An area of low pressure moved northeastward across southeastern Wyoming and brought moderate to heavy snow to some areas East of the Divide. In the lower elevations, the highest totals were in Casper, Wyoming, where up to 11 inches of snow fell and near Lander, Wyoming, where up 10 inches fell.
Every event with a place to put it; 4 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.
An area of low pressure moved northeastward across southeastern Wyoming and brought moderate to heavy snow to some areas East of the Divide. In the lower elevations, the highest totals were in Casper, Wyoming, where up to 11 inches of snow fell and near Lander, Wyoming, where up 10 inches fell. Southern Johnson County also had up to 7 inches of new snow. The highest total in the mountains was 15 inches of Casper Mountain.
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 Mar 9, 2:37 PM MST, 2 hours 23 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 237 PM MST Tue Mar 9 2021 .SHORT TERM...(This afternoon through Wednesday night) Issued at 237 PM MST Tue Mar 9 2021 Despite cooler temperatures in the wake of a cold front, most areas have climbed to around or above seasonal averages this afternoon. Jet-induced mid- and upper-level cloudiness has developed over west- central Wyoming with the overall trend toward increasing cloud cover this evening. The jet streak will track northeast late this afternoon and evening. Right entrance region of the jet streak will be a brief player this evening as precipitation begins to develop across central Wyoming toward 06Z/Wednesday. Models consistent with a 700mb circulation forming over Sweetwater County by that time. However, this forecast trends a bit slower with the development of precipitation, especially in central Wyoming. Model trends have been slower and this seems very plausible given current location of the main shortwave. Therefore, expect an initial period of light rain after mainly 03Z/Wednesday before a changeover to snow by 06Z/Wednesday. Recent warm temperatures have warmed the surface, so accumulations prior to 06Z will be limited. The exception will be the I-80 corridor across southern Wyoming, where higher elevations and proximity to the moisture and shortwave will allow for light accumulations by midnight. The primary period for heavier snow will be the 06Z-15Z/Wednesday time frame. The 700mb circulation will make its way slowly north into Natrona County by 12Z. Enhanced snowfall will occur with this feature. Also, there should be at least a 6-hour period of better upslope flow of 20-30kts, which otherwise is generally unimpressive with this storm. Q-vector convergence is solid across central Wyoming during this 9-hour period. 700mb temperatures never get much colder than -11C, so the lack of dendritic growth could be a limiting factor. Jet energy will also be limited as the track keeps it too far south. Much will depend upon the circulation and this storm making efficient use of the available moisture. Dynamics and upslope begin to decrease by 18Z/Wednesday as the 700mb circulation pulls east. The best wrap- around snow will remain across the I-25 corridor, but intensity will decrease and surface temperatures will warm enough to limit afternoon accumulations. A more broad area of wrap-around snow is likely to remain across a good chunk of the forecast area through the day. Coupled with the 500mb cold air advection, instability will be sufficient to support ongoing showers. As a result, Wednesday afternoon may end up being a high PoP, low QPF forecast at many locations west of the I-25 corridor. Westerly surface flow will kick-in across Sweetwater County for the afternoon. Additional snowfall across the south in the afternoon would mainly be confined to eastern Sweetwater County. Models have been consistent with lingering a plume of wrap-around moisture from northwest to central Wyoming in the evening and it is difficult to discount. Expect all areas to be dry early Thursday morning. Clouds will decrease from the northwest early Thursday and this will need to be watched. Too much clearing could play havoc with morning lows in areas with freshly fallen snow. Jet stream to our south will allow for moisture to sneak into Sweetwater County as shortwave energy tracks just to our south as we head into Thursday. Overall, forecast remains consistent with impacted locations and those areas where the heaviest snow is expected. Similar to yesterday, QPF was tempered somewhat. However, looking at ensemble model QPF and snow accumulation plumes, believe that this may be on the right track with regard to mean storm totals. Natrona County into far southern Johnson County has the longest residence time for the moisture and the most favorable dynamics. Still looking like 6 to 10 inches in the lower elevations, with the heaviest snow perhaps falling north and west of Casper. Fremont County totals look to be in the 3 to 5 inch range, with 5 to 8 inches closer to the foothills. All totals still well within the current highlights and forecast amounts. The challenge may come in the Bighorn Basin where the demarcation may be sharp. Upslope and upper flow will favor the highest amounts in Hot Springs and Washakie counties with lesser amounts over Park and Big Horn. .LONG TERM...(Thursday through Tuesday) Issued at 237 PM MST Tue Mar 9 2021 Thursday will see a chance of showers pushing into the southwest areas from a low sitting across NV/UT. That low looks to be absorbed into the stronger low that starts off CA coast and slowly moves to the four corners region Saturday. This low will continue into southeast Colorado Saturday night. After that, the models can't agree on what direction this low will take. GFS wants to move it northward to the Dakotas, while the other models continue the eastward track. We are keeping with the drier solution for the weekend now, but will continue to watch that track. It does look like Friday will see chances of precipitation across the south and east central areas. Models are indicating another system moving into the northwest on Monday for another round of precipitation. This will eventually spread snow across most of the region Monday night. Timing of this system is still variable across the models, so we have left the forecast for now. Temperatures on Thursday and Friday will be on the cool side, with warming over the weekend. There will be some gusty winds along the southern corridor on Friday and Saturday afternoons. Otherwise, winds should not be an issue throughout the long term forecast. && .AVIATION...