textproduct: Western and Central Wyoming

KEY MESSAGES

- An active monsoonal pattern continues today with more showers and thunderstorms, especially west of the Divide. Stronger storms with heavy rain, strong wind and hail are possible.

- Turning somewhat drier and cooler Sunday and Monday. Patchy frost is possible in the western valleys Monday morning as temperatures drop into the 30s.

- Monsoonal moisture returns for much of next week, with a continuing chance of mainly afternoon showers and thunderstorms.

DISCUSSION

Issued at 257 AM MDT Sat Aug 29 2026

I have returned after a break and this pattern reminds me of one of my favorite types of candy from my youth and the movie it was from. The movie was "Willie Wonka and the Chocolate Factory", the original with Gene Wilder. And the candy was the "Everlasting Gobstopper." And this reminds me of the pattern we have been in since the middle of July, expect I would title this "The Everlasting Monsoon."

And this is no exception this morning, as we continue to have a plume of monsoonal moisture streaming northward over the area, mainly the northwest half of the region. Dew points are also rather high, most locations in the 50s. I know people from the East Coast or Gulf Coasts are laughing at this statement, but that is rather high in this part of the country. Precipitable water values also remain rather high, topping out at over an inch in some locations, which is close to 200 percent of normal. Most guidance continues to put the main impact West of the Continental Divide. The HREF ensemble gives a greater than 1 in 2 chance of a half an inch or rain or more across much of western Wyoming through 7 am Sunday. And, with the rain being enhanced by a shortwave and approaching cold front, the forcing gives me greater confidence in the solution. Exact positioning of the storms is still not certain though. There are a couple of factors against excessive rainfall though. One is cloud cover, it may be fairly substantial today, this would limit instability and possible thunderstorm development. The other is that we do have some decent flow at 500 millibars, as much as 40 knots on some models. This should keep the storms moving. There could still be some training though. We continue to have a marginal risk of excessive rainfall across this area as well. So, we will continue the flood watch as is for now. Like tonight, showers and possible thunderstorms will continue through the nighttime hours as well.

We also have to talk about the possibility of severe thunderstorms. The entire state is under a marginal risk with western Sweetwater and southern Lincoln County under a slight risk of severe thunderstorms. Hodographs are generally of the unidirectional variety which would lead to wind and hail being the main threat. We did have some wet microbursts on Friday and this looks to once again be the main threat today. The Storm Prediction Center has roughly the southern half of Wyoming in a Conditional 1 area for convective wind (wind gusts to 75 mph), and this looks reasonable given the largely straight hodographs and decent mid level forcing and wind. Again, the main concern against the severe weather may be cloud cover which could limit instability. So, severe storms are not definite but the possibility is definitely there.

The cold front will move across the area on Sunday. Much drier air will move in behind it through the area, bringing below normal precipitable waters from west to east across much of the area by sunset on Sunday evening. This should also help shut off or limit the amount of convection, The main area of concern on this day would be southern and eastern portions of the area. We still have a marginal risk of excessive rainfall on this day, roughly south of an Evanston to Casper line. As for stronger thunderstorms, it would be more limited and largely across the usual suspects of Johnson and Natrona Counties where more instability would build before the cold front arrives. And then, to quote Monty Python, "Now for Something Completely Different".

Much cooler air will move in behind the front, and with a clearing sky, light wind and the longer nights of late August, it will get quite cool. And this brings the possibility of potential below freezing temperatures and possible frost. The best chance would be across the western valleys. The NBM ensemble gives around a 1 in 2 chance of portions of the Jackson Valley falling to 32 degrees Monday morning. Most other locations are less than 1 out of 3 though. The chance of temperatures below 28 remains very low though, less than 1 out of 10. At this point, if there is any frost it would be patchy. There is still time to evaluate though so we will let the day shift take another look.

On Monday, monsoonal moisture will make another push northward although it looks to be a later arrival at this point except in far southern Wyoming. With the night time arrival could limit excessive rainfall though with little instability and most precipitation just being showers despite precipitable waters again rising to over 150 percent of climatological normals. Tuesday looks like more widespread activity. Wednesday may be rather active as well although another approaching shortwave and weak cold front and drier air behind it may dry things out somewhat. Timing is still uncertain this far out though. Thursday may also be drier but given how poor the models have been with the monsoonal moisture in the medium to long range, confidence is low with the details. As for temperatures, the general trend will be for above normal but not hot temperatures with a possible cooler day on Wednesday or Thursday depending on the timing of the cold front. So, it looks like the year of the monsoon will roll on through much of next week, although with a few breaks possible.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1010 PM MDT Fri Aug 28 2026

The active period will continue as moisture arrives within southwesterly flow. This will keep scattered showers and thunderstorms going across the area through much of the period. Confidence is highest on impacts across western terminals, including KJAC, KBPI/KPNA and KRKS. Regardless, all terminals have a chance of seeing this activity; the uncertainty lies more with the timing. All terminals have at least PROB30 groups to account for this. Brief heavy rain may bring occasional MVFR conditions, again with the best chances across terminals west of the Divide. After a relative lull after 12Z, another round of showers will arrive by 16Z and become more widespread over western WY by 19Z. These will spread eastward through the afternoon, impacting terminals east of the Divide. Other than rain, strong outflow wind gusts to 35 knots will be possible with this activity. Showers and storms will end for much of the area by 00Z, with the focus shifting to southern portions as another wave shifts northeastward through the end of the TAF period. Low clouds will lead to frequent mountain obscuration through the period, especially across western Wyoming.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts.

RIW WATCHES/WARNINGS/ADVISORIES

Flood Watch through Sunday morning for WYZ001-002-012>014-023- 024.