textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms are expected throughout the day today, with the greatest coverage across western and southern Wyoming. Moderate to heavy rainfall could result in localized instances of flooding.

- An active, rainy pattern continues Friday through late Saturday. The most active day is forecast to be Friday where a few strong storms are possible along with instances of flooding.

- A Flash Flood Watch has been issued for the Absarokas, the Cody Foothills, the Owl Creek and Bridger Mountains, the Bighorn Mountains, and Johnson County from 12PM Friday through 6PM Sunday.

- Fall-like temperatures are forecast through the weekend with plenty of clouds. Anticipate highs ranging from the middle 60s to upper 70s.

UPDATE

Issued at 1240 PM MDT Thu Sep 17 2026

Noon radar imagery shows an area of showers and thunderstorms across western Wyoming. This convection is supported by a shortwave disturbance and will progress to the north and east through the afternoon. However, with the dense morning cloud cover keeping the atmosphere stable across central Wyoming, thunderstorm coverage is expected to decrease. Another round of showers and thunderstorms is forecast to develop across far southwest Wyoming late this afternoon (3-5 PM MDT time range). A few strong thunderstorms are possible before sunset due to increased instability (500 to 700 J/kg of BLCAPE) and moderately unstable mid-level lapse rates. The threats with any strong thunderstorms will be hail up to 1 inch, gusty 40 mph winds, and moderate to heavy rainfall that is capable of isolated flooding. The strong thunderstorm threat is expected to be relatively short-lived and come to an end by sunset. As mentioned in the morning discussion, rain showers could linger through the overnight hours.

The active and rainy pattern continues Friday through Sunday as an upper level low slowly treks across Wyoming. Precipitable water amounts with this system have consistently remained above the 98th percentile of climatology across northern Wyoming. The track of the low is directly across Wyoming, west to east, and puts the highest rainfall amounts across southern and central Montana. However, the Absarokas, the Cody Foothills, Owl Creeks, Bighorns, and Johnson County are forecast to receive anywhere from 0.75" to 1.5" of rain from Friday afternoon through Sunday afternoon. Thus, a Flash Flood Watch has been issued for the aforementioned areas Friday 12PM through Sunday at 6PM. There will be breaks between thunderstorms and it will not rain for two days straight, but the thunderstorms are expected to have moderate to heavy rainfall rates. The areas of most concern are steep terrain and poor drainage areas. It is important to note that the totals noted above represent the mean. If a thunderstorm has higher rainfall rates, these values may be underdone. Additionally, if the upper low deviates from its forecasted path these values may be unrepresentative. For example, if the low tracks father to the south, anticipate higher totals.

DISCUSSION

Issued at 206 AM MDT Thu Sep 17 2026

The story today (and for the next few days, as it has been for the past two and a half months) is monsoonal moisture. Satellite imagery shows an area of moisture advancing northward with showers and embedded thunderstorms 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 will be across western Wyoming where there will be more forcing, and also across areas east of the Bighorns, with the least chance in the center, especially the central basins. There continues to be one factor that makes me 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 as 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 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 /06Z TAFS THROUGH 06Z SATURDAY/

Issued at 1000 PM MDT Thu Sep 17 2026

Showers will remain possible near KJAC through 18Z. Currently, there is a line of convection over the eastern Snake River Plain moving toward the northeast. Most of this is expected to stay well to the north of the terminal, but there will be enough instability for possible showers. Confidence is still too low to include into the forecast. Showers will become widespread across the west by sunrise, spreading eastward through 20Z. Thunderstorms will become possible within the showers by this time, with an additional wave moving into the west. All terminals have PROB30 groups through about 02Z. Conditions will stay VFR, with a 30% chance for MVFR ceilings at KCPR through 13Z. Increasing chances for mountaintop obscurations over the western mountains after 18Z and will continue through the rest of the TAF period.

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

Flood Watch from Friday afternoon through Sunday afternoon for WYZ002-003-007>011.