Area Forecast Discussion

KEY MESSAGES

- Widespread showers and thunderstorms are expected through sunset today before chances become confined to areas along and east of the Continental Divide overnight and Sunday.

- The primary threat in any shower or thunderstorm will be heavy rainfall, with localized flash flooding possible, through Sunday. The best chances (60% to 80%) for flooding are along and north of a Powell to Kaycee line.

- Cooler temperatures are expected to close out the weekend. A warm up and overall drier weather is looking more favorable for the upcoming work week beginning Tuesday.

UPDATE

Issued at 1218 PM MDT Sat Sep 19 2026

Showers and thunderstorms continue to be the forecast story as a potent upper level low treks across the state over the next 24 hours. 18Z satellite water vapor imagery shows the upper level low centered over eastern Idaho. As the low progresses east today, shower and thunderstorm potential will increase across much of the state with the most numerous thunderstorms occurring along and east of the Continental Divide and across Yellowstone National Park. The track of the low puts the higher moisture content across northern Wyoming, so the showers and thunderstorms with the greatest heavy rain, and associated flash flooding, threat will also be in northern portions of the state. With that being said, the potential for heavy rain and localized flooding across west and southern Wyoming cannot be ruled out but chances are significantly less (20%). This much lower heavy rain threat is due to the fact that drier air has begun to shift into those areas this morning (evident by morning sunshine!)

The sunshine mentioned above has allowed the atmosphere destabilize with widespread BLCAPE values above 500 J/kg, and even up to 1000 J/kg. Additionally, 0-6km shear values of 35kts to 45kts and unstable lapse rates are expected this afternoon. This means that thunderstorms could contain small hail, with some 1 inch hailstones not out of question. This threat will be greatest across southern and central Wyoming roughly from 2PM to 7PM. Tonight and Sunday morning, the shower and thunderstorm chances, and heavy rainfall potential, are largely confined to locations along and east of the Continental Divide. Given the rain over the past few days and forecasted clear overnight skies, low clouds and fog development is possible across western and southern Wyoming Sunday morning.

DISCUSSION

Issued at 243 AM MDT Sat Sep 19 2026

The monsoon has decided to give me a brief break tonight, as the heaviest showers are either in Montana or across Colorado. There are still a few showers around as I write this, but everything, at least at the time of publication is on the light side. The main show still looks to be today though, especially this afternoon and into tonight.

The big bowling ball, AKA the Upper Level low continues to spin and slowly move eastward. Getting close to the Wyoming border around sunset. This will lead to another day of showers and thunderstorms across the area. One difference is that the most numerous will likely shift to the East of the Divide, especially after sunset as the low moves eastward and drier air moves in behind the low. Now for the main concern, which we have been talking about for the past several days, the potential for heavy rain and flooding. It will still be present today, although the main threat will again be across northern Wyoming. Precipitable waters one most guidance top out at around an inch across the Big Horn Basin and Johnson County, approaching twice of climatological normals, then gradually decreasing as you head south and west. There two areas in particular that are concerning, the Cody Foothills and Absarokas as well as Johnson County and the eastern slopes of the Bighorn Range. The main concern is if the low can take a further south track, this would set up an upslope flow in these areas, bringing greater precipitation efficiency. The main time of concern would be this afternoon and evening in the Cody Foothills, and later tonight and possibly into Sunday morning for the Bighorns and Johnson County as the low makes it's closest approach. The Weather Prediction Center agrees as well, as the Excessive Rainfall Outlook has been upgraded to slight roughly north of a Cowley to Sussex line, with Marginal north of a Lake Yellowstone to Casper Mountain line. The chance of flooding is less elsewhere but not zero. As for stronger thunderstorms, the best instability will be east of our area. However, I could see some hail with low wet bulb zero levels at times. Precipitation should finally end in western Wyoming later tonight as the low moves eastward.

There is one more day to get through on Sunday. The low will be still be moving eastward, finally crossing into South Dakota and Nebraska Sunday night. Some wrap around moisture will still be around in northern Wyoming though, especially along and east of the Bighorns. The threat of heavy rain will likely linger another day here, especially in the morning before gradually tapering off in the afternoon. Chances of showers decrease further west, with most of western and southwestern Wyoming being dry by noon on Sunday, As for the flood watch, we will continue it as is this morning. We may be able to trim some of the western zones out for Sunday if the low moves far enough east, but we will cross that bridge when we get there.

Ridging builds over the area on Monday, bringing a nice last full day of astronomical summer with mild temperatures and a good deal of sunshine for most locations. Models are still split from later Tuesday through Wednesday with the next potential system in the form of a trough moving in from the southwest. Some guidance, like the European, brings it across the state bringing another chance of showers. Other guidance keeps it to the south with mainly dry conditions. The current forecast leans toward the wetter solution, but this is far from certain. Mainly dry weather then likely returns for Thursday and Friday. And, with largely south to southwest flow continuing, temperatures will likely average above normal through much of next week as we head into astronomical autumn, which begins Tuesday at 6:04 pm.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 518 PM MDT Sat Sep 19 2026

Scattered rain showers and thunderstorms continue this evening, with most ending west of the Continental Divide after 06Z. Conditions will drop to MVFR/IFR when showers and storms impact terminals this evening. Rain will continue east of the Continental Divide through Sunday afternoon and mainly impacting only KCPR beyond Sunday morning as the weather system pushes east into the Plains. Low clouds are likely (70%) east of the Continental Divide tonight into Sunday morning, with prevailing MVFR conditions in general. Skies should mostly clear out west of the Continental Divide tonight, which will allow for fog development, especially where rain fell today. The best chance will be at KBPI and KPNA with mentions of VCFG and SCT001 at this time. This will be re-evaluated for the 06Z TAF issuance.

Wind will average 5-10 knots through the TAF period, with gusts up to 20 knots at KCPR Sunday afternoon. Mountain obscuration will improve across the west tonight as clouds clear out and deteriorate east as low clouds develop overnight.

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

Flood Watch through Sunday afternoon for WYZ001>003-007>011.