A abnormally strong ridge and dry soil brought record heat to western and central Wyoming. Many areas saw record high temperatures, with a few stations reaching 100 degrees.
Every event with a place to put it; 6 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.
A abnormally strong ridge and dry soil brought record heat to western and central Wyoming. Many areas saw record high temperatures, with a few stations reaching 100 degrees. All time monthly high temperatures were set or tied at Big Piney, Casper, Lander, Riverton, Rock Springs and Worland.
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 Sep 5, 10:40 AM MST, 1 hour 20 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 1140 AM MDT Sat Sep 5 2020 .SHORT TERM...Today through Sunday A strong upper ridge of high pressure continues over the area. Record or near record high temperatures are expected today, as 700mb temperatures warm to between 16 and 18 degrees Celsius, especially with dry air and clear skies. The hazy/smoky skies look to remain through the day, as the HRRR shows some smoke pushing into the area from the west. The main concern for the period will be elevated to critical fire weather conditions for most of the Labor Day Weekend. A Red Flag Warning is in effect for western and central Wyoming today, from noon until 8 PM MDT this evening. On Saturday night, a weak cold front will push south across areas along and east of the Divide. This will result in temperatures at least 5 to 10 degrees cooler, while areas over the west will only be a couple degrees cooler. Humidities in the north will be a bit higher and winds in most places look to be marginal. Thus, have decided to keep a Fire Weather Watch for much of the area east of the Divide. .LONG TERM...Sunday night through Friday The pattern change that has been discussed for the last several forecast shifts will have begun with the cold front Saturday night/Sunday morning. This will prime the Cowboy State ahead of the strong cold front that will enter northern portions by Labor Day morning. Temperatures over far northern portions will hold steady from their morning lows and slowly fall through the day. The front, at this point, looks to reach the Divide by midday Monday. Precipitation, in the form of rain showers, will begin over the north by this time and steadily make its way southward over the Bighorn Basin and Johnson County through the rest of the afternoon. Ahead of the front across southwestern and southern WY, westerly flow will respond to the pressure gradient and increase out of the west. Gusty winds, along with RH values dropping into the lower teens, will result once again in these areas. Current Red Flag Warnings over these areas will probably be extended into Labor Day no later than Sunday. By 6pm Monday, 700mb temps are expected to drop below 0C across northern WY, bringing snow to the higher elevations and foothills. From 6pm to 10pm Monday night, models are indicating the mid-level forcing to catch up with the surface forcing and push the front strongly southward. North winds will increase quite a bit and moderate to heavy rain will push from northern into central WY. The surface front will also push southward into Sweetwater County after 6pm and across southwest WY through 10pm. Models are showing a very good upslope signature for areas east of the Divide by midnight, as well as, strong lift from frontogenesis from central to eastern WY. Expect snow levels to drop to the basin floors across northern WY during the early evening and across central WY just after midnight. Temperatures will likely hover right around freezing at the lower elevations for accumulating snowfall for a few hours through sunrise. Tuesday morning will see the core of the system move quickly down the Front Range, with residual showers into the afternoon hours and winds weakening toward late afternoon. Strong northeast wind will continue across southern WY through the day though. Overall, the forecast area should see precipitation totals ranging from about 0.3" west of the Divide to over 1 inch in some places east of the Divide for the storm. The Absarokas, Bighorns, and eastern Wind Rivers, as well as Casper Mountain, are likely to see 3-6 inches, with the lower elevations receiving anywhere from a trace around Greybull to over 2 inches around Lander. With 700mb temps remaining -6C to -9C through the day Tuesday, high temps will be record-breaking cold in the 40s east of the Divide. As the storm moves southeastward Tuesday night, clouds will decrease and clear for temperatures to drop overnight Tuesday night. This could be a hard freeze for many areas which have not yet seen the freezing temperatures from a week ago. The temperatures will likely