textproduct: Western and Central Wyoming

KEY MESSAGES

- A weather system treks across Wyoming today and brings showers and thunderstorms to the area, especially the northern half of Wyoming, as early as 12PM and continues into Sunday. A few stronger thunderstorms could have hail.

- 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.

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 /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 417 AM MDT Sat Sep 19 2026

Areas that saw moisture west of the Divide have seen some low clouds and patchy fog develop early this morning, with some IFR to occasional very low IFR. This should mix out between 14Z and 16Z, although lower ceilings may linger until 17 to 18Z, especially in vicinity of KBPI and KPNA and possibly KCOD. Mountain obscurations will also be possible during this time.

Otherwise, largely VFR conditions should continue until around 19Z to 20Z. An upper level low will then move eastward and bring some showers and thunderstorms, starting in the west and then gradually moving eastward in the afternoon and evening. Some showers may persist through the night as well, especially in vicinity of KLND, KRIW, KCPR and KWRL. MVFR ceilings will be around at times with the heavier showers. Occasional to frequent mountain obscurations will also be possible through this period, especially East of the Continental Divide.

RIW WATCHES/WARNINGS/ADVISORIES

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