textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms are expected across the state through at least 9PM, with a few strong to severe thunderstorms possible across west and southwest Wyoming. The primary threats will be frequent lightning, small hail, and breezy winds.

- Showers and thunderstorms are likely Saturday and Sunday, with the greatest coverage and chances across northern and central Wyoming.

- Heavy rain and localized flooding will be possible in any shower or thunderstorm through Sunday afternoon, especially across northern Wyoming. Thus, a Flash Flood Watch is in effect for portions of northern Wyoming, including Yellowstone National Park, through Sunday afternoon.

UPDATE

Issued at 144 PM MDT Fri Sep 18 2026

The forecast remains on track with an active weather pattern through Sunday as an upper level low, currently over northern Nevada and southern Oregon, shifts east into and across Wyoming. As of 1PM, showers and thunderstorms are spreading northeast across Wyoming as a shortwave rounds the parent upper low. Strong to severe thunderstorms remain possible this afternoon and evening, mainly across western Wyoming, with height rises aloft, BLCAPE values of 700 to 1000 J/kg, and 30 to 45 kts of 0-6km shear. The threats in any strong to severe thunderstorms will be hail and gusty winds up to 45 mph. The severe thunderstorm threat should diminish by 9PM as the shortwave moves out of the region. More of note, abundant moisture continues to be present across the state (the RIW 18Z launch indicates PWATs of 0.86"). Thus, any shower or thunderstorm will be capable of heavy rain that causes localized flash flooding, especially in poor drainage and urban areas and steep terrain. Although shower and thunderstorm coverage is expected to decrease overnight, there will likely be at least a few showers across the area.

Model guidance continues to be in agreement that the upper level low will progress east across Wyoming on Saturday and Sunday. The position of the low will focus the most numerous shower and thunderstorm activity across northern Wyoming. The Flash Flood Watch remains in effect for higher elevations of northern Wyoming and Johnson County. An axis of rain is forecast to position itself over northwest Wyoming, including Yellowstone National Park. Given the steep terrain and low drainage areas across Yellowstone and current (1PM) 24 hour rainfall amounts ranging from 0.12" and 1.00", the Flash Flood Watch has been extended to include Yellowstone National Park. Through Sunday, many locations could see 0.75" to 1.75" of rain, with a majority of this rain falling quickly in thunderstorms. If a thunderstorm has higher rainfall rates, these values may be underdone. Additionally, if the upper low deviates from its forecasted path these values may be unrepresentative. For example, if the low tracks further to the south, anticipate higher totals. All in all a rainy and stormy weekend is in store.

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 /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1155 AM MDT Fri Sep 18 2026

Multiple rounds of widespread showers and thunderstorms will move through the region over the next 24 hours. Although VFR conditions will prevail, any thunderstorms in the vicinity of airports will be a threat, and if a storm passes directly over an airfield, brief MVFR to IFR conditions are possible as the heavy rain passes over. Winds today will generally be light, but could gust into the 25 to 35 kt range near any storm downdraft/outflow. These winds would be erratic, and gusty VRB winds groups are prevalent in TEMPO and PROB30 groups at individual TAF sites.

Storms will not end around sunset (which is normally the case), because tonight the weather system supporting storms will still be overhead. However, they should be less widespread and less robust overnight.

Low clouds( and IFR to MVFR conditions) look to push into the Bighorn Basin overnight into Saturday morning, including for COD. These low clouds will likely persist until around 19/1700Z, after which time thinning of low clouds is expected, while surface mixing help to lift cloud decks slightly.

RIW WATCHES/WARNINGS/ADVISORIES

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