get into the low 20s in areas where snow remains on the ground overnight with the clear skies, such as around Lander, Casper and Jeffrey City. Weak high pressure then starts building over the area with NNW flow on Wednesday, warming the temperatures slightly into the 50s, though western WY will reach the 60s. Far western WY will rebound more quickly, as that area will not be as influenced by the storm Monday and Tuesday. More seasonal conditions return Thursday and Friday, though winds may shift to the north late Friday due to a system moving into eastern MT. && .AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 1139 AM MDT Sat Sep 5 2020 VFR conditions are expected through the forecast period. Gusty winds have already developed at KCPR, and will continue through the afternoon. Winds will increase and become gusty between 18Z and 21Z across the rest of the area. The breezy conditions will occur through 02Z, then decrease. A weak cold front will move into northern WY after 06Z Sunday, with a wind shift to the north or northwest. Dry weather will continue through the period. Areas of smoke aloft will continue over most of WY. && .FIRE WEATHER... High pressure continues across the area, so expect another day of warm, dry and hazy conditions. Near record high temperatures are expected today, along with a Haines index of 6, and widespread RHs in 8 to 12 percent range. Southern areas will also see some westerly gusts in the 25-30 mph range during the afternoon. A Red Flag Warning is in effect starting at noon today across the entire area. West of the Divide, the Red Flag will continue through Sunday evening. East of the Divide, the Red Flag will end this evening. A weak, dry cold front will push into the area Sunday morning, lowering temperatures and raising RHs. Thus, a Fire Weather Watch is still in effect for Sunday. A major cold front on Labor Day will bring much colder temperatures, some precipitation, and higher humidity. Across the southwest, ahead of the cold front, elevated to possibly critical fire weather conditions could occur. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning until 8 PM MDT Sunday for WYZ277>279-288-289- 414>416. Red Flag Warning until 8 PM MDT this evening for WYZ140-275-276- 280>283-285>287-300. && SHORT TERM...Hulme LONG TERM...LaVoie AVIATION...Fisher FIRE WEATHER...Hulme
Hazardous Weather Outlook National Weather Service Riverton WY 1047 AM MDT Sat Sep 5 2020 WYZ001>020-022>030-061230- 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- 1047 AM MDT Sat Sep 5 2020 ...Red Flag Warning across the entire area this afternoon and Evening... ...Red Flag Warning continues through Sunday evening across the far west and southwest... This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight. Critical fire weather conditions across the entire area. Areas of smoke will linger across the area. Near record high temperatures expected. .DAYS TWO THROUGH SEVEN...Sunday through Friday. Sunday...Elevated to critical fire weather conditions across the entire area. Please see the red flag warning product for more details. Labor Day...Elevated to possibly critical fire weather conditions across southern portions. Labor Day and Tuesday...A strong cold front will push across the Cowboy State. Significant precipitation is expected with this system east of the Divide, especially Monday night. Significant accumulating mountain snow along and east of the Continental Divide Monday afternoon into Monday night. Some snow accumulation is also expected in the lower elevations. The bulk of the snow is expected to end late Monday night into Tuesday morning. Widespread freezes are expected Monday night and Tuesday night. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions, according to standard operating procedure. TEAM RIVERTON
Special Weather Statement National Weather Service Riverton WY 1159 AM MDT Sat Sep 5 2020 WYZ001>020-022>026-070400- 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- Including the cities of Lake, Mammoth, Old Faithful, Clark, Cody, Meeteetse, Basin, Greybull, Lovell, Powell, Shell, Thermopolis, Ten Sleep, Worland, Buffalo, Kaycee, Jackson, Dubois, Riverton, Shoshoni, Lander, Jeffrey City, Casper, Afton, Alpine, Star Valley Ranch, Thayne, Pinedale, La Barge, Big Piney, and Farson 1159 AM MDT Sat Sep 5 2020 ...AIR QUALITY ALERT FOR WILDFIRE SMOKE UNTIL 10 PM SUNDAY... The following message is transmitted in collaboration with the Wyoming Department of Environmental Quality, Air Quality Division and the Wyoming Department of Health. * WHAT...Air Quality Alert for Wildfire Smoke. * WHERE...Big Horn, Hot Springs, Fremont, Johnson, northern Lincoln, Natrona, Park, Sublette, Teton, and Washakie counties. * WHEN...Until 