A strong Canadian cold front moving in from the north combined with Pacific moisture moving in from the south and brought heavy snow to portions of northern and central Wyoming. In the mountains, amounts of over a foot were recorded in the Big Horn and Wind River Range.
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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 strong Canadian cold front moving in from the north combined with Pacific moisture moving in from the south and brought heavy snow to portions of northern and central Wyoming. In the mountains, amounts of over a foot were recorded in the Big Horn and Wind River Range. In the lower Elevations, the highest amounts fell in Fremont County where close to a foot fell around Lander with 6 to 9 inches around Riverton, Wyoming.
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 11, 2:01 AM MST, 6 hours 59 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 301 AM MDT Sat Apr 11 2020 .SHORT TERM...Today through Monday night Things will get rather active over the next 36 hours or so as a cold front drops southward out of Montana and brings an end to nice stretch of mild weather we have had over the past few days. Radar is already showing some returns this morning but the precipitation in the lower elevations is mainly in the form of rain with snow confined to the higher elevations. This will change as the front drops southward across the area and rain changes over the snow from north to south through the day and into the evening. This does have some instability with it and there could even be a thunderstorm in a few spots this afternoon and evening. Timing still looks decent with the system with the biggest impacts and heaviest snow from mid afternoon in the far north and then tonight across most areas, with an emphasis on this evening for the worst conditions with the maximum frontogenical forcing and upper level forcing over the area. With lee cyclogenesis over Colorado increasing the pressure gradient, it will become rather windy in some areas as well, and blow the snow around tonight as temperatures drop into the 20s and the snow becomes drier. At this point, the highest amounts look to fall in the favored upslope locations with north to northeast flow. This includes places like Lander, Thermopolis and Casper. All of the watches were upgraded to warnings this morning, and many other areas have advisories. Just about all areas will see snow though, and we will issue a special weather statement to cover for this. High temperatures will be a pain in the butt today as well, as some areas will have falling temperatures as the front passes and others won't. The steady snow looks to end after sunrise on Monday. However, with cold air in place and cyclonic curvature remaining, areas of snow will linger through the day, although they will become lighter into the afternoon. The highlights may be run a little long, but they are more for impacts. Sunday afternoon should see improving conditions, as the strong April sun should begin to bring better roads conditions as the snow lightens despite the cold temperatures. All snow should end late Sunday night as the front moves out of the area. The main story will then shift to how cold will it get. We will have two of the required ingredients with fresh snow and a cold airmass. However, there is still a question if there can be enough clearing to get good radiational cooling. Record lows will still be possible though. Monday looks to be a cold day with a mix of clouds and sunshine. Another shortwave will swing across the state on Monday. However, with the source region from central Canada it has very limited moisture to work with. The NAM is by far the wettest model but it usually has a wet bias this far out and is also overdone on QPF in the mountains as it overestimates the orographic effects usually. The GFS and NAM look much more reasonable. So we have gone with largely slight chance to chance POPs with low snowfall amounts. There could always be some snow banding, but these are almost impossible to pinpoint this far out. .LONG TERM...Tuesday through Saturday In a strong, moist north northwest flow aloft Tuesday, a few snow showers are expected mainly over the northwest/northern mountains. Models then show the potential for a distinct shortwave trough sweeping southward out of SW Canada in the Wednesday/Wednesday night time frame. Global models have significant differences on the evolution/timing of this system, but overall there will be decent chances of snow Wednesday and/or Wednesday night. Also another cold front will surge across the area reinforcing the cold air already in place, but not to the degree as what will occur this Saturday night and Sunday. Models diverge significantly after Wednesday, but continue to show some chances of snow continuing Thursday into Thursday night. After Thursday, the placement of systems and their associated sensible weather elements is low to very low. The main