A fast moving system brought heavy snow to portions of the Tetons. At the top of the Jackson Hole Ski Resort, 12 inches of new snow fell.
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The episode as NWS RIW wrote it up.
A fast moving system brought heavy snow to portions of the Tetons. At the top of the Jackson Hole Ski Resort, 12 inches of new snow fell.
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 Jan 4, 2:40 PM MST, 20 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 240 PM MST Mon Jan 4 2021 .SHORT TERM...(This evening through Tuesday night) Issued at 240 PM MST Mon Jan 4 2021 Upper level ridge axis will move across the forecast area and be to our east by early Tuesday morning. Warm advection precipitation will increase across western Wyoming by early Monday evening ahead of a trough moving onshore Monday afternoon in northern California and the Pacific Northwest. Isentropic lift relatively impressive with an 8-hour period of heavier snow anticipated between 7PM Monday and 3AM Tuesday over the far west mountains and valleys. Instability is not overly impressive, but moisture looks plentiful overnight. Have increased mountain wind speeds overnight given current wind trends and projected 700mb winds of 30-40 mph. Overall, snow totals of 8 to 14 inches in the Tetons, with most other mountain locales seeing 6 to 12 inches. The western valleys should see 2 to 4 inches with totals up to 6 inches around Moose and also Alpine. Shortwave energy looks to track just to our south late tonight providing the potential for accumulating snow across the I-80 corridor through Sweetwater County. This will be aided by the left-exit region of an 80kt 300mb jet max. However, this period of snow appears to be brief across the southwest as dynamic forcing wanes and isentropic downglide takes over around sunrise Tuesday in the southwest. Trough axis will make its way to the western border by 12Z/Tuesday, with a quick end to the snow by midday Tuesday. Have left highlights as is for now, but suspect the accumulating snow will be over by 18Z/Tuesday. Gusty west to northwest wind will blow across Sweetwater and Sublette counties Tuesday afternoon in the wake of this system. To the east, conditions remain dry tonight with southwest wind increasing across the Wind Corridor ahead of the incoming storm system. Would not be surprised to see a few 50 mph gusts along Wyoming Boulevard in Casper late tonight as 700mb wind jumps to 40-50kts by 12Z/Tuesday. The upper flow will veer to the northwest during the day Tuesday with a cold front pushing south through the day. The chance of light snow will increase from sunrise through mid-morning in the Bighorn Basin. The northwest flow favors accumulating snow in the Upper Nowood Valley. A better chance of snow will spread south along I-25 during the mid- morning into the early afternoon. Temperatures and precipitation type may get a bit tricky at the onset. Current thinking is that a changeover to snow will occur quickly by midday Tuesday as the precipitation intensity increases and dynamic cooling takes over. 700mb temperatures should drop to between -6C and -8C, so enough cold air should be present. The potential for heavier snow over a period of 3 to 5 hours, gusty northwest wind, and temperatures falling to around freezing could cause travel problems across Natrona and southern Johnson counties. There could also be some initial frontogenetic forcing and a 500mb cold pool aloft to enhance snowfall rates around midday. In the end, snowfall of 1-2 inches with local amounts to 3 inches is likely in this area. The northwest flow does not favor Fremont County and believe it should be areas east of Shoshoni that have the best shot at accumulations. Conditions will improve rapidly around or shortly after sunset. A ridge will fold-over the region Tuesday night with warmer air moving in aloft. Despite the late day snow and the warm air advection to strengthen inversions, lingering wind should inhibit fog formation across central Wyoming. A clearing sky and new snow cover should allow for good radiational cooling Tuesday night across the entire forecast area. Current low temperatures may be too optimistic, especially in the Jackson and Star Valleys, and will need to be watched on subsequent shifts. .LONG TERM...(Wednesday through Monday) Issued at 240 PM MST Mon Jan 4 2021 By Wednesday, the trough is more or less history as it races east across the Plains. Ridging builds in behind it, leaving the region in generally tranquil conditions through at least the next 24 to 36 hours. Winds may continue to be gusty in northwest flow with a tight gradient behind the system for Wednesday, but otherwise dry conditions are expected through Thursday morning. Both the GFS and the ECMWF are in agreement that the long term forecast will be unsettled with a series of weak shortwaves moving across the region through the early part of next week, but this is where the agreement ends. Models prog the next system to move across the area for Thursday night. The GFS and the ECMWF are not in much agreement when it comes to location and amount of precip, with the ECMWF hanging onto the trough longer and therefore bringing more precipitation east of the Divide. However, will likely be more of a western Wyoming event with precip spilling east of the Divide Thursday night and potentially lingering into Friday morning. The next system is progged for the weekend, but again, the models are not showing any real agreement or lock on what or where the impacts will be. The weak, unsettled pattern looks to continue into the early part of next week, but take the forecast with a grain of salt, due to the copious amount of uncertainty in the models at this time. Due to the uncertainty in the forecast, just let the NBM ride through the period and only noteworthy changes to the grids were to bring down WPC QPF amounts for the Thursday system by 10 percent in the higher elevations, as the WPC QPF amounts tend to be well overdone in the western mountains and Bighorns. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon) Issued at 1044 AM MST Mon Jan 4 2021 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals Backed off a couple hours on the timing of snow showers from the previous forecast based on current observations. Snow showers will be increasing in intensity and coverage today, with KJAC seeing likely -sn after 00Z through the end of the period. Highest impact times will be between 00Z and 12Z Tuesday. Some of these snow showers will approach KBPI and KPNA, with the greatest chances again happening after 00Z. Winds for these sites will remain light through the period. KRKS will see lighter winds today, with only a brief period of gusts possible this afternoon. Snow chances at KRKS remain low, occurring between 06Z and 15Z. LLWS will occur overnight due to strong flow aloft at some sites. Gusty winds will mix down at KRKS. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals VFR conditions are expected through the TAF period. Winds will remain light except for KCPR, which will see gusty winds through much of the TAF period. Gusts up to 40kt will be possible this afternoon. Gusty winds will also occur at KCOD. Snow showers from the west are expected to move east of the Divide after 12Z Tuesday with only VCSH in the forecast for now. Also included LLWS for KCPR and KLND due to strong winds aloft. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued AT 240 PM MST Mon Jan 4 2021 Gusty southwest wind will continue across the Casper Dispatch area overnight ahead of an incoming weather system. This system will bring snow to the mountains and valleys of western Wyoming tonight. Widespread totals above 6 inches are expected in the western mountains with 2 to 4 inches in the valleys. Some light snow will spread into the High Desert District by sunrise with accumulations of one inch or less. A push of colder air and gusty northerly wind will accompany snow sliding south through areas east of the Continental Divide on Tuesday. The wintry switch will occur in the Bighorn Basin and northern Johnson County during the morning before impacting central Wyoming around midday. Accumulating snow is likely, with accumulations of 1 to 2 inches possible over the lower elevations of Natrona County. North-northwest wind 25-40 mph with potentially higher gusts will be found in the norther portions of the Bighorn Basin and Johnson County by midday Tuesday. The snow and wind will gradually end from west-to-east through the day, lingering longest in Natrona County. Clearing continues overnight with a dry day in store for Wednesday. Basin and valley inversions, especially where new snow falls Monday and Tuesday, will limit smoke dispersal Wednesday. Temperatures will be seasonal except along the east slopes of the Absaroka and Bighorn ranges. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 3 PM MST Tuesday for WYZ001-013- 014-023>025. Winter Storm Warning until 3 PM MST Tuesday for WYZ012. && SHORT TERM...CNJ LONG TERM...Fisher AVIATION...Fisher FIRE WEATHER...CNJ
Hazardous Weather Outlook National Weather Service Riverton WY 146 PM MST Mon Jan 4 2021 WYZ001>020-022>030-051400- 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- 146 PM MST Mon Jan 4 2021 ...Winter Storm Warning and Winter Weather Advisories for far Western Wyoming through Tuesday Afternoon... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Rest of Today and Tonight Snow will increase across western Wyoming this evening and continue overnight. The snow will be heavy at times through early Tuesday morning. Morning commuters should be prepared for hazardous travel over Teton Pass and through Snake River and Hoback canyons. Light snow is also expected along the I-80 corridor through Sweetwater County late tonight. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Tuesday...Snow will decrease during the morning across western Wyoming, ending by early afternoon. East of the Continental Divide, there will be a chance of snow beginning in the north during the morning. The snow then spreads into central Wyoming by early afternoon with a brief period of intense snow possible across Natrona and southern Johnson counties. Expect gusty west to northwest wind much of the day in Sweetwater County and northern portions of the Bighorn Basin and Johnson County. Thursday and Friday...A chance of snow in the west Thursday, spreading eastward Thursday night into early Friday. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report overnight snowfall totals Tuesday morning.
Special Weather Statement National Weather Service Riverton WY 233 PM MST Mon Jan 4 2021 WYZ027>030-051300- South Lincoln County-Rock Springs and Green River-Flaming Gorge- East Sweetwater County- Including the cities of Kemmerer, Cokeville, Rock Springs, Green River, and Wamsutter 233 PM MST Mon Jan 4 2021 ...Light Snow and Gusty Wind Likely Early Tuesday... This is a special weather statement from the National Weather Service in Riverton. * WHAT...Light snow accumulations up to one inch along with gusty west wind of 20 to 30 mph. * WHERE...Sweetwater and south Lincoln counties. * WHEN...Early Tuesday morning prior to sunrise. * ADDITIONAL DETAILS...The combination of snow and wind could create local areas of reduced visibility and a slick road surface. These conditions could be especially hazardous at higher speeds along I-80 and US Highway 30. WYZ010-051300- Northeast Johnson County- Including the city of Buffalo 233 PM MST Mon Jan 4 2021 ...Snow and Gusty Northwest Wind Arriving Tuesday Morning... This is a special weather statement from the National Weather Service in Riverton. * WHAT...Light snow accumulations up to one inch along with gusty northwest wind of 25 to 40 mph during the morning. The snow will end by midday, but a northwest wind of 25 to 35 mph with gusts of 40 to 50 mph will persist through the afternoon. * WHERE...Northern Johnson County. * WHEN...Tuesday. * ADDITIONAL DETAILS...The combination of snow and wind could create local areas of reduced visibility and a slick road surface. These conditions could be especially hazardous at higher speeds along I-90. WYZ011-020-022-051300- Southeast Johnson County-Natrona County Lower Elevations- Casper Mountain- Including the cities of Kaycee and Casper 233 PM MST Mon Jan 4 2021 ...Snow Likely by Late Tuesday Morning... This is a special weather statement from the National Weather Service in Riverton. * WHAT...Snow could be heavy at times. Lower elevation accumulations of 1 to 3 inches, with 3 to 5 inches possible on Casper Mountain. * WHERE...Natrona County and southern Johnson County. * WHEN...9 AM to 3 PM. * ADDITIONAL DETAILS...The snow is likely to be heaviest between 11 AM and 2 PM. Surface temperatures may limit accumulation at the onset, but a wet snow should develop by midday. Roads may become slick on I-25 north of Casper and on US 20/26 to the west of town.
The national radar mosaic for the moment the first event began.
1 winter storm. Local standard time, as filed.
After NWS Directive 10-1605, the rules offices file Storm Data by.
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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: 932198.