A snow squall moved across eastern Sweetwater County. The squall brought visibility to under one-quarter mile and resulted in dangerous travel conditions, mainly from Wamsutter eastward along Interstate 80.
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The episode as NWS RIW wrote it up.
A snow squall moved across eastern Sweetwater County. The squall brought visibility to under one-quarter mile and resulted in dangerous travel conditions, mainly from Wamsutter eastward along Interstate 80.
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 27, 3:43 AM MST, 5 hours 7 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 343 AM MST Mon Dec 27 2021 .SHORT TERM...(Today through Tuesday) Issued at 332 AM MST Mon Dec 27 2021 Another day and another round of snow for western Wyoming. The persistent active pattern continues today with a robust shortwave rotating through the longwave trough centered over the western CONUS. Another few inches of snow are likely for western Wyoming valleys, and the higher terrain and mountain passes are likely to see another 6 to 10 inches by midnight tonight. Winter Weather Advisories and Special Weather Statements cover the areas most likely to see notable accumulations. Similar to yesterday (Sunday), high-resolution models show a persistent band of heavy snow that pushes southeastward between noon and midnight. This is associated with the incoming cold front with winds switching to northwesterly behind the front. This feature will usher in a drier air mass, thus ending most snow showers after it passes. As the front passes expect strong wind gusts, quickly accumulating snow, and a sudden drop in visibility. Currently it looks like the front pushes through western valleys during the afternoon, reaching the Upper Green and Wind River Basin during the evening. It's not until the early overnight hours that south central Wyoming and the Powder River Basin see this feature move through. Most locations can expect a snowy and blustery half hour period at sometime over the next 24 hours; however, accumulations should be light (under an inch) for most lower elevations in the north central, south central, and central part of the state. The reprieve in snow Tuesday will come at a cost, and it's temperatures that will take the hit. Cold air advection behind the front bring some of the coolest temperatures we have felt so far this season, although they are not overly anomalous for late December. That being said, for those east of the Divide that have felt record mid-December warmth, the change is likely to be noticed. Lows in the single digits (above zero east of the Divide, below zero west) are likely Tuesday morning. Those out west struggle to get into the teens during the day, whereas temperatures east of the Divide have a hard time breaking 20. .LONG TERM...(Tuesday night through Sunday) Issued at 332 AM MST Mon Dec 27 2021 Any snow behind the Arctic front will have moved out by Tuesday evening, with the exception of a few orographic snow showers across the west. The million dollar question will be how cold will it get. This will be one of the coldest air masses so far this winter, although that's not saying much since it has been very mild so far. Also, a lot depends on snow cover and since the snow from the previous night will be of the banded variety East of the Divide, there could be quite a bit of variation. Some lingering cloud cover may also keep things a bit warmer. Many areas will be below zero on Wednesday morning so the biggest impact when people walk outside may be teeth chattering. Another shortwave will approach the west Wednesday and bring the next chance of snow showers. This system does not have a lot of moisture to work with so amounts at this point look to remain generally on the light side except in any banding that develops. The most numerous showers would be late Wednesday afternoon into Wednesday evening as the trough axis swings through. In areas East of the Divide, this will likely bring the return of breezy conditions to the areas prone to southwesterly flow, like Casper. The models are already showing some 700 millibar winds around 50 knots. This should moderate overnight lows somewhat in these areas. Inversions may hold in the basins, so highs were lowered a few degrees. Downsloping flow should keep areas East of the Divide dry. A messy system will then move in for Thursday night into Friday. The greatest impacts once again will be across the west where another round of winter weather highlights may be necessary. There is more uncertainty East of the Divide. For now, we kept PoPs and snowfall amounts on the lower side of guidance until there is more certainty. This may again end up bring more of a banded snow that is tricky to nail down this far out. Quieter weather should then move in for the weekend as flat ridging builds across the Rockies, bringing a welcome break to the active weather. Flow aloft will remain fast so it will remain breezy to windy in the favored spots. Temperatures stay below normal for this timeframe. && .AVIATION...(For the 12Z TAFS through 12Z Tuesday morning) Issued at 332 AM MST Mon Dec 27 2021 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals A Pacific moisture feed continues across the west today. KJAC will be MVFR and even IFR at times through most of today. Mountains will be frequently obscured. Light snow and MVFR may spread into KBPI and KPNA as early as 15Z Monday with a better chance of snow in the afternoon. Cold front pushes east during the late afternoon and early evening Monday. The best chance for KRKS to see a snow shower and MVFR would be shortly after 00Z Tuesday. Conditions gradually improve from west-to-east between 02Z and 06Z/Tuesday. Mainly VFR conditions are expected after that although occasional mountain obscurations will still be possible. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals VFR to prevail at all terminals through 00Z/Tuesday. A few snow showers accompanied by MVFR conditions may begin to sneak into KCOD after 21Z Monday and into the basins around 00Z Tuesday in advance of a cold front. A shallow Arctic front slips south through KCOD during the afternoon and into KWRL just prior to 00Z/Tuesday. This leads to lowering cloud decks at these terminals. KCPR needs to be watched for the potential for brief IFR/MVFR between 03Z-06Z Tuesday with snow showers. As for wind, a southwest surface wind persists at KCPR Monday with gusts around 35 kt during the day. KRIW will see a period of gusty northwest wind behind passage of the cold front between 00Z and 03Z Tuesday. Mountain tops will be frequently obscured. Conditions will then improve from northwest to southeast starting at 06Z Tuesday. && .FIRE WEATHER... Issued AT 332 AM MST Mon Dec 27 2021 Snow continues in the west today with several inches of accumulation. Ventilation will be very good east of the Divide as southwesterly winds gust in the 40 to 50 mph range. A cold front this afternoon and evening will bring a short period of intense snow and gusty winds to most locations. Winds shift to northwesterly behind this feature, and a quick inch of snow east of the Divide is possible as the front passes. Tuesday will be drier, with much cooler temperatures. The active pattern persists with snow returning to the west Wednesday night into Thursday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 8 PM MST this evening for WYZ012- 014-024. && SHORT TERM...VandenBoogart LONG TERM...Hattings AVIATION...Hattings FIRE WEATHER...VandenBoogart
Hazardous Weather Outlook National Weather Service Riverton WY 351 AM MST Mon Dec 27 2021 WYZ001>020-022>030-281100- 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- 351 AM MST Mon Dec 27 2021 ...Winter Weather Advisories in effect across portions of western Wyoming until 8 PM this evening... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Light to moderate snow will prevail in the western mountains and valleys. Gusty southwest winds continue across much of the region during the afternoon, later switching to northwesterly as a cold front passes. A half-hour period of intense snowfall, low visibility, and gusty winds are likely area-wide as the cold front blasts southeastward this afternoon and evening. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday. Tuesday and Wednesday...Cooler temperatures are forecast to move into the state, with below normal readings each day. Thursday and Friday...More snow in the west and strong winds area-wide are possible. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report overnight snowfall totals.
The national radar mosaic for the moment the first event began.
1 winter weather. Local standard time, as filed.
A snow squall accompanied by estimated wind gusts of 30 mph reduced visibility on I-80 in eastern Sweetwater County resulting in hazardous travel conditions.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2021Every state in 2021This day in weather historyWarnings in Wyoming, December 2021
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: 997102.