exception is for a warming trend Friday and Saturday. && .AVIATION...12Z Issuance West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals Westerly winds will increase this morning for KRKS due to approach of the next front. Rain will be turning to snow at KJAC shortly. The rain will then turn to snow around 13Z at KBPI and PNA today. Rain showers will likely not arrive at KRKS until around 21Z this afternoon. West winds at KRKS will increase to 25 to 35 knots by around 21Z this afternoon. IFR/MVFR conditions along with mtn obscn are expected in far western Wyoming, especially at KJAC, with mostly MVFR conditions at KPNA and KBPI through 03Z Sunday. There could be some thundersnow with the front this afternoon with the front. Expect gradually improving conditions after 06Z Sunday. East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Winds will blow from the north for KCOD and KWRL, as the front begins to progress south and will reach KRIW/KLND just before 15Z this morning. Precipitation will be on the increase, but but with the colder air lagging behind a bit, it will start as rain, with snow switchover not expected until the daytime hours today. Precipitation will begin around 15Z at KCPR and by 20Z at KRIW and KLND. Once the precipitation sets in on Saturday, expect IFR/MVFR conditions along with mtn obscn. Conditions will gradually improve after 06Z Sunday, except in KLND and KCPR where upslope snow will keep conditions IFR through 11Z Sunday. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... A cold front will drop southward across the state today and tonight and bring an increasing chance of showers across the area which will turn to snow from north to south. A few spots could even see an isolated thunderstorm as well. A gusty to strong north wind will develop behind the front, especially in areas East of the Divide. Relative humidity will remain above critical levels. Mixing and smoke dispersal will range from fair to poor in the north to excellent in the south. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning from 6 PM this evening to 6 PM MDT Sunday for WYZ015-018>020-022. Winter Weather Advisory from noon today to 6 AM MDT Sunday for WYZ001-002-012. Winter Storm Warning from noon today to noon MDT Sunday for WYZ010-011. Winter Weather Advisory from 6 PM this evening to 6 PM MDT Sunday for WYZ016-017. Winter Weather Advisory from 6 PM this evening to noon MDT Sunday for WYZ014-024. Winter Storm Warning from 6 AM this morning to noon MDT Sunday for WYZ008-009. Winter Weather Advisory from 3 PM this afternoon to 3 PM MDT Sunday for WYZ005>007. Winter Weather Advisory from noon today to noon MDT Sunday for WYZ003. && SHORT TERM...Hattings LONG TERM...Murrell AVIATION...Murrell FIRE WEATHER...Hattings
Hazardous Weather Outlook National Weather Service Riverton WY 946 AM MDT Sat Apr 11 2020 WYZ001>020-022>030-121600- 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- 946 AM MDT Sat Apr 11 2020 ...Winter Storm Warnings and Winter Weather Advisories remain in effect for much of the area Today into Sunday... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Rain changing to snow across much of the area through the day today, with potentially impactful snow accumulations. Please see winter weather products, statements and forecasts for details. .DAYS TWO THROUGH SEVEN...Easter through Friday. Easter...Snow gradually tapering off. Temperatures will be 20 to 30 degrees below normal. Please see winter weather products, statements and forecasts for details. Wednesday and Thursday...Scattered snow showers. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions, according to standard operating procedures.
The national radar mosaic for the moment the first event began.
7 winter storm. Local standard time, as filed.
Heavy snow fell in portions of the Eastern Big Horn Range. At the Cloud Peak SNOTEL, 13 inches of new snow fell.
Heavy snow fell in portions of the Big Horn Mountains. At the Middle Powder SNOTEL site, 12 inches of new snow fell.
Heavy snow fell in portions of northern Johnson County. A trained spotter measured 6 inches of snow to the southwest of Buffalo, Wyoming.
A trained spotter measured 7 inches of snow in Casper, Wyoming.
Heavy snow fell in portions of the Wind River Mountains. At the Deer Park SNOTEL site, 13 inches of new snow fell.
Heavy snow fell around the Lander, Wyoming Foothills. At the Lander, Wyoming Airport, 9.1 inches of snow fell. Amounts around Lander, Wyoming generally ranged from 8 to 12 inches.
Heavy snow fell in portions of the Wind River Basin. Around Riverton, Wyoming, 6 to 9 inches of snow fell with 8 inches of snow at Hudson, Wyoming. Amounts gradually decreased to the north and west of Riverton, Wyoming.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2020Every state in 2020This day in weather historyWarnings in Wyoming, April 2020
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: 885230, 885228, 885231, 885233, 885235, 885237, 885239.