A unusually strong cold front and upper level low combined with a deep stream of Pacific moisture to produce the first snowstorm of the year. The hardest hit area was the Absarokas.
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The episode as NWS RIW wrote it up.
A unusually strong cold front and upper level low combined with a deep stream of Pacific moisture to produce the first snowstorm of the year. The hardest hit area was the Absarokas. Some areas received close to a foot of snow. The Beartooth highway was shut down for several hours due to winter conditions.
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 Sep 15, 1:15 AM MST, 1 hour 45 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 215 AM MDT Fri Sep 15 2017 .SHORT TERM...Friday through Sunday night Some needed rain is now falling across much of the area, especially across portions of northern Wyoming. So far, all of the populated areas are staying rain, although there is some changeover in the higher elevations as cooler air slowly works into the area. The models continue to show a cloudy and chilly with periods of rain continuing, although there are some differences in timing with the heavier periods of precipitation with timing of some mesoscale features and the movement of the 700 millibar low. On average, new guidance for QPF does not look that much different so we only made cosmetic changes to continuity. In addition, there will be just enough instability across southern portions of Wyoming for a couple of thunderstorms to fire this afternoon. The mashing of teeth is with precipitation type in some areas. The mountains should change over the snow and the warnings and advisories will be continued as is. However, the NAM guidance has come in a bit cooler and is concerning in places like Cody, with temperatures in the middle 30s by 9 am. The concern is not really with roads, since recent warm temperatures and warm ground should melt any potential accumulation unless the snow is very heavy. The concern is with the trees, which still have full leaf coverage and could cause some problems with limbs breaking and possibly falling on things that people really care about like cars and power lines. However, the other models are warmer and keep things a cold rain. In addition, a 700 millibar temperature of minus 3 would normally but snow levels around 6500 to 7000 feet. The truth is probably somewhere in the middle like it usually is. For now, the preponderance of the evidence looks to keep things just warm enough for rain with some wet snow mixing at times, but this will have to be watched very closely. Other locations in the lower elevations should remain largely rain through the day. There is one more concern tonight with one last batch of precipitation moving in from the southwest riding the left front quadrant of a jet streak. The NAM keeps most of the energy to the south and east of the area and hence most of the precipitation. The GFS brings a bit further west and north which may put a burst of rain or possible snow over portions of Natrona County as well. However, there should be a downward trend in intensity of the precipitation tonight, especially late as the low and upper level support begin to move away to the north and east. Saturday should see improving conditions as the trough moves away and precipitation ends from south to north. It still looks rather cool though. Temperatures could be rather tricky however. If the precipitation moves out quicker and clouds clear it could get a bit warmer than expected. High pressure should bring a nice end to the weekend with partial sunshine and warmer temperatures. although still a bit below normal for this time of year. .LONG TERM...Monday through Friday Mean longwave trough settles in across the Inter-Mountain west through the period. Models struggling with the details of of the next developing upper low. General consensus tonight is the first trough in the extended swings through on Tuesday with the main low well north. After that, models generally have a second potent shortwave dropping down into the mean trough, forming at least a sharpened trough over the Great Basin Wednesday into Thursday before swinging across later Thursday into Thursday night. Another potentially cold shortwave may dive down from the north on Friday to keep the area unsettled. It will likely take the models a few days to grab hold of this pattern change and developing mean trough that peaks around Thursday of next week before slight weakening towards next weekend. The overall pattern is set for another round of cold and unsettled late 'summer' (and start of fall) weather with more mountain snow at times and cold rain (with some lower elevation snow possible similar to the current system). Initial front approaches the nw late Monday into Monday night with an increasing chance of showers/higher elevation snow. Widespread rain and snow showers in the mountains Tuesday/Tue night with a chance of showers/isold tstms moving east across areas east of the divide. Next system digging into the Great Basin Wed/Wed night will increase the threat of more widespread rain/mtn snow in most areas through Thursday with falling temps. Unsettled and cool through Friday with general trough hanging around through the end of this medium range period. The end of summer (Thursday) looks