textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms are possible (15 to 40 percent chances) this afternoon, primarily east of the Continental Divide.

- Showers and thunderstorms again possible Monday. A few could be strong, especially around Johnson nd Natrona Counties. Large hail and gusty outflow winds are the primary hazards.

- A warming trend occurs through rest of the week, with daily chances of showers and thunderstorms for some locations.

UPDATE

Issued at 1247 PM MDT Sun Jun 21 2026

No notable changes made to the key messages for today's forecast with this update. Have included the latest hi-res guidance for placement of precipitation. Generally 15 to 40 percent chances occur this afternoon and evening for locations around and east of the Divide. Rain chances linger overnight for the Bighorn Basin and Johnson County. Currently, radar showers a mix of showers, and a few thunderstorms, developing over the mountains. Some are moving off into the surrounding basins, but are not lasting long.

DISCUSSION

Issued at 224 AM MDT Sun Jun 21 2026

We are now entering my favorite season the year, summer, which arrives at the exact time I am sending this discussion. I was feeling rather nostalgic before I came into work and put on the song the many fellow Gen Xers that are Hip Hop fans is the perfect summer song. And of course I am referring to the classic "Summertime" by DJ Jazzy Jeff and the Fresh Prince, now 35 years old (I'm getting old). And the main weather concern over the next few days will be the typical summertime weather concern, thunderstorms, with a bit of fire weather mixed in.

It was quite active yesterday, with many reports of high wind gusts and even a landspout tornado. One thing we didn't see a lot of, unfortunately, was rain, which we need badly. Things are fairly quiet this morning, although we still have a few slight showers across the northern third of the area. Today will see more showers and thunderstorms as another weak shortwave moves through the area. However, they should not be as numerous or as strong as yesterday. For one, instability parameters are less, with CAPE anywhere from 20 to 30 percent less than yesterday. Also, there is less shear and most jet energy will remain north and south of the Cowboy State. We could see a couple of stronger storms, but not nearly as widespread as yesterday. Precipitable water values East of the Divide remain above normal though, approaching the 90th percentile in some locations (nearly 0.75 inches, around 150 percent of normal). There is still steering flow though, and this should mitigate the chance of flooding. Most storms should be East of the Divide today, with less than 20 percent coverage in the west and south. And like tonight, a few showers could linger into the night.

I still have concerns about Monday. Another shortwave will be moving in from the west. However, there are a couple of other factors that could make for some stronger storms today. For one, an almost 100 knot jet will be passing close to the Wyoming-Montana border in the afternoon, enhancing upper level forcing and upper level divergence. There will also be a push of rather dry air moving in from the west, with precipitable water values in western Wyoming falling to as low as 0.25 inches. At the same time, weak low pressure passing to the south will bring easterly flow into eastern portions of the area, enhancing upslope flow over Natrona and especially Johnson Counties. Instability parameters look rather impressive as well, with much of northern Wyoming having over 500 J/Kg of CAPE in the afternoon with as much as 1500 J/Kg in Johnson County. Lifted indices fall as low as minus 5 as well. With Storm Relative Helicity rising to over 200, we could see rotating storms. How far west storms can get still looks dependent on how far east the dry air can get, as the pseudo dryline could be the place where storms will fire, as well as off the Bighorns with low level upslope flow. The Storm Prediction Center already has a slight risk into eastern Johnson County, with a marginal generally along and east of Interstate 25. This pattern generally favors large hail, but all hazards are possible. This is still far from a certainty but needs to be watched. If the drier solution wins out, fire weather will be the concern. At this time, relative humidity should remain above critical levels in areas where fuels are critical though. Lower humidity will be found across the south though and with a gusty wind from the proximity of the jet, elevated fire weather looks like at least a 2 out of 3 possibility Monday afternoon, especially in central and southern Wyoming.

Coverage of storms should decrease somewhat on Tuesday as ridging moves across the area and the deeper moisture gets pushed further east out of the area. The best chance of storms this day would again be in the eastern counties. The models then diverge somewhat. Most show a warming trend starting Wednesday with many areas possibly seeing 90s by Friday. A few shortwaves will move through the area as well with chances of showers and storms, but guidance has different timing of the waves so confidence on timing remains low.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1118 AM MDT Sun Jun 21 2026

Conditions remain VFR through midday Monday with the possible exception of brief MVFR with a thunderstorm east of the Continental Divide. Showers and thunderstorms erupt over the Absaroka, Bighorn, and Wind River Mountains between 18Z-20Z/Sunday before moving east-southeast into the lower elevations through the afternoon. The convective wind hazard will be much less Sunday than it was Saturday. Potential outflow gusts of 25-35kts occur with stronger convection. Gusty westerly surface wind over the southwest terminals fades around 02Z/Monday before returning between 16Z-18Z/Monday. Light showers linger across the northwest quadrant of Wyoming overnight.

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.