, with mountain top obscurations becoming widespread in the wake of the cold front. Mid-level circulation will push east during the midday hours Tuesday with drier air pushing in from the north. Gusty northerly wind will prevail as conditions dry and become VFR across the forecast area Tuesday afternoon. && .FIRE WEATHER... Issued AT 131 PM MDT Mon Apr 24 2017 Fire danger low across the forecast area through (at least) the next week. All fuels currently in green-up. Periods of rain and high country snow will occur periodically through next Sunday. Isolated Thunderstorms will also be possible this afternoon and early evening...with light to moderate snow expected across the area's mountains through Tuesday with lighter snow generally over the lower elevations. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until noon MDT Tuesday for WYZ001-002- 012-014-015-024. Winter Weather Advisory from 6 PM this evening to 6 PM MDT Tuesday for WYZ008-009. Winter Weather Advisory from midnight tonight to 6 PM MDT Tuesday for WYZ010. && SHORT TERM...Braun LONG TERM...Jones AVIATION...Jones FIRE WEATHER...Braun
Hazardous Weather Outlook National Weather Service Riverton WY 245 AM MDT Mon Apr 24 2017 WYZ001>020-022>030-251200- 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- 245 AM MDT Mon Apr 24 2017 ...Winter Weather Advisories in effect for the Western Mountains, Big Horns and Northern Johnson County from today through Tuesday... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight... Snow likely in the western Mountains, Bighorns from late this morning through Tuesday. Snow is likely in northern Johnson County late tonight and Tuesday. Please see Winter Weather Products and forecasts for details. Isolated thunderstorms possible this afternoon and evening. .DAYS TWO THROUGH SEVEN...Tuesday through Saturday... Tuesday...Snow in the mountains and far north into the morning with a cold rain or rain and snow mix across the lower elevations of Central Wyoming. Wednesday...Mountain snow west of the divide. Thursday and Friday...A slowly evolving cold storm system will develop over the Great Basin and spread widespread cold rain or snow across much of the area. Significant snowfall is possible across parts of the area, especially around Central Wyoming. Saturday...Snow ending with improving conditions. .SPOTTER INFORMATION STATEMENT... Spotters are urged to report snowfall amounts or other significant weather. Team Riverton
The national radar mosaic for the moment the first event began.
2 winter storm. Local standard time, as filed.
Heavy snow fell across portions of the east slopes of the Bighorns. Numerous locations picked up over a foot of new snow. Some of the highest amounts included 18 inches at the Hansen Sawmill SNOTEL site and 15 inches at the Bear trap Meadow SNOTEL site.
Rain changed to a heavy, wet snow across northern Johnson County around midnight and quickly accumulated from 5 to 11 inches. The heavy, wet snow combined with gusty winds that knocked out power to much of Buffalo for several hours. At the height of the storm, portions of Interstates 25 and 90 were closed due to winter conditions.
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, April 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: 684368, 684366.