Mid-level atmospheric moisture moved over Wyoming causing scattered high-based showers and thunderstorms to develop. With dew point depressions greater than 50 degrees, wind gusts of 50 knots or higher were reported in portions of Johnson, Natrona, Sublette, and Sweetwater Counties.
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The episode as NWS RIW wrote it up.
Mid-level atmospheric moisture moved over Wyoming causing scattered high-based showers and thunderstorms to develop. With dew point depressions greater than 50 degrees, wind gusts of 50 knots or higher were reported in portions of Johnson, Natrona, Sublette, and Sweetwater Counties.
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 Aug 13, 10:35 AM MST, 2 hours 45 minutes earlier. Its key messages:
Area Forecast Discussion National Weather Service Riverton WY 1135 AM MDT Wed Aug 13 2025 .KEY MESSAGES... - Critical fire weather conditions this afternoon due to dry and warm air, gusty afternoon winds and thunderstorm chances. Red Flag Warnings are in effect through 8PM tonight. - A thunderstorm could become strong across Johnson County, northern Natrona County, and the Bighorn Mountains. The primary threat will be gusty winds. - Elevated to critical fire weather conditions remain for Thursday, especially across southwest and central Wyoming where Red Flag Warnings will be in effect Noon to 8PM Thursday. - Increasing moisture brings daily rain chances through the weekend, with the highest chances over southwest Wyoming. && .UPDATE... Issued at 1132 AM MDT Wed Aug 13 2025 Current (17Z) radar and satellite imagery shows an area of showers and thunderstorms moving into the Upper Wind River Basin. This convection is at the nose of a 500mb wind and vorticity maximum that is associated with the upper-level high pressure axis. This convective area will shift east through the afternoon, as the upper- level axis shifts east, with additional convective initiation across higher elevations through the afternoon and early evening. Showers and thunderstorms are still expected to be scattered to widespread so exact timing and location remains uncertain. Thunderstorms will be capable of frequent lightning and gusty winds around 40-50 mph. Additionally, 17Z surface observations show higher dewpoints (upper 40s) across Johnson County, northern Natrona County, and the eastern Bighorn Basin. These dewpoints are expected to drop a few degrees with daytime heating and mixing. However, with additional moisture to work with, mean CAPE across the aforementioned areas is forecast to be around 500 J/kg. With this afternoon instability along with the upper-level wind maximum overhead, there will be support for an isolated strong to severe thunderstorm with gusts up to 60 mph. Thus, these locations are in a SPC Marginal Risk for severe weather this afternoon. Otherwise the forecast remains on track with critical fire weather conditions and Red Flag Warnings across much of the forecast area. 17Z observations already show relative humidity values below 20% across southern and central Wyoming. Relative humidity values will continue to drop with increasing winds across the area as daytime mixing increases. Nearby showers or thunderstorms could also create gusty outflows. Limited moisture across much of the area will result in low chances of wetting rain. With any thunderstorm there is a threat of new fire starts and erratic fire behavior. && .DISCUSSION... Issued at 1230 AM MDT Wed Aug 13 2025 The focus for today is on fire weather, which consists of low relative humidities (RH), gusty winds, and shower/thunderstorm chances. Starting with RH. The axis of high pressure moves over today, and with it, the warmest and driest air compared to the past few days. Afternoon minimum RHs will generally be around or under 25 percent for all locations (both higher and lower elevations) today. Most low elevations will be under 15 percent. Driest locations will be in central and southern Wyoming, with RHs down into the single digits. High temperatures today will be in the upper 80s to mid-90s, which are a bit above normal for mid-August. Next element is gusty winds. A shortwave moving through northern Wyoming, in combination with an approaching low system tightening the pressure gradient, afternoon, synoptic winds increase. Gusts 25 mph or greater should occur across much of central and southern Wyoming. In addition, the higher elevations, namely the western mountains, will see gusts over 35 mph. The least windy spots will be the Bighorn Basin and northern Johnson County. The other wind factor comes from thunderstorms chances. Showers and thunderstorms are possible (at least 15 percent) across the entire area today. Hi-res models show convection developing over the far western mountains as early as about 9am MDT, though would be more likely with typical diurnal trends starting 11am to 1pm. Chances than become very difficult to pinpoint. Each model has differing locations and timings of showers and storms through the afternoon and evening. Have blended them altogether for the forecast, and "broad-brushed" to give the entire area at least a 15 percent chance. Not everywhere will get a shower or storm, but given the widespread coverage models depict, there is at least a chance everywhere. The main hazard with the showers/storms is wind in the form of localized outflow gusts. Given dew points, gusts over 40 mph are very possible, with the HRRR producing some gusts