textproduct: Western and Central Wyoming

KEY MESSAGES

- Deeper moisture moves over the area today with an increased chance of showers and thunderstorms. The most numerous showers will be in western and southern Wyoming.

- Showers will remain likely through at least Saturday ans possibly into Sunday, although the focus of the most numerous and heaviest showers will shift to northern Wyoming starting on Saturday. The most numerous storms should be on Friday.

- Temperatures will be cooler today with more cloud cover. Friday will be a warmer day before cooler temperatures move in Saturday and especially on Sunday.

DISCUSSION

Issued at 206 AM MDT Thu Sep 17 2026

The story today and for the next few days, as it has been for seemingly forever (actually the past two and a half months) is monsoonal moisture. Satellite imagery shows an area of moisture advancing northward with showers and embedded thunder moving toward Sweetwater County as I write this around 1 am. This area of showers will spread northward tonight. Meanwhile, most of northern Wyoming should remain dry tonight.

This deeper moisture will overspread the entire area today, with precipitable water levels rising to 150 to 175 percent of normal. Just about all areas will have at least a 1 in 6 chance of a shower or thunderstorm today. The best chances though will be across western Wyoming where there will be more forcing, and also across areas East of the Big Horns, with the least chance in the center, especially the central basins. There continues to be one factor that makes my think coverage of thunderstorms may be less though. There will be a decent amount of cloud cover around the area, and this could limit surface instability somewhat. And it is we are now almost to the autumnal equinox, not the middle of summer. So, the sun is weaker and daytime heating will be less. This may also limit the chance of heavy rain and any flooding potential. Steering flow is less than yesterday, but we still have 10 to 20 knots and this should be enough to keep storms moving somewhat. However, with the high precipitable waters there could still be isolated minor flooding, especially west of the Continental Divide. Most showers should end after sunset, but a few showers will likely linger through the night. And it will probably be more than expected, the models have been horrible this season with nocturnal monsoonal convection.

An upper level low that has been helping to funnel the moisture into the area will finally begin to move eastward today, drifting toward Idaho by tomorrow. This may be the most active day for convection of the period, especially thunderstorms. Most guidance is showing a clearer sky in the morning, allowing for more daytime heating and surface instability. Instability parameters are decent for this time of year, with CAPE approaching 700 J/Kg and lifted indices as low as minus 4. With the approaching upper level flow, there will also be some decent directional shear, especially in western Wyoming. The result will be a chance for some stronger thunderstorms, again especially in western Wyoming where there is a marginal risk of severe weather. The main threat would be hail given the low wet bulb zero levels. With more sun and southwest flow continuing, this will also be a warmer day, especially East of the Divide where warmer spots may get close to 80 for a high. Showers will continue into the night as well. With the added instability and a shortwave rotating around the low, if there are any flooding problems, this would be the time for it.

The latest guidance continues to slow the progression of the low, only slowly moving it over Wyoming by Saturday. On this day, the best chance of showers and storms would be across northern Wyoming. Most guidance is in decent agreement on this. The deepest moisture would be over Montana, but most areas will have a chance. There may also be a dry slot that moves in portions of the area, but exactly where is still in question. The low will slowly cross the state then on Sunday, but will still be close enough for some wrap around moisture to bring more showers, mainly to northern and eastern portions of the area. This also looks like a cooler day as flow turns to the northwest, with temperatures anywhere from 5 to 10 degrees cooler than on Saturday. Following this, there is more divergence in the models with some bringing another system across the area early next week with another chance of showers and others suppressing it to the south, keeping the area largely rain free. All guidance shows south to southwesterly flow returning though, so temperatures look to be back above normal as we head into the start of autumn, which arrives on Tuesday, September 22nd at 6:04 pm.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 452 AM MDT Thu Sep 17 2026

Moisture will spread across the area today with all TAF sites having a chance of a shower or thunderstorm. The chance is the lowest in vicinity of KWRL (20 percent), so we left an mention of showers out of the forecast for now. An area of showers will move out of Colorado and move northward through the morning, impacting mainly KRKS, KCPR, KRIW and KLND through around 18Z. Other showers will impact KJAC until around 16Z this morning. Additional showers will be possible later this afternoon and evening and possibly again late tonight although there is still high uncertainty with timing and position of the afternoon and nighttime showers. Wind should generally remain less than 11 knots. The exception will be in and around any thunderstorms or even showers where a gust to 35 to 40 knots can't be ruled out. Conditions should be largely VFR, although occasional MVFR conditions are possible around showers and thunderstorms. Mountains will be obscured at times around showers at times, especially above 9000 feet.

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.