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.

- A few stronger thunderstorms could have hail this afternoon, mainly across central and southern Wyoming. The hail threat diminishes around sunset this evening.

- 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 is 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.

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 /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1110 AM MDT Sat Sep 19 2026

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

Low clouds should diminish within the first couple hours of the period. However, low-to-mid-level clouds will continue through the rest of the day associated with the increased moisture over the area. Scattered shower and thunderstorm chances (25-60%) occur through the afternoon and early evening, possibly bringing brief MVFR conditions with moderate to heavy rain and lowered ceilings. Rain chances decrease between 00z and 12z as the main upper-level low shifts eastwards. CLouds will continue to linger, however, and low clouds and fog may develop for locations that recently received rain. Confidence remains low of how impactful these low clouds could be. Very gradually improvements in cloud coverage occurs into Sunday morning.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

A complex next 24 hours as an upper-level low tracks eastwards, directly over the area. Rain chances will be persistent at all locations through the TAF period, with TAFs including TEMPO, PROB30, and prevailing groups for rain where chances are best. Otherwise, chances are 15 to 20 percent. VFR to MVFR conditions are likely, with lowering ceilings and moderate to heavy rain. For Saturday afternoon, showers and thunderstorms are also possible, with lessening chances through Saturday night.

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.