over 50 mph. Combining the previous paragraphs' weather elements, elevated to critical fire weather is in place today. Red Flag Warnings are in effect for much of the area, where the synoptic winds, low RHs, and thunderstorms chances best align. RH recovery is fairly poor Wednesday night, especially across central and southern portions of the area, and with winds increasing again Thursday, Red Flags continue for some locations (fire zones 279, 280, 289, 300) into Thursday. On Thursday, moisture increase for the area as the high shifts east and advects moisture from the south. RHs begin to improve a little, especially across western Wyoming where moisture gets to first. This also brings the chances of showers and thunderstorms, primarily for southwest Wyoming, though there are still some chances into central and northern Wyoming. The monsoonal moisture push keeps daily rain chances into the weekend. Temperatures remain very similar through the rest of the week as well. The upper-level pattern remains messy through the weekend, with long range models showing the ridge building back into the start of next week. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1119 AM MDT Wed Aug 13 2025 Showers and thunderstorms are currently moving across northern Wyoming, but have been posing minimal and only brief interruptions at local airports. Expect more to develop this afternoon during peak heating, with minimal impacts. With the overall small size of showers, have kept to prob30 notation in TAFs, but have expanded these to include KBPI/KPNA for the afternoon. Some showers may also return near KJAC this afternoon, but chances remain low, around 10%. && .FIRE WEATHER... Issued at 1230 AM MDT Wed Aug 13 2025 Winds increase late morning today, with critical RHs in the afternoon. Therefore, Red Flag Warnings are in effect across portions of southwest and central WY today. Across Fire Zones 277, 278, 279, 280, 283, 289, and 300, RHs as low as 8 to 12 percent, coupled with west winds 10 to 15 mph gusting 25 to 30 mph in the afternoon, will result in critical fire weather conditions. Notably for zones 279, 280, 289, and 300, similar conditions will return Thursday afternoon, so Red Flag Warnings for those zones are in effect through 8 PM Thursday. Conditions are expected to improve Wednesday night with winds decreasing to mostly light and variable, however, overnight recoveries will be fairly poor, with RHs only recovering to around 30 to 45 percent. Across mountain zones in western and northern WY, including zones 414, 415, and 416 along with 286 and 288, RHs will be much more marginal, generally 14 to 16 percent in the valleys and around 20 percent across higher elevations. Despite this, winds gusting 30 to 40 mph across higher terrain and the thunderstorm threat (20 to 40 percent chances) across those regions this afternoon necessitated the issuance of Red Flag Warnings for those zones. In particular, there is increased concern for new fire starts and erratic fire behavior with any new starts, given the thunderstorm chances. Though there should be some rain with any thunderstorms, precipitation will not be enough to extinguish any lightning starts that may occur, as wetting rain chances remain near zero. Thursday, the thunderstorm threat shifts to southwest WY, but storms should have more notable moisture with them on Thursday as monsoonal flow begins to become more dominant. && .RIW WATCHES/WARNINGS/ADVISORIES... Red Flag Warning until 8 PM MDT this evening for WYZ277-278- 283-286-288-414>416. Red Flag Warning until 8 PM MDT Thursday for WYZ279-280-289- 300. && UPDATE...Gerhardt DISCUSSION...Wittmann AVIATION...Straub FIRE WEATHER...Hensley/Wittmann
Hazardous Weather Outlook National Weather Service Riverton WY 218 AM MDT Wed Aug 13 2025 WYZ001>020-022>030-141300- Yellowstone National Park-Absaroka Mountains-Cody Foothills- North Bighorn Basin-Southwest Bighorn Basin-Southeast Bighorn 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- 218 AM MDT Wed Aug 13 2025 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and tonight. Critical fire weather for portions of the area due to warm temperatures, low relative humidities, gusty winds, and thunderstorm chances. Please see Red Flag Warnings for details. Elevated fire weather possible many other locations. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday. Thursday...Elevated to critical fire weather, especially across southern Wyoming. Thursday through Sunday...A chance of afternoon and evening thunderstorms each day, with Friday looking to have the greatest coverage. .SPOTTER INFORMATION STATEMENT... Weather spotters are encouraged to report significant weather conditions according to Standard Operating Procedures.
The Riverton/Lander radar (KRIW), 174 km away, was scanning. The volume nearest the first event began at 20:19 UTC.
6 thunderstorm wind. Local standard time, as filed.
Wind gusts of 58 mph occurred at Smokey Gap along Interstate 25 near Midwest.
Wind gusts as high as 65 mph occurred at sensors near I-25 in southern Johnson County.
Wind gusts as high as 65 mph occurred at sensors near I-25 in northern Natrona County.
Wind gusts as high as 65 mph occurred along US 20 26 in Natrona.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2025Every state in 2025This day in weather historyWarnings in Wyoming, August 2025
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: 1292290, 1292293, 1292294, 1292296, 1292298, 1292300.