1000 PM Sunday September 6th. IMPACTS...Thick smoke from wildfires in other states has been observed across much of western and central Wyoming. The Smoky conditions are expected to last through Sunday. HEALTH INFORMATION...It is recommended by the Wyoming Department of Health that the elderly, young children, and individuals with respiratory problems avoid excessive physical exertion and minimize outdoor activities during this time. Wildfire smoke is made up of a variety of pollutants, including particulate matter and ozone, which can cause respiratory health effects. Although these people are most susceptible to health impacts, the Department of Health also advises that everyone should avoid prolonged exposure to poor air quality conditions. CURRENT CONDITIONS...The Wyoming Department of Environmental Quality, Air Quality Division offers near real-time air quality data for Wyoming's monitoring stations and health effects information to help the public interpret current conditions. Current air quality conditions across the state of Wyoming can be found at http://www.wyvisnet.com/
Special Weather Statement National Weather Service Riverton WY 1229 PM MDT Sat Sep 5 2020 WYZ002>011-015>020-022-061200- 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-Wind River Mountains East- Upper Wind River Basin-Wind River Basin-Lander Foothills- Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain- Including the cities of Clark, Cody, Meeteetse, Basin, Greybull, Lovell, Powell, Shell, Thermopolis, Ten Sleep, Worland, Buffalo, Kaycee, Dubois, Riverton, Shoshoni, Lander, Jeffrey City, and Casper 1229 PM MDT Sat Sep 5 2020 ...Major Change to Cold and Snow Monday Afternoon and Monday Night... This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...A very strong cold front will pull southward Monday and Monday night with significant mountain snowfall likely, and even some snow accumulation in the lower elevations. Strong northerly winds will also accompany the front. * WHERE...Northern and Central Wyoming east of the Continental Divide. * WHEN...Monday through Tuesday morning. * IMPACTS...People recreating in the mountains need to prepare now for a sudden change to wet, cold and snowy conditions. * ADDITIONAL DETAILS...Precipitation will begin over the north Monday afternoon spreading south into central Wyoming Monday evening. Mountains will begin as snow, but in the lower elevations the precipitation is expected to begin as rain before changing to snow after a few hours depending upon location. The snow ends from north to south late Monday night into Tuesday morning. WYZ001-012>014-023>030-061200- Yellowstone National Park-Teton and Gros Ventre Mountains- Jackson Hole-Wind River Mountains West-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- Including the cities of Lake, Mammoth, Old Faithful, Jackson, Afton, Alpine, Star Valley Ranch, Thayne, Pinedale, La Barge, Big Piney, Farson, Kemmerer, Cokeville, Rock Springs, Green River, and Wamsutter 1229 PM MDT Sat Sep 5 2020 ...Major Change to Colder Temperatures with some Snow Monday and Monday Night... This is a special weather statement from the National Weather Service Office in Riverton. * WHAT...A very strong cold front will pull south into northwest areas Monday and then pull southward across the entire area Monday evening ushering in much colder temperatures. There looks to be light amounts of snow in the mountains and even a little in the lower elevations. * WHERE...Western and southwest Wyoming. * WHEN...Monday afternoon and Monday night. * IMPACTS...People recreating on this holiday weekend need to be prepared for a sudden change to cold and wet conditions. 21
The national radar mosaic for the moment the first event began.
6 heat. Local standard time, as filed.
An all time September record high temperature of 91 degrees was set at Big Piney, Wyoming. This broke the old record of 87 degrees.
The Natrona County, Wyoming airport had a record high temperature of 98 degrees. This was a record high for the day and also tied the all time September high temperature.
The Lander, Wyoming airport had a high temperature of 96 degrees. This broke the all time September record of 95 degrees.
The ASOS at the Riverton, Wyoming airport reached a high of 96 degrees, tying the all time September high temperature.
The Sweetwater County, Wyoming airport had a high temperature of 91 degrees, breaking the all time September record high of 90 set in 2019.
The Worland, Wyoming airport reached a high temperature of 100 degrees, tying the all-time September record.
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, September 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: 915951, 915952, 915953, 915954, 915955, 915956.