textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms have become more widely scattered this evening, with most locations having a 50% to 80% chance of measurable rainfall. Convection has diminished some and will continue through sunset from west to east with the westerly steering flow.

- Moderate to heavy rainfall and localized flash flooding are the primary threats with showers and thunderstorms the remainder of the evening hours through midnight, especially in steep terrain east of the Winds through the Rattlesnakes and to Casper Mountain.

- Scattered shower and thunderstorm potential exists Friday through Sunday with overall cooler temperatures.

UPDATE

Issued at 135 PM MDT Thu Aug 13 2026

The forecast for the afternoon and evening remains on track with widespread showers and thunderstorms across the state. Around 11AM this morning, a slow moving thunderstorm tracked over Casper and dropped 0.5" to 1.5" of rain in less than an hour which prompted a Flood Advisory to be issued. This is expected to be the slowest moving storm/storms of the day due to that convection being focused along a weak stationary boundary and overall slow storm motion (model soundings signaled 14 to 15kts of cloud layer wind/motion). However, if the Casper area sees another thunderstorm that produces moderate to heavy rain rates today, the ground may not be able to handle too much additional rain. This will need to be monitored throughout the afternoon and evening.

As of 1PM local time, showers and a few embedded thunderstorms have started to push into southwestern and western Wyoming. These showers and thunderstorms are rooted to a vorticity maximum/shortwave axis and a surface frontal boundary. Shower and thunderstorm coverage is expected to increase as the system progresses to the northeast this afternoon, with any given location having a 60% to 90% chance of rain by 9PM. The 18Z balloon sounding in Riverton shows 16kts of cloud layer wind/motion, a deep warm cloud layer, and precipitable water amounts of 1.01" (well above the 90th percentile). This suggests that the atmosphere is primed for shower and thunderstorm activity. Storms are expected to move somewhat faster compared to what Casper saw this morning due to the system being more progressive. It is important to note that model soundings show an increase in the cloud layer wind/motion this afternoon as well. That being said, instances of moderate to heavy rainfall rates are possible as this system traverses across the state, so isolated flash flooding is possible. The areas of most concern would be urban areas, poor drainage areas, and any steep terrain. It is important to stay vigilant and not pass through any roads or drainage areas that may have minor flooding. A bulk of the shower and thunderstorm activity is forecast to be out of the forecast area/Johnson County by 2AM Friday. Lingering light rain showers remain possible (up to 30% chance) through the overnight hours across northwestern Wyoming.

DISCUSSION

Issued at 152 AM MDT Thu Aug 13 2026

Chances for precipitation will be widespread across the CWA today. The threat for heavy rain and localized flash flooding is still in the forecast, with areas east of the Divide having the better chances. Precipitable water values will be higher here, ranging between 1 and 1.3". WPC Excessive Rainfall Outlook continues to have a Slight risk in the forecast for Johnson County and portions of Natrona County (mainly along and east of I-25). Showers will move northward over Lincoln County and the Green River Basin after sunrise and spread over western portions through the rest of the morning. Scattered showers and thunderstorms will develop across the rest of the CWA between 18Z and 20Z, moving toward the northeast through the afternoon. Convection will begin to decrease by 00Z, ending from southwest to northeast through the evening. Light showers will still be over northern portions by 06Z and linger through the rest of the night. Thunderstorms will be possible across the area this afternoon, with portions of Lincoln and Sublette counties having the best chances as a shortwave from NV/UT moves over this region.

Convection will not be as extensive Friday, being more scattered through the day along and north of a Afton to Midwest line. Heavy rain and localized flash flooding will continue to be the main threat from these storms as PWAT values remain abnormally high, ranging between 0.7" to 1.1". The hazy/smoky skies looks to no longer be an issue by Friday, as the southwest flow aloft cuts off the source region and the atmosphere becomes more mixed.

The cooler and wetter pattern will continue through the weekend, with showers and thunderstorms possible each day. The threat for heavy rain will not be as strong by Sunday, as a bit more drier air filters into the region. More drier conditions look to return by Monday, but temperatures are trending to remain seasonal for the early part of next week.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

Issued at 1030 PM MDT Thu Aug 13 2026

All TAF sites should remain VFR for this cycle as rain has pushed north and east as it dissipates through the rest of the overnight. Low ceilings at JAC/COD/CPR through Friday morning but VFR keeping a small chances ~10% for some periodic MVFR ceilings just below the 3kft mark. A light rain still lingers around JAC through 12Z Friday morning but limited in impact. Otherwise, light winds around 10kts through the period with a brief light gust at CPR during peak heating Friday after 16Z. Another round of thunderstorms expected but much more isolated, with only BPI/PNA/JAC west of the Divide affected. Gusty outflows the main threat but otherwise, brief in nature between 20-01Z. Scattered mid level clouds there after with light winds into the overnight hours Friday night into Saturday morning.

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