textproduct: Western and Central Wyoming

KEY MESSAGES

- After a mainly dry day today, monsoonal moisture returns tonight into Tuesday with a scattering of showers and thunderstorms. Locally heavy rain is possible with any thunderstorm, especially in western Wyoming.

- Mainly dry, warm and breezy to windy Wednesday through Friday. With low humidity, elevated to critical fire weather is possible each afternoon, especially East of the Divide.

- Monsoonal moisture returns again for the weekend, with an increased chance of showers and thunderstorms.

DISCUSSION

Issued at 1244 AM MDT Mon Aug 31 2026

I have arrived at work for another shift and for the first time in a while the radar is clear, no showers in the county warning area as drier air has spread across almost all of the area. However, the monsoon machine has been unrelenting this year, going almost nonstop for the last six to seven weeks with only a few stops for maintenance. We are in one of the those now, but it won't last. The lock out / tag out on it will be removed later today and will again be fired up across the area tonight.

But today looks like a fairly nice start to the week for most, now the last day of August and of meteorological summer. After a rather chilly start, with some frost possible in the deeper valleys in the west with temperatures dropping close to freezing, it should be comfortably warm this afternoon. Temperatures look to be fairly close to normal with light to moderate wind for most locations. The monsoonal moisture will begin to creep northward late today, with a few showers and thunderstorms south of roughly an Afton to Wamsutter line before sunset. Moisture then spreads northward tonight, across much of the area by sunrise. As for the potential of heavy rain and possible flooding, we once again have some factors in favor of it and some against it. Well above normal precipitable water values will again move across the area, as much as 175 percent of normal. The moisture will also be riding a shortwave lifting from the southwest, enhancing lift and mid level forcing. There are some factors against it as well. Most of the precipitation will fall at night, with limited surface instability and less of a chance for thunderstorms. There will also be some mid level flow, so any showers or storms will not be stationary. At this point, the marginal risk of excessive rainfall looks reasonable for now, mainly in the west and south where the greatest lift will be. Chances of showers will continue into Tuesday as well, although chances of showers and heavy rain should decrease through the day as the deeper moisture moves away to the east. Most showers should end by midnight Tuesday night. As for amounts, most of the ensembles are giving at least a 2 in 3 chance of a quarter of an inch or rain across most of the western mountains and valleys. The best chance of over a half an inch or rain will be in the Wind River Range and Tetons with a greater than 1 out of 2 chance. Some locations will probably end up with an inch of rain but with the nature of convection this is impossible to pinpoint.

Much drier air will the move across the area for the middle of the week as the moisture gets pushed to the east. However, we have another concern for these days. On these days, we will be between an upper level low over the Pacific Northwest and ridging over the southeast. The jet stream will also be nearby, as much as 90 knots, a decent speed for this time of year. The result will be a gusty wind developing across much of the area. Most areas will have at least a 3 out of 5 chance of wind gusts over 30 mph on Wednesday, Thursday and even into Friday. Some of the more wind prone areas could even see gusts to 50 mph, especially on Wednesday when the wind is expected to peak. The limiting factor will be humidity. The main area of concern looks to be East of the Divide where most locations have at least a 1 in 2 chance of relative humidity below 15 percent. We will likely (around a 3 out of 4 chance) need some fire weather highlights for some if not much of the period. It is still a bit too early right now though. And, we can't completely rule out a stray shower over the mountains but the chance is less than 1 out of 8 each day. Temperatures should be well above normal on these days, with the warmer spots, like the eastern Bighorn Basin, having at least a 1 in 2 chance of 90 degree highs each day.

The monsoon looks to make a return for next weekend as the ridge over the southeast retrogrades and moves back toward the southern plains. At the same time, an upper level low will be moving into the central Pacific coast. This may create a moist conveyor belt of moisture into the area for Saturday or Sunday with some possible adding of moisture from a possible tropical storm or hurricane off of the western Mexico coast. The GFS is very moist with precipitable water, showing around 200 percent of normal Saturday night into Sunday. This model does tend to run a bit hot on this, so reality is probably 20 to 25 percent less. However, the chance of showers and storms really increases on then. With the ridge remaining fairly stationary for a few days, this pattern looks to continue into early next week. And with this, temperatures will remain above normal, generally around 7 to 12 degrees above normal. So, the monsoon continues to stick around like an house guest that overstayed its welcome. However, it will be leaving for a few days at least.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 359 AM MDT Mon Aug 31 2026

Mainly VFR conditions will continue through most of the forecast period. Moisture will increase from the south as a weak weather system approaches and brings a chance of showers and thunderstorms from south to north. The main push of moisture will be at terminals West of the Divide, with progression from south to north. Confidence is highest at KRKS, where we continue to prevail showers after 02Z until around 10Z when the moisture moves northward. We continued PROB30 groups at KBPI and KPNA and added one to KJAC as well. Brief periods of MVFR ceilings or visibilities are possible if heavier showers move over the airports. East of the Divide, PROB30 groups were added in vicinity of KCPR, KLND and KRIW after 06Z tonight for showers. Any rain should hold off until after 12Z at KWRL and KCOD. Wind should remain 11 knots or less for the most part, except in vicinity of KCPR where wind gusts to 20 knots are possible this afternoon. Occasional mountain obscurations are possible in western Wyoming in any lower ceilings and rain showers.

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

RIW WATCHES/WARNINGS/ADVISORIES

None.