textproduct: Western and Central Wyoming

KEY MESSAGES

- Today will be the hottest day of the week with isolated thunderstorms mainly in southern and western Wyoming, with a 15-20% chance of a couple isolated storms pushing down the Wind River Basin after 4-5PM.

- Monsoonal moisture increases on Saturday with more numerous showers and thunderstorms, some may produce heavy rain and local flooding. Marginally severe weather looking more possible in the afternoon across Natrona/Johnson Counties, with damaging winds and small hail the main threats at this time.

- Lingering moisture will bring a daily chance of showers and thunderstorms into early next week, with temperatures more seasonable for this time of year pushing towards the end of August.

UPDATE

Issued at 1245 PM MDT Fri Aug 21 2026

Hottest of the afternoon still in progress, bumped up a few spots a couple degrees east of the Divide as it has already warmed past the previous noon forecast. Otherwise, much of the forecast still intact and on schedule. Monsoonal moisture visible on IR pumping up from the south, but more limited and a bit west than it was yesterday currently. This should extend into western parts of the state by mid-afternoon that will spark thunderstorm activity. HiRes models haven't been very reliable the past couple of days missing the morning showers across Johnson County and a bit late on development thus far. PoPs will be adjusted as needed once things kick off, but main concerns (if you want to call it that) is gusty outflows over 40 mph. A rogue storm or two could push down the Wind River Basin between 4-6PM with some of the models picking up on this still, although chances low around 15-20%. Even some isolated storms developing currently near South Pass with slow eastward movement due to weaker steering flow aloft. Not much should come from it but a mere breeze.

Otherwise, weekend precipitation looks on track for now but maybe a bit stronger severe weather wise across eastern counties. HiRes models do indicate better instability and stronger multicellular formations across Natrona to Johnson County during the afternoon and early evening hours. Even though SPC pulled the Marginal further east, this should be back in place east of the Bighorns by Saturday morning if not having a slight closer to the eastern parts of the CWA. Stay tuned for updates overnight into early Saturday morning.

DISCUSSION

Issued at 241 AM MDT Fri Aug 21 2026

We are now returning to my favorite (note sarcasm, this is by far the thing I hate the most about the weather around here) thing to forecast, the monsoon. And, the models are off to a bad start. We have some showers across northern Wyoming that none of the models had. So, they are 0 for 1 right now. And this isn't baseball where batting .300 makes you an all star.

Other than these light showers, things are quiet right now, and it should stay this way through the morning. However, we will see another round of isolated showers and thunderstorms today, probably in similar positions to yesterday. Coverage should remain less than 1 out of 3 for all areas though, as some warm air aloft should prevent widespread convection. This is mainly across the western half of the state. Dewpoint depressions remain fairly high and we still have inverted-V soundings, so this will likely be another day of strong wind gusts from any shower or thunderstorm with only light rain. There could be a stray shower further east, but the chance is less than 15 percent so we left the forecast dry. But, let's just say my confidence is not high with how poorly the models handle the monsoon with weak forcing. Some showers will linger through the night across the west as well, but the core of the moisture likely holds off until Saturday.

The other story today will be the heat, as temperatures will peak today as 700 millibar temperatures climb to around 20 Celsius. All areas below 6500 feet have at least a 4 out of 5 chance of high temperatures over 90 this afternoon. Areas below 5500 feet have at least a 1 in 2 chance of highs over 95. As for triple digits, it is smaller. However, temperatures have been running higher than guidance so I give it around a 1 in 3 chance in places like Greybull and Worland. The warmest temperatures would be east of the Divide with the lower elevations and more sunshine.

The monsoon returns with a vengeance on Saturday. Precipitable water values climb to around 150 to 200 percent of normal across most of the area. We will also have a shortwave moving across the area to bring some additional forcing. The result will be a chance for all areas to see a shower or thunderstorm, but most guidance favors western Wyoming. There is a Marginal risk of Excessive Rainfall across areas west of the Divide as I write this around 1:15 am. This could be expanded east, we will know around 2:30 am. There is some steering flow, especially at 500 millibars, so this should mitigate the flooding threat somewhat but it will be there. Most convection should end by midnight, but some showers will continue after midnight. Confidence on placement of showers Saturday night is very low, as this is where the models have struggled the most with the monsoon. The best chance will be in western Wyoming, but exactly where is still somewhat uncertain. As for temperatures, they will remain very warm east of the Divide where there will be some morning sunshine, but about 5 degrees cooler than Friday. Areas west of the Divide should see a greater drop, with more cloud cover and a greater coverage of showers. And, some storms could become strong. We do have a Marginal risk of severe weather east of the Bighorn Range. It may be a bit overdone, since there is only weak shear. The main threat would be strong wind gusts, like most showers or storms in Wyoming.

Monsoonal moisture then looks to stick around into the first part of next week. This will lead to daily chances of mainly diurnal showers and thunderstorms each day. Guidance has poor consistency with the details though. So, it is still difficult to nail down details on where the most numerous convection will be each day and timing of the storms. So, we kept things broad brushed for the most part. Temperatures will decrease somewhat with more cloud cover. However, with the mean ridge remaining in place, above normal temperatures should continue into next week. There are some indications that drier air may try to move in toward the end of next week, bringing a return to drier conditions for a few days through the weekend.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 354 PM MDT Fri Aug 21 2026

Virga, showers, and thunderstorms will be present across western and central Wyoming, including in the area of or at KJAC, KRIW, KRKS, KLND, KPNA, and KBPI, through 06Z at the latest. Gusty outflows of 35 to 45kts are possible in and around any virga, showers, and thunderstorms. KCOD, KCPR, and KWRL have less than a 20 percent chance during this timeframe. Between 07Z and 18Z, model guidance is showing a cluster of showers moving across western and central Wyoming. Rain shower chances exists at all terminals except KRKS during this timeframe. However, confidence in exact timing and impacts is rather low so continued with the PROB30 groups. More widespread showers and thunderstorms are forecast Saturday afternoon at all terminals with the primary hazards being lightning, breezy winds, and heavy rainfall. Brief periods of MVFR flight conditions cannot be ruled out in any shower or thunderstorm this TAF period. Otherwise, VFR flight conditions are anticipated through 00Z Sunday with winds remaining 12kts or less. KCPR could see occasional gusts around 20kts Saturday afternoon outside of showers or thunderstorms.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts.

RIW WATCHES/WARNINGS/ADVISORIES

None.