A large area of persistent moderate rain caused fast rising streams and creeks, leading to minor flooding of rural roads across the southern 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 large area of persistent moderate rain caused fast rising streams and creeks, leading to minor flooding of rural roads across the southern 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 Sep 4, 3:42 PM MST, 3 hours 58 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 442 PM MDT Mon Sep 4 2023 .DISCUSSION...(This evening through Monday) Issued at 1258 PM MDT Mon Sep 4 2023 At 1100L, the low center is positioned over far southwestern WY. This low will continue to shift northeast across the state through the day. Showers and occasional thunderstorms will rotate through much of the region through the rest of the day. Most of us across the western half of the state will remain overcast with periods of light to moderate rain today as the low pushes through the region. Clear skies remain across the eastern half of WY this morning, which will help to create instability as we head into the afternoon hours. The location of the axis of stronger storms will largely depend on the track of the low as it moves northeastward this afternoon. To the areas east of the low (but not directly on top of the low center), CAPE will increase to 1000 to 1500J/kg and could get as high as 2000J/kg with slightly negative LIs, as instability increases through the day. Storms will be quick-moving and as soon as areas begin to cloud up, the air will quickly stabilize. Strongest storms will be more likely to occur across far northeastern WY and into the Dakotas later this evening after several hours of afternoon heating, however, eastern portions of our CWA (Natrona and Johnson Counties) do have a chance of seeing development of stronger storms this afternoon. SPC has gone with a marginal severe risk in line with this. Gusty winds and small hail will be the main threats with stronger storms. The low will rotate out of the region overnight and most of the CWA will be dry by the time you are headed to work Tuesday morning. Some lingering showers may be possible across the eastern most zones, however, these will end by early afternoon. Skies will begin to clear and drier air will move in behind it. The much cooler air that is in place today (temps will be 10 to 15 degrees below normal today) will also start to be replaced by a warmer airmass as a longwave ridge begins to build. Tuesday's temps will still be 5 to 10 degrees below normal, but by Wednesday, temperatures will warm back up to seasonal normals. The seasonal temps will remain in place through the weekend, despite the building ridge. Ridging will keep overall dry and sunny weather over the region, though an occasional brief afternoon shower does not look out of the question as a few disturbances do exist in the flow. The next more significant chances of showers and/or thunderstorms do not look to be until next weekend when models hint at a shortwave dropping out of Canada. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 442 PM MDT Mon Sep 4 2023 A moisture-laden system currently rotating across the state will keep showers and thunderstorms for all terminals through the afternoon. MVFR to even IFR ceilings are expected with this activity at times. Otherwise, gusty winds are expected at several terminals, especially at KCPR which could see gusts to 40 knots this afternoon. Precipitation will begin to decrease first across areas west of the Divide by around 06Z, then across remaining terminals through 12Z. Fog is a possibility at KJAC Tuesday morning if clouds clear out, but low confidence precludes mentioning more than a low scattered deck for now. A few mountain showers in the vicinity of KJAC are expected after 20Z Tuesday, with mostly clear skies expected elsewhere. Frequent mountain obscuration is likely through tonight. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued at 1258 PM MDT Mon Sep 4 2023 As a Pacific low pressure system moves across the region today, most areas will see off and on periods of showers and occasional thunderstorms through the day, with rain and storms ending overnight. Stronger thunderstorms are possible this afternoon across zones 280/281. Small hail and gusty winds are the main threats with these stronger storms. Other than maybe a lingering shower Tuesday morning across eastern zones, Tuesday will be slightly warmer and drier. By Wednesday, a ridge builds and seasonal temps and dry weather will prevail through the rest of the week. Though minimum RHs across 279/280/289 will approach critical late in the week, light winds are expected, which will keep fire weather concerns to a minimum through the next 7 days. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && DISCUSSION...Hensley AVIATION...Myers FIRE WEATHER...Hensley
Hazardous Weather Outlook National Weather Service Riverton WY 227 AM MDT Mon Sep 4 2023 WYZ004>007-011-016>020-022-026>030-051300- North Big Horn Basin-Southwest Big Horn Basin- Southeast Big Horn Basin-Owl Creek and Bridger Mountains- Southeast Johnson County-Upper Wind River Basin-Wind River Basin- Lander Foothills-Green Mountains and Rattlesnake Range- Natrona County Lower Elevations-Casper Mountain- Upper Green River Basin-South Lincoln County- Rock Springs and Green River-Flaming Gorge-East Sweetwater County- 227 AM MDT Mon Sep 4 2023 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and tonight. Widespread showers and thunderstorms. Moderate to heavy rainfall is possible. A few stronger storms are possible around Johnson County. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday. Tuesday...Isolated showers and thunderstorms, with cool temperatures. Wednesday through Sunday...Mainly dry, with slight afternoon chances of showers and thunderstorms over higher terrain. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to Standard Operating Procedures.
The Riverton/Lander radar (KRIW), 127 km away, was scanning. The volume nearest the first event began at 02:40 UTC.
2 flash flood. Local standard time, as filed.
Reports to Sheriffs office of minor flooding across some of the dirt roads in southwestern portions of Johnson county.
Deputies reported some minor flooding over roadways in southeast portions of Washakie county.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2023Every state in 2023This day in weather historyWarnings in Wyoming, September 2023
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: 1142701, 1142702.