(For the 00Z TAFS through 00Z Wednesday Evening) Issued at 237 PM MST Tue Mar 9 2021 East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals VFR conditions will start the period. Between 00Z and 03Z, snow showers are expected to develop across central areas, before becoming widespread across Fremont, Johnson, and Natrona counties between 03Z and 06Z tonight. Widespread MVFR ceilings and IFR visibilities in snow will likely occur in those areas for a prolonged period. KCPR has a decent chance of seeing airfield minimums at times especially after 06Z Wednesday. Some of the snow will likely push into the Bighorn Basin, with snow and MVFR conditions at KWRL after around 09Z. Confidence is not high enough to prevail snow at KCOD, but did prevail MVFR ceilings. West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals VFR conditions to start the period. Scattered snow showers are expected to develop over the south/southwest between 00Z and 03Z this afternoon, and continue through the night. KRKS will be most impacted by this activity with a decent chance for a period of IFR conditions. MVFR conditions are expected at times at KBPI and KPNA mainly after 06Z. KJAC will be on the fringe of the snow showers and will keep them dry for now. 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 237 PM MST Tue Mar 9 2021 While cooler today, temperatures have still managed to climb above normal for most areas. Gusty northwest wind behind a departing cold front will be on the decrease the rest of this afternoon. Attention then turns to an approaching batch of Pacific moisture and the development of low pressure over the forecast area tonight and Wednesday. Widespread snow will develop later this evening across southwest and central Wyoming. The heaviest snow is anticipated through mid-morning Wednesday with accumulations of at least three to five inches inches for many locations. The BLM High Plains District can expect higher totals of six to ten inches in Natrona County and southern Johnson County. Low pressure over central Wyoming at the start of the day will trudge east during the afternoon. Snow will continue to wrap around this low and across northern and central Wyoming through the afternoon. Accumulations during the afternoon will be minimized by warming temperatures and decreasing snowfall rates. Most areas will be dry by late Wednesday evening. The storm track then takes the next few systems to our south from Thursday into the weekend. This will provide the best chance for snow showers and gusty northeast wind of 15-25 mph across the southern half of the BLM High Desert District Thursday. A storm system Saturday could wrap rain and snow showers into central Wyoming during the afternoon. Temperatures will be cooler than normal Wednesday with a gradual increase to more seasonal averages by Saturday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 8 PM this evening to 5 PM MST Wednesday for WYZ007>010-015>019-022. Winter Storm Warning from 8 PM this evening to 5 PM MST Wednesday for WYZ011-020. && SHORT TERM...CNJ LONG TERM...Swanson AVIATION...Swanson/CNJ FIRE WEATHER...CNJ
Hazardous Weather Outlook National Weather Service Riverton WY 156 PM MST Tue Mar 9 2021 WYZ001>020-022>030-101400- 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- 156 PM MST Tue Mar 9 2021 ...Winter Storm Warning Tonight through Wednesday Afternoon for Most of Natrona County and Southern Johnson County... ...Winter Weather Advisory Tonight through Wednesday Afternoon for Other Areas of Central and Northcentral Wyoming... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Rest of Today and Tonight Snow will develop in southern and central Wyoming late this evening, with the heaviest snow coming after midnight. Moderate to heavy snow during the pre-dawn hours will create hazardous travel conditions. Most susceptible will be I-25, US 20/26, US 287, and Wyoming 220 in central Wyoming. Please see the Winter Weather Message for more information on specifics and timing. .DAYS TWO THROUGH SEVEN...Wednesday through Monday Wednesday...Moderate to heavy snow continues in central Wyoming through the morning commute. Many areas of central and northcentral Wyoming can then expect light snow through midday before a gradually decrease in the afternoon. .SPOTTER INFORMATION STATEMENT... Spotters are encouraged to report overnight snowfall to the National Weather Service Wednesday morning.
Special Weather Statement National Weather Service Riverton WY 207 PM MST Tue Mar 9 2021 WYZ028>030-101400- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- Including the cities of Rock Springs, Green River, and Wamsutter 207 PM MST Tue Mar 9 2021 ...Snow to Develop this Evening and Persist through Wednesday Morning... This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Periods of light snow developing. Total snowfall of 2 to 3 inches. * WHERE...Sweetwater County. * WHEN...Tonight through Wednesday. Most of the accumulating snow will have ended by late Wednesday morning. * ADDITIONAL DETAILS...Area roads, including I-80 across Sweetwater County, will become slick or snow covered early Wednesday morning. WYZ005-006-101400- Southwest Big Horn Basin-Southeast Big Horn Basin- Including the cities of Thermopolis, Ten Sleep, and Worland 207 PM MST Tue Mar 9 2021 ...Snow to Develop Overnight and Persist through Wednesday Morning... This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...Periods of light snow. Snowfall of 1 to 3 inches. Higher amounts possible along the foothills of the Bridger Mountains. * WHERE...Southern Bighorn Basin, including Hot Springs and Washakie counties. * WHEN...Midnight through Wednesday morning. * ADDITIONAL DETAILS...Area roads will become slick or snow covered late tonight through Wednesday morning.
The national radar mosaic for the moment the first event began.
4 winter storm. Local standard time, as filed.
The SNOTEL on Casper Mountain reported 15 inches of new snow.
To the east of Kaycee, Wyoming, 6.5 inches of new snow fell.
Snowfall totals around Casper, Wyoming ranged from 6 to 11 inches. Amounts dropped further north and west.
New snowfall in Lander, Wyoming ranged from 6 to 9 inches; including 7.7 inches at the Lander airport.
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, March 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: 940923, 940921, 940922, 940924.