A plume of Pacific moisture moved into Western Wyoming ahead of an Arctic Cold front and brought heavy snow to portions of the area. The highest amounts fell across the Tetons as well as the Salt and Wyoming Range where over a foot of new snow was common with some areas eclipsing the two foot mark.
Every event with a place to put it; 3 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 plume of Pacific moisture moved into Western Wyoming ahead of an Arctic Cold front and brought heavy snow to portions of the area. The highest amounts fell across the Tetons as well as the Salt and Wyoming Range where over a foot of new snow was common with some areas eclipsing the two foot mark. Heavy snow bands also developed in the Star Valley with many areas having over a foot of new snow.
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 Dec 21, 3:44 PM MST, 3 hours 16 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 344 PM MST Thu Dec 21 2017 .SHORT TERM...This evening through Sunday Snow has ended across the area this afternoon, as mid to high level clouds begin to stream over the NW. This trend will continue this evening, as the first flakes of new snow begin to fall over Yellowstone and northern portions of the Absarokas by 06Z Friday. The northern and southern branches of the jet will start to come together over central ID during the day Friday, forming a channel jet. Unfortunately, an upstream shortwave in the northern branch will push this to the south. Regardless, there will be decent UL dynamics over the far west as well as added upslope components. Thus, the far west zones will have a long period of light to moderate snow Friday and Friday night. Most areas will have advisory amounts of snow, while the Tetons and Gros Ventres receive 12-16". Models differ on whether there will be any moisture to work with east of the Divide, as the arctic front makes its way southward Friday and Friday night. The GFS is certainly the most bullish, but still only went with sub- advisory amounts for now. The best place that could receive advisory amounts would be the Cody Foothills. The arctic front will stall out near the Green Mountains and Rattlesnake Range during the day Saturday. Snow will also end from north to south after 12Z Saturday, with only lingering snow showers/flurries expected during the afternoon. Gusty winds will occur across the south Friday afternoon and again Saturday afternoon as the front stalls out. Thus, winds will be even stronger, with WNW gusts up to 50 mph possible across the I-80 corridor. Temperatures will be cold, but not as cold as they could be. The H7 cold pool of -20C temps continue to trend eastward Saturday and Saturday night. Highs will range from the upper teens to middle 20s through the weekend, with the coldest night being Saturday night. Widespread subzero readings will occur across all but extreme southern portions. .LONG TERM...Sunday night through Thursday By Sunday night, it appears that the coldest weather for the time being will be behind us, as the 1045mb sfc high slides off to the southeast toward the Southern Plains. Then by Monday, we establish a steady fast northwest flow aloft with the long wave trough over Hudson Bay. This pattern will more or less remain through Wednesday with weak disturbances in the northwest flow throwing periods of occasional light snow onto the northwest and northern mtns. Temperatures will not be quite as cold, given the surfacing northwest to west winds and clouds to keep radiational cooling at bay. This is a far cry from the frigid pattern as suggested by the models last week for this time frame. Then on Wednesday, a southern stream shortwave will align itself with a northern stream shortwave in the northwest flow, so have pulled down chances of snowfall farther south along the Wind River Mtns and Salt River Range for Thursday. && .AVIATION...For the 00Z TAFS through 00Z Saturday West of the Continental Divide...KJAC, KBPI, KPNA and KRKS Expect VFR conditions to prevail across the entire area through this evening. The next cold front will bring areas of MVFR/IFR light snow to a vicinity KJAC-KDUB and north line by 12z Friday and continuing through the day, as the area of snow and low clouds move south with the arctic front. However, the wind shift to the north associated with the front will never quite make it to KRKS Friday. East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL VFR conditions will prevail across the area tonight. The next cold front will begin to spread areas of MVFR/IFR ceilings and -SN south to a vicinity KCOD-KBYG line by 12z Friday. This area of light snow will spread south during the day on Friday. The wind shift to the north will occur by 18Z at KCOD and KWRL, and in KCPR by 00Z. The wind shift at KRIW will just be a gradual veering of the wind to the northwest. && .FIRE WEATHER... Clear skies tonight, as conditions begin to reset this evening ahead of an arctic front. Clouds will increase across the north tonight, with snow reentering the NW by midnight. This snow will spread from north to south during the day Friday, with the higher accumulations occurring across the west. Snow will end by late Saturday afternoon across the area, with temperatures remaining cold into early next week. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 11 PM this evening to 5 AM MST Saturday for WYZ001-002. Winter Weather Advisory from 2 PM Friday to 5 PM MST Saturday for WYZ014-023>025. Winter Weather Advisory from 5 PM Friday to 5 PM MST Saturday for WYZ027. Winter Weather Advisory from 5 AM Friday to noon MST Saturday for WYZ013. Winter Storm Warning from 5 AM Friday to noon MST Saturday for WYZ012. && SHORT TERM...LaVoie LONG TERM...Lipson AVIATION...Lipson FIRE WEATHER...LaVoie
Hazardous Weather Outlook National Weather Service Riverton WY 258 PM MST Thu Dec 21 2017 WYZ001>020-022>030-221200- 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- 258 PM MST Thu Dec 21 2017 ...Winter Storm warnings for the Teton and Gros Ventre Range in effect from Friday morning through Saturday morning... ...Winter Weather Advisories in effect from late tonight through the day Saturday for much of the rest of the far western portion of Wyoming... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...This Afternoon and Tonight. No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Friday through Wednesday. Friday and Saturday...An arctic cold front and storm system will spread snow and bitter cold air across the entire area. Significant snowfall is expected over the far west along with lesser accumulations east of the divide. Please see the the winter storm warning and winter weather advisories for more details. .SPOTTER INFORMATION STATEMENT... Spotters are urged to report any snowfall amounts. Thank you. TEAM RIVERTON
Special Weather Statement National Weather Service Riverton WY 202 PM MST Thu Dec 21 2017 WYZ028>030-221100- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- Including the cities of Rock Springs, Green River, and Wamsutter 202 PM MST Thu Dec 21 2017 West winds will increase on Friday in Sweetwater County out ahead of an approaching arctic cold front approaches from the north, but stalls out just to the north of Sweetwater County. With the close proximity of the stationary arctic front just to the north, west winds will continue Friday night and into Saturday. For Friday, west winds will blow at 25 to 35 mph, but by Saturday will increase to 30 to 40 mph with some gusts to 50 mph. Sudden, strong cross winds may enhance the blow over risk for light weight and high profile vehicles, including travel on Interstate 80, especially on Saturday.
