textproduct: Western and Central Wyoming

KEY MESSAGES

- Increased moisture brings an increased chance of showers and storms across the area today. Some storms could have heavy rain, strong wind or hail.

- The chance of showers and storms lingers through much of the week, although today looks the most active.

- Temperatures will gradually decrease to near normal levels for much of the week.

DISCUSSION

Issued at 141 AM MDT Sat Aug 22 2026

The monsoon has once again arrived across the area, I believe this is the third or fourth appearance so far. So far, it has behaved. We do have some light showers across mainly the western half of the area but nothing heavy so far. The main concern in the forecast will be the potential for heavy rain and possible strong thunderstorms today. Right now, in this meteorologists opinion, potential is mixed for if we will see widespread of either.

In regards to heavy rain, the main factor will be the amount of moisture in the atmosphere. Dew points have been slow to rise so far, but they should this afternoon, rising into the 50s in portions of the Big Horn Basin as well as Johnson and Natrona Counties. Precipitable water values should also be rather high. The GFS, for example, has precipitable water values about 175 to 200 percent of normal. This had been overdone by around 15 to 20 percent the past few events though. Even with this, precipitable water values over 150 percent of normal look fairly likely. There will also be some forcing in the form of a shortwave moving in from the west. This should enhance chances for all areas, so everyone will have a chance. The best chance still looks to be in northwestern Wyoming, generally a 1 in 2 chance, and there is decent model agreement in this. As for severe thunderstorms, there looks to be a decent amount of instability. This is especially so in the favored portions of our area, mainly East of the Bighorn Range in Johnson and Natrona Counties. Lifted indices fall as low as minus 4 to minus 5 with close to 1500 J/Kg of surface CAPE here. There is also a decent amount of directional shear in this area. This, plus the ample moisture, may increase the hail threat in this area. We do have a marginal risk of severe weather in portions of Johnson and Natrona Counties, and this could be expanded westward with the 13Z update. All areas could have a thunderstorm but with less instability and shear most storms would be sub severe.

However, there are some factors against possible hazards. In regards to flooding and thunderstorms, it is cloud cover. Much of the area, especially western Wyoming, is covered by clouds as I write this around midnight. If it stays mostly cloudy for much of the day, this could limit instability somewhat and decrease the chance of stronger thunderstorms. There will likely be some areas that will be sunnier though, especially further east. And there is one more mitigating factor for heavy rain and possible flooding. With the shortwave moving through, there will be some steering flow today. It won't be strong, but most areas will have 15 to 25 knots at 500 millibars which should keep storms moving somewhat. We continue to have a marginal risk of excessive rainfall, and persistent storms could still cause flooding. But it may decrease the chance somewhat. The shortwave should clear the area by midnight, but some showers will likely linger through the night. As for temperatures, more cloud cover should lower them around 5 to 8 degrees, but it will still be above normal for almost all areas.

The forecast from Sunday through much of next week looks fairly similar. We will remain on the edge of the ridge. Some monsoonal moisture will stick around, with precipitable water values remaining above average through much of the period. However, they do look lower than today, so the chance of convection should be somewhat lower. Most guidance favors western Wyoming at this time. However, given the poor performance of the models in regards to timing and placement of the showers and storms, confidence in the details remains low. Temperatures will also cool off somewhat, back down to near to just slightly above normal levels for late August. There continue to be some indications of drier air moving in for the end of next week or next weekend, possibly shutting off the monsoon and bring a period of warm, but not really hot weather for the end of August.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 414 AM MDT Sat Aug 22 2026

Rain showers will spread into western Wyoming during the morning hours, with PROB30 groups at all terminals West of the Continental Divide (KJAC, KPNA, KBPI AND KRKS). These showers then spread East of the Divide after 13 to 14Z and move eastward. Additional showers are possible through the day with thunderstorms possible mainly after 19Z although exact timing of the periods of showers and thunderstorms is uncertain. Most storms should end after 04Z tonight although some showers will linger through at least 08Z tonight and some even until sunrise on Sunday. The main threat from most storms would be heavy rain that could bring brief MVFR conditions with the heavier periods of rain and wind gusts over 35 knots. Stronger storms are possible in Johnson and Natrona Counties, including KCPR where storms may become strong to severe. Locally stronger storms are also possible in northwestern Wyoming, mainly in vicinity of KJAC.

RIW WATCHES/WARNINGS/ADVISORIES

None.