textproduct: Western and Central Wyoming

KEY MESSAGES

- A weather system will bring some showers and thunderstorms to northern Wyoming today. Some storms could be strong to severe with large hail and strong wind, with the highest chance in Johnson County.

- Low humidity and a gusty wind will bring elevated fire weather to much of central and southern Wyoming this afternoon.

- Chances of showers and thunderstorms continue through much of the week, with another possibility of strong to possible severe thunderstorms on Wednesday.

UPDATE

Issued at 1232 PM MDT Mon Jun 22 2026

The latest models keep conditions favorable for strong storms from around a Worland-Powell line eastwards through Johnson County. The best conditions for stronger storms will be over northern Johnson County. Large hail continues to be the main hazard, though non-zero chances of strong wind and tornadoes remains given the favorable conditions in place. Peak timing of this looks to be between noon and about 7pm. Current observations have a few showers/storms starting to develop off the Bighorn Mountains.

DISCUSSION

Issued at 243 AM MDT Mon Jun 22 2026

It is now June 22nd, and it is a sad day for summer lovers like myself. It is now when days begin getting shorter. But enough of this, we do have things to talk about today, as we will continue to have the chance of convection, and possibly a few stronger ones.

We have been talking about today for a couple of days now, and we continue to have some uncertainty. The main uncertainty is how far west will the threat of stronger storms move. The slight risk area has moved little at this point, covering mainly far eastern Johnson County. The Marginal Risk as shifted a bit further west, right now mainly covering an area East of a Powell to Greybull to Kaycee to Casper line. This makes sense. A cold front will be moving through the area, and areas further west will likely see more stable and air move in behind it. The other factor will be moisture. Precipitable water values will drop behind the front, leading to little to no convection across the western and southern two thirds of the area. Instability parameters look impressive across northern areas East of the Divide though, with CAPE values as high as 1500 J/Kg and lifted indices to minus 4. The uncertainty is that some of the models, both of the synoptic and high resolution varieties, have little coverage in our area. In addition, the jet core, which was previously predicted to be over the Wyoming-Montana border, has shifted further north. For now, we have elected to largely go with the Storm Prediction Outlook, putting convection in this afternoon across the Bighorn Basin to northern Natrona County and northward. We have kept chances generally less than 1 out of 4 in any particular location though given the uncertainty. Further south and west, the atmosphere may be too dry. A high based shower storm or virga shower can't be ruled out, but with a chance of less than 1 out of 8 we kept them out of the forecast for now.

Now for the threats. For areas further to the east, the main threat looks to be large hail. Lapse rates remain rather impressive, approaching 9 C/Km in the late morning and early afternoon. Shear values of over 50 knots over the 0-6 km layer are sufficient for this as well. The main threat for this would be over Johnson County, where deeper moisture will be present and low level upslope southeasterly flow should aid in convective initiation, with diminishing chances further west and south as you move into the drier air. As for strong wind gusts, this is possible with any shower or thunderstorm. This includes areas further west and south. Many model soundings further east show inverted Vs with steep lapse rates. Any convection here would be higher based with little rain. Dew point depressions do approach 50 degrees in places like Riverton so in the unlikely chance that convection can form, there could be a downburst when the clouds collapse. And, as for tornadoes, this is by far the least likely. There is a 2 percent area in Eastern Johnson County where cloud bases are lowest and shear is the highest. Chances are not zero, but very small. And there is one more thing. Further south, humidity will fall into the teens and with a gusty wind elevated fire weather is likely. This will be in areas where fuels are not critical, so no Red Flag Warnings. However, any fire starts could spread rapidly.

Most strong convection should end by sunset, but like tonight, a few showers and storms will likely linger through the night. Tuesday at this point looks like a cooler day behind the cold front. There will still be a few showers and storms around, but coverage will be less as shortwave ridging and some upper level convergence moves over the Cowboy State. Chances of convection then increase again as another shortwave passes over the area. This time, a jet streak will be passing by and this could increase the chance of strong to severe thunderstorms again. The Storm Prediction Center already has a Marginal Risk over Natrona County, where northeasterly upslope flow is predicted and where the maximum instability and moisture is present. This is another day we are going to have to watch closely. The active pattern looks to continue into next weekend with more chances for convection, although timing of the individual shortwaves remains uncertain. With the area near the top of a ridge, temperatures should average close to normal with no heat waves expected through next weekend.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1049 AM MDT Mon Jun 22 2026

VFR conditions are expected to prevail across nearly all terminals through the TAF period. Northerly/northwesterly winds increase shortly after the start of the period with speeds of 10 to 15 knots with occasional gusts nearing 25 knots. A shift to a more easterly direction is expected to occur at most terminals this evening around 01-07Z Tuesday. Some convection will develop this afternoon but most terminals are likely to remain dry. The best chances for a nearby shower or thunderstorm look to be at KCOD, KWRL, and KCPR. Other terminals have a non zero chance to see a brief shower or thunderstorm. A brief shift to MVFR or even IFR conditions cannot be ruled out especially if any strong storms move directly overhead of a terminal. Low clouds may linger around KCPR Tuesday morning, which would lead to a period of MVFR conditions persisting to the end of the TAF period. Mountains may be obscured at times this afternoon/evening as a result of developing convection.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts.

RIW WATCHES/WARNINGS/ADVISORIES

None.