Special Weather Statement National Weather Service Riverton WY 246 PM MST Thu Dec 21 2017 WYZ003>008-221100- Cody Foothills-North Big Horn Basin-Southwest Big Horn Basin- Southeast Big Horn Basin-Owl Creek and Bridger Mountains- Bighorn Mountains West- Including the cities of Cody, Meeteetse, Thermopolis, and Worland 246 PM MST Thu Dec 21 2017 An arctic cold front will be pushing south across the Big Horn Basin Friday night. Northerly winds will increase to 10 to 20 mph with the onslaught of the front. Shortly afterward, light fluffy snowfall will ensue. With the dry nature of the snow, it will only take a 10 to 20 mph wind to blow the snow around which will effectively reduce visibility significantly at times. By mid day Saturday, expect 1 to 3 inches of snowfall to accumulate across most of the basin, 2 to 3 inches in the Cody Foothills, 4 to 8 inches on the West Slope of the Big Horn Mountains, and 2 to 5 inches in the Owl Creek and Bridger Mountains. Expect a fair amount of drifting snow as well. The heaviest snowfall and strongest breezes will likely occur Friday evening. Make sure you check the latest weather forecasts and road conditions before heading out for the holidays. WYZ009>011-221100- Bighorn Mountains Southeast-Northeast Johnson County- Southeast Johnson County- Including the cities of Buffalo and Kaycee 246 PM MST Thu Dec 21 2017 An arctic cold front will be pushing south across Johnson County and the adjacent East Slope of the Big Horn Mountains Friday night. Northerly winds will increase to 10 to 20 mph with the onslaught of the front. Shortly afterward, light fluffy snowfall will ensue. With the dry nature of the snow, it will only take a 10 to 20 mph wind to blow the snow around which will effectively reduce visibility significantly at times. The heaviest snowfall and strongest breezes will likely occur Friday evening. Total snowfall by mid day Saturday should amount to 1 to inches at the lower elevations of Johnson County, including 2 inches at Buffalo, 4 to 8 inches of snow are expected on the East Slope of the Big Horn Mountains. There may be some isolated pockets of up to a foot of very fluffy snow at the highest elevations of the Big Horn Mountains. Make sure you check the latest weather forecasts and road conditions before heading out for the holidays. WYZ019-020-022-221100- Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain- Including the cities of Jeffrey City and Casper 246 PM MST Thu Dec 21 2017 An arctic cold front will be pushing south across Southeast Fremont County and Natrona County, including the city of Casper, as well as Casper Mountain. Northerly winds will increase to 10 to 20 mph with the onslaught of the front. Shortly afterward, light fluffy snowfall will ensue. With the dry nature of the snow, it will only take a 10 to 20 mph wind to blow the snow around which will effectively reduce visibility significantly at times. The strongest breezes will likely occur Friday evening with the heaviest snowfall occurring around midnight. Total snowfall by mid day Saturday should amount to 1 to 2 inches, including the city of Casper, to up to 5 inches on Casper Mountain. Make sure you check the latest weather forecasts and road conditions before heading out for the holidays. WYZ015>018-221100- Wind River Mountains East-Upper Wind River Basin-Wind River Basin- Lander Foothills- Including the cities of Dubois, Riverton, Shoshoni, and Lander 246 PM MST Thu Dec 21 2017 An arctic cold front will be pushing south across the Wind River Basin and surrounding foothills and mountains. Northerly winds will increase to 10 to 20 mph with the onslaught of the front. Shortly afterward, light fluffy snowfall will ensue. With the dry nature of the snow, it will only take a 10 to 20 mph wind to blow the snow around which will effectively reduce visibility significantly at times. The strongest breezes will likely occur Friday night, with the heaviest snowfall occurring after midnight Friday night and early on Saturday morning. Total snowfall by mid day Saturday should amount to an inch or less across most of the Wind River Basin, 1 to 2 inches in the Lander Foothills and around the Dubois area, and 2 to 5 inches along the East Slope of the Wind River Mountains. Travel will likely be especially hazardous over South Pass where considerable blowing and drifting snow will likely take place where wind gusts will likely reach 35 mph. Make sure you check the latest weather forecasts and road conditions before heading out for the holidays. TEAM RIVERTON
The national radar mosaic for the moment the first event began.
3 winter storm. Local standard time, as filed.
Heavy snow fell throughout the Star Valley. Total snowfall amounts ranged from around 8 inches in Afton to over 20 inches in some areas near Thayne.
Heavy snow fell in the Salt and Wyoming Range. Some of the higher amounts included 26 inches at Box Y Ranch and 19 inches at the Spring Creek Divide SNOTEL.
Heavy snow fell across the Tetons. Some of the highest amounts included 21 inches at the Jackson Hole Ski resort and 19 inches at the Grassy Lake SNOTEL site.
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, December 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: 728399, 728400, 728398.