quite cool and unsettled with the first day of fall (Friday) seeing modest improvement but still quite unsettled and cooler than normal with at least some mountain rain or snow showers. && .AVIATION...12Z Issuance A weather system will move through the region today and tonight. Rain with isolated thunderstorms will occur over the region. Snow will occur in the mountains with the snow level lowering to 6000 to 7000 feet MSL tonight. Areas of MVFR and IFR conditions will occur through tonight. Please see terminal forecasts for more details. Please see the Aviation Weather Center for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Expect a cold and wet day today as low pressure moves across the state. Rain will fall across many locations with several inches of snow likely for the mountains. All areas have a good chance of receiving a wetting rain. Any lightning would be confined to southern Wyoming today. A gusty north wind will blow at times today. Mixing and smoke dispersal will be generally fair to poor. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning until noon MDT Saturday for WYZ002-008-009- 015. Winter Weather Advisory until noon MDT Saturday for WYZ001-012- 014. && SHORT TERM...Hattings LONG TERM...Skrbac AVIATION...Ross FIRE WEATHER...Hattings
Hazardous Weather Outlook National Weather Service Riverton WY 220 AM MDT Fri Sep 15 2017 WYZ001>020-022>030-161200- 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- 220 AM MDT Fri Sep 15 2017 ...Winter Storm Warnings and advisories for the western and northern mountains late Thursday night into Saturday morning... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Significant mountain snowfall possible over western and northern mountains. Sharply cooler with lowering snow levels. Snow levels will lower to near or at the valley floor out west Friday night and into the foothills east of the divide. Please see winter weather products and forecasts for details. .DAYS TWO THROUGH SEVEN...Saturday through Thursday Saturday...Lingering showers and mountain/foothill snow Saturday with gradual improvement during the day. Monday...Isolated thunderstorms in the far west. Tuesday...A strong cold front will sweep east across the area with a chance of snow in the mountains with isolated thunderstorms possible in most areas. Wednesday and Thursday...Unsettled with a good chance of some mountain snowfall at times with possible snow down to the valley and basin floor out west and far north similar to our current storm system. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report snowfall and significant rainfall amounts. Thank you! Team Riverton
Special Weather Statement National Weather Service Riverton WY 1247 AM MDT Fri Sep 15 2017 WYZ003-152300- Cody Foothills- Including the cities of Cody and Meeteetse 1247 AM MDT Fri Sep 15 2017 A large early Autumn storm will sweep southeast toward Northwest Wyoming during the next few days. This storm bring much colder weather then the recent summer months. This will result in the first cold widespread precipitation event in a few months. A steady chilly rainfall will persist through Saturday. High temperatures may not even rise above 40 today in the Cody area. Rain will mix with and turn to snow at times today through tonight including the town of Cody where 1 to 3 inches of snow are possible. Precipitation will diminish by Saturday afternoon. Outdoor interests should prepare for a major change toward more winter-like conditions. WYZ016-152300- Upper Wind River Basin- Including the city of Dubois 1247 AM MDT Fri Sep 15 2017 A large early Autumn storm will sweep southeast toward Northwest Wyoming during the next few days. This storm bring much colder weather then the recent summer months. This will result in the first cold widespread precipitation event in a few months. A steady chilly rainfall will persist through Saturday. High temperatures will remain in the 40s today in the Dubois area. Rain will mix with and turn to snow tonight and continue Saturday morning in the Upper Wind River Basin, including the Dubois area, where 1 to 2 inches of snow may accumulate. Precipitation will diminish by Saturday afternoon. Outdoor interests should prepare for a major change toward more winter-like conditions through Saturday. WYZ013-152300- Jackson Hole- Including the city of Jackson 1247 AM MDT Fri Sep 15 2017 A large early Autumn storm will sweep southeast toward Northwest Wyoming during the next few days. This storm bring much colder weather then the recent summer months. This will result in the first cold widespread precipitation event in a few months. A steady chilly rainfall will persist through Saturday. High temperatures will remain in the 40s today. Rain will mix with and turn to snow tonight and continue Saturday morning in the Jackson Valley, including the town of Jackson, where an inch or less of snow will is possible. Precipitation will diminish Saturday afternoon. Outdoor interests should prepare for a major change toward more winter-like conditions through Saturday.
The national radar mosaic for the moment the first event began.
1 winter storm. Local standard time, as filed.
The Beartooth highway was closed for several hours 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, September 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: 718102.