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 /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1147 AM MDT Sat Aug 29 2026

Numerous showers and thunderstorms are expected through the period as a moist southwesterly flow continues. All terminals are expected to see impacts, most persistently at KBPI, KJAC, KPNA, and KRKS. Brief heavy rain and low clouds will lead to occasional MVFR conditions. The more widespread impact will likely be strong outflow wind gusting 30 to 40 knots. There is also a more isolated threat of gusts to around 50 knots at KCPR, KLND, KRIW, and KRKS between 18Z and 00Z as stronger thunderstorms move through the area. After a relative lull around 00Z Sunday, the next wave of showers will follow quickly, first across KRKS with chances increasing to the north overnight into Sunday morning. Showers and storms will then move from west to east through early Sunday afternoon, with most of this ending by the end of the period. Low clouds will lead to frequent mountain obscuration through the period, especially across western Wyoming through 18Z Sunday.

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.