textproduct: Western and Central Wyoming

KEY MESSAGES

- Deep moisture and a weather system approaching from the west will bring more numerous showers and thunderstorms today and tonight, mainly in western Wyoming this afternoon then northern Wyoming tonight.

- Further showers and thunderstorms are likely Saturday and Sunday, with the best chance across northern Wyoming with lessening chances across southern Wyoming.

- Heavy rain and local flooding is possible around any shower or thunderstorm, so the Flood Watch will be continued as is for now.

DISCUSSION

Issued at 240 AM MDT Fri Sep 18 2026

It looks like quite an active few days across the area as my nemesis, the monsoon, with an assist from any upper level low, will continue to bring rounds of showers and thunderstorms for the next few days. We continue to have convection tonight, especially in western Wyoming where some storms have been quite strong. They should slowly weaken as we head into the night. However, many of the storms have produce very rain back in Idaho and some local flooding will be possible before sunrise, especially north of Jackson and into Yellowstone Park.

As for today, some showers and an isolated thunderstorm will be around in the morning. The main action looks to be this afternoon and tonight though as we get some daytime heating to destabilize the atmosphere and a shortwave around the parent upper level low and brings some additional lift in the afternoon and evening. The result will be a good coverage of showers and thunderstorms across the area. As for timing, the most numerous storms should be west of the Divide and across southern Wyoming in the afternoon, then moving East of the Divide later in the afternoon and into the evening. There is fair model agreement in the most numerous storms to be in the northern counties East of the Divide this evening, with lessening chances further south. This lines up well with the flood watch, which we will continue as is. Precipitable water values remain abnormally high, generally 150 to as much as 200 percent of normal, especially East of the Divide. So, as would be expected, any storm could produce heavy rain and local flooding. But there is another aspect. There could be some strong to possible severe thunderstorms. There is close to 1000 J/Kg of mid level CAPE as the trough approaches the western portions of the state. There is obviously plenty of moisture. Directional shear will also be increasing as the upper level low moves closer. The main threat other than heavy rain would be hail, especially with fairly low wet bulb zero levels. Showers and storms should continue into the night, although coverage should gradually decrease.

The upper level low continues to drift eastward on Saturday, moving into Wyoming later in the day. On this day, the most numerous showers and storms will shift to northern Wyoming. Some drier air may try to wrap into the southern portions of the area, possibly shutting of convection for a while. We have maintained at least a 1 in 5 chance for all areas though. A better chance of showers likely returns Saturday night though as low pressure moves across the area. Again, despite decent steering flow, the amount of moisture will be sufficient for locally heavy rain and possible flooding.

As we head into Sunday, conditions may begin to improve as the low moves east into the Dakotas. There will still be some showers around from the wrap around moisture though. The main threat on this day shifts to northeastern portions of the area, with decreasing chances to the south and west, with mainly dry conditions expected across southwestern Wyoming. With flow turning northwest, it should be a cooler day as well. Showers should then end for all areas by Sunday evening.

There continues to be more uncertainty regarding next week. Some guidance brings another system up from the southwest with another chance of showers on Monday or Tuesday, others keep it south of the area, primarily impacting Utah and Colorado. Given the concerns in the short term, we didn't have time to take a good look this far out. One thing are more confident is that flow will turn south to southwesterly again, and bring another period of above normal temperatures for the middle and end of next week.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 438 AM MDT Fri Sep 18 2026

Deep moisture will move over the area today, and bring some rounds of showers and thunderstorms. Most convection will start across the western terminals, especially in vicinity of KJAC, KPNA and KBPI this morning. Showers then increase in coverage across the west through the morning, spreading East of the Continental Divide this afternoon, especially after 21Z. Tonight, the main focus of showers and thunderstorms will shift to northern Wyoming, especially in vicinity of KCOD and KWRL and northward, with some drying out across the southern terminals. Confidence in exact timing of periods of showers and any thunderstorms is remains rather low. MVFR conditions are possible around any shower or thunderstorm. Mountain obscurations will be possible through the day, especially in the western mountains. Wind should remain under 12 knots except in and around any shower or thunderstorm.

RIW WATCHES/WARNINGS/ADVISORIES

Flood Watch from noon MDT today through Sunday afternoon for WYZ002-003-007>011.