A cold front swept southeastward across Wyoming and brought heavy snow to Johnson County. Up to 7 inches of snow fell in the vicinity of Buffalo, Wyoming with up to 15 inches of new snow in the eastern Bighorn Mountains.
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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 cold front swept southeastward across Wyoming and brought heavy snow to Johnson County. Up to 7 inches of snow fell in the vicinity of Buffalo, Wyoming with up to 15 inches of new snow in the eastern Bighorn Mountains.
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 Apr 18, 9:44 PM MST, 16 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1044 PM MDT Sun Apr 18 2021 .SHORT TERM...(This evening through Monday night) Issued at 131 PM MDT Sun Apr 18 2021 Cold front continues to track southward across Montana currently, and should be entering northern Wyoming around 6pm. The colder air is lagging behind slightly, so initial precipitation will be rainfall, before transitioning to snow by mid-evening. The colder air will continue to trek southward through the night, spreading snowfall into central WY around sunrise. The front will be exiting into CO around sunrise, but with the cold air lagging behind in the morning, initial precip across southern and southeast WY will be rainfall,before also transitioning to snow in the afternoon. Overall highest accumulations will be across the Bighorn Range tonight and tomorrow, and along the eastern Wind River Range. Snow accumulations for lower elevations will highly depend on when it falls. Areas of Johnson County will get a good accumulation during the night, and will be accompanied by gusty winds, so WSW issued for the I-90 corridor. Most lower elevations east of the divide will see some snowfall, but warmer ground temperatures will help travel conditions. With the front exiting southward quickly on Monday, most areas of snow will be ending, except for the highest elevations of the Winds and Bighorns Monday evening. Colder temperatures will move into the state quickly Monday night, with lows in the teens for most areas and single digits for the western Parks and mountains. .LONG TERM...(Tuesday through Sunday) Issued at 131 PM MDT Sun Apr 18 2021 Cold, northerly flow will be in place over the region Tuesday, as a weak shortwave starts to dig over the Northern Plains. This trough will produce some some minor activity as it passes over the forecast area Tuesday and Tuesday night. Moisture will not have a chance to advect back into the area, so any snow showers will be very isolated and likely confined to the northern mountains and possibly Johnson County. Temperatures will remain colder than normal, with highs 10 to 20 degrees below normal. Lows Tuesday night could be close to setting new records, with readings in the teens for much of the forecast area. Conditions begin to moderate Wednesday, as the region is under weak steering flow aloft. This will continue Thursday, with winds remaining relatively light across the region. A shortwave trough will clip the northern half of the state Thursday night into Friday. Gusty winds will be possible over northern Johnson County and areas north of Cody. Additionally, snow showers will be possible over far northern portions as the trough exits over the Plains. An upper level ridge will build over the region, in the wake of this trough. This ridge could remain in place through the weekend, but model solutions diverge in the relative strength of the ridge. Some solutions prog a flat ridge, while others have it strengthening and building into southern Canada. && .AVIATION...(For the 06Z TAFS through 06Z Tuesday) Issued at 1040 PM MDT Sun Apr 18 2021 East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Snow with MVFR to IFR conditions will develop in the north and continue through 18Z. Then, the snow will gradually decrease after 18Z through 00Z. After 00Z, scattered snow showers will linger through 06Z with local MVFR to IFR conditions. In the central, snow with MVFR to IFR conditions will develop after 10Z and by 14Z should encompass all the terminal. The snow will occur through 00Z, then gradually decrease through 06Z. Expect some mountain obscuration through the period. Please see the terminal forecast for details. West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals The terminals will see VFR conditions overnight into Monday morning. An area of snow will move from north to south Monday with MVFR to IFR conditions. KJAC will see the snow first, around 14Z, then KBPI and KPNA a few hours later, around 16Z. KRKS looks to see the snow around 20Z. Conditions will improve from north to south after 18Z through 00Z. KRKS will see improvement around 02Z. Expect some mountain obscuration today as the snow moves through the mountains. Please see the terminal forecasts for details. 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 131 PM MDT Sun Apr 18 2021 The cold front will race across the state tonight and tomorrow, bringing much colder temperatures and a chance for rain and snow for most of the state. Wind will shift to the north at 15 to 20 mph behind the front, which will continue through the day Monday. Most precip will end Monday evening, with winds decreasing to around 5 mph. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 6 AM to 11 PM MDT Monday for WYZ015- 022. Winter Weather Advisory until 6 PM MDT Monday for WYZ002. Winter Weather Advisory from 11 PM this evening to 6 PM MDT Monday for WYZ005>007. Winter Storm Warning from 9 PM this evening to 6 PM MDT Monday for WYZ010. Winter Storm Warning until 11 PM MDT Monday for WYZ008-009. Winter Weather Advisory from 9 PM this evening to 6 PM MDT Monday for WYZ011. && SHORT TERM...Straub LONG TERM...LaVoie/Hulme AVIATION...AR FIRE WEATHER...Straub
Hazardous Weather Outlook National Weather Service Riverton WY 600 PM MDT Sun Apr 18 2021 WYZ001>020-022>030-191200- 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- 600 PM MDT Sun Apr 18 2021 ...Winter Storm Warnings and Winter Weather Advisories are in effect for portions of northern Wyoming for tonight and Monday... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Tonight. A strong cold front will bring another round of snow and cold air overnight. Snow is likely along and east of the Continental Divide tonight. .DAYS TWO THROUGH SEVEN...Monday through Saturday. Monday...Snow will continue across areas east of the Divide through the day. The best accumulations will occur during the morning and on grassy surfaces during the day. Much below normal temperatures and gusty northerly winds will also occur. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions.
The national radar mosaic for the moment the first event began.
2 winter storm. Local standard time, as filed.
Snowfall amounts around Buffalo, Wyoming generally ranged from 4 to 7 inches, with a maximum of 7.3 inches four miles south-southwest of Buffalo.
Several locations around the eastern Bighorn range had snowfall amounts over a foot, including 15 inches at the Hansen Sawmill SNOTEL site.
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, April 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: 953884, 953885.