textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms will continue over the western mountains into the Bighorn Basin this evening, with isolated storms in Johnson County. Locally heavy rain and localized flash flooding will remain a threat with the stronger storms for the next few hours.

- The threat for thunderstorms will be more widespread across the forecast area Saturday, with central and southern portions having better chances than have occurred through the week so far.

- The chance of showers and thunderstorms continues each day through the next seven days, with the chance of local flooding each afternoon and evening.

UPDATE

Issued at 238 PM MDT Fri Jul 17 2026

Thunderstorm activity was delayed slightly this morning due to midlevel cloud cover holding back daytime heating, but has now begun to bubble along the western mountains and the Bighorns. Storm motion remains low, and storms have been tapping deeper monsoon moisture, so thunderstorms have been producing ample rainfall. PW values from 18Z sounding noted 0.94" still, so the main threat with any storms remains localized downpours. The Flash Flood Watch continues through 10PM tonight, and was expanded west to include Greater Yellowstone National Park, where models have noted showers and thunderstorms may last well into the evening. Remaining forecast looks on track for today. Remember: if you see flooded roads, Turn Around, Don't Drown!

DISCUSSION

Issued at 157 AM MDT Fri Jul 17 2026

Have you ever been stopped at a train crossing watching a really long freight train go by? One of those really long ones that tend to always show up when you are running late for something. Well, I am comparing that long freight train to the monsoonal pattern that we look to be locked in for at least another week. There will be day to day fluctuations, but there will be at least some chance of showers and thunderstorms each day, and possibly the chance of flooding.

We will have one of those fluctuations today, as the ridge builds back to the west somewhat. This will lead to somewhat higher heights and warmer temperatures. The biggest impact will be that precipitable waters will drop about 15 to 25 percent roughly west of a Buffalo to Riverton to Rock Springs line. This means that the chance of storms and the chance of flooding should be somewhat reduced today in this area. However, this will not be the case for the Bighorn Basin as well as the western mountains. So, the main area of showers and storms will be here, with tapering chances further south and east. And with the continued chance of flooding, we will issue another Flood Watch for the Absarokas and much of the Bighorn Basin given the saturated soils. We thought about one for the western mountains but rainfall hasn't been as heavy here and we will hold off for now. However, much like yesterday, any shower or storm could have heavy rain given the abnormally moist atmosphere. Like yesterday, most storms will be over shortly after sunset, but a few may linger until after midnight.

The moisture axis will shift a bit further to the east on Saturday, and as a result so will the chance of showers and thunderstorms. Precipitable waters climb back to levels similar to Thursday, so the chance of storms should be similar as well. Again, the Bighorn Basin looks to be the area under the gun as well as the Absarokas as the low level easterly upslope flow continues. Far eastern areas, like Kaycee and Casper, who have been wondering where their rain is, will have another dry day on this day. This will also be a warmer day, and some locations in the eastern Bighorn Basin might make a run at 100 degrees if they can get enough sunshine.

It still looks like the transition day will be on Sunday. On this day, some drier air will begin to move into western Wyoming, drying things out somewhat. The main focus then moves to east of the Divide, including locations like Casper that have remained hot and dry through much of the week. Precipitable water values will again average around 175 percent of normal, and again, sounding like a broken record, the threat of heavy rain and localized flooding. With more clouds shifting eastward, temperatures should begin to cool down as well on this day, although they will remain well above normal.

There is a wrinkle for Monday, as there is some split in the guidance. Some ensemble members, as well as the deterministic ECMWF model run, now builds the ridge over the state, shutting off most of the convection. Others keep things quite active though. For now, the compromise was to dry out northern Wyoming and have the highest PoPs across southern Wyoming. There is still plenty of moisture in the atmosphere so it would not take much for a few storms to fire.

Following Monday, there is greater model agreement in bringing the deep monsoonal moisture back over the area, especially east of the Divide. Precipitable water values look exceptionally high on some model runs. This is still 5 or more days out, so take this with a grain of salt. For example, at Riverton, our mean precipitable water value is 0.63 inches. On Wednesday, some ensemble members and the deterministic GFS is showing precipitable waters of over 1.3 inches. This is over 200 percent of normal. It is also close to the record high precipitable water value of 1.46 inches that was set back in 2003. Again, it is a long way out but there is the potential for some very heavy rain from any thunderstorm that could develop for the middle of the week. The abnormally high precipitable water values look to stick around through at least Thursday. So, the monsoonal freight train will continue to transport moisture into the area, along with the chance of drenching thunderstorms, for the next several days.

AVIATION /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 745 PM MDT Fri Jul 17 2026

The first wave of thunderstorms continues to move northward across NW WY this afternoon, bringing brief localized downpours in the vicinity of KJAC and KCOD. More redevelopment is expected farther south, which has the potential to reach KBPI/KPNA early this evening. Light showers may continue into the early overnight hours, but most areas will see just a midlevel cloud deck through the night, with clear skies farther east.

Another day of afternoon thunderstorms expected for Saturday. The main tap of monsoonal moisture shifts east slightly. This would lower thunderstorm chances for KJAC, but bring better chances into KLND, possibly KRIW and KRKS. KCPR remains out of the main moisture, but may finally see a bit more cloud cover during the afternoon.

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 until 10 PM MDT this evening for WYZ001>005-007.