textproduct: Western and Central Wyoming

KEY MESSAGES

- A weather system and a plume of monsoonal moisture will bring scattered showers and storms today, most numerous in the morning and in western Wyoming.

- Mainly dry and warm from Wednesday through Friday. Low humidity and a gusty wind will bring elevated to potentially critical fire weather each afternoon.

- The next better chance of showers moves in this weekend, most likely on Sunday with the next plume of monsoonal moisture.

UPDATE

Issued at 1200 PM MDT Tue Sep 1 2026

To go in line with what models did with the overnight into this morning in terms of rain shower activity, PoPs have been cut with this next weakening wave from the southwest. Limited in coverage for isolated storms as they push northward west of the Divide into the evening hours through about midnight or so. Nothing significant of note will come from these under weak flow and instability. Going forward, warming trend and increasing fire weather concerns for the remainder of the week and into the weekend. Thursday looks to be the strongest in terms of fire weather danger, and will likely push critical thresholds with strong winds and low minimum relative humidity values. Stay updated overnight with fire weather products to be issued going forward.

The system in the long term for Sunday and into next week looks to be trending a bit slower as this past system did. Expect impacts to follow the same line but stay tuned in the coming days for early next week's forecast.

DISCUSSION

Issued at 134 AM MDT Tue Sep 1 2026

This morning we say goodbye to meteorological summer for another year. And we enter what is the favorite month for many Wyoming residents, September. The number of tourists drops substantially, especially after Labor Day, so we have the state more to ourselves. It is usually a fairly dry month (with some exceptions). And temperatures are usually pretty nice, with comfortably warm days and cool nights. However, I have witnessed a few snowstorms in September in my almost 15 years at the office. But, none of those on the horizon, at least for the next week.

We are seeing some needed moisture this morning, in the form of a plume of monsoonal moisture and a shortwave lifting northward across the area. We do have some showers across the area this morning but as of right now, the heaviest showers are across Idaho. Most guidance does shift this eastward toward dawn and into the morning, with decent model agreement showing northwestern Wyoming seeing the steadiest and heaviest showers. Ensemble guidance is giving at least a 1 in 2 chance of a quarter of an inch of rain from through midnight tonight across the Tetons, Wind River Range and northern Salt and Wyoming Range, with the adjacent valleys having a 4 in 5 chance of at least a tenth of an inch of rain. The chance of an inch or rain is small though, generally less than 1 out 3 and this is only at the highest elevations. As for flooding potential, it looks small at this point given that there is steering flow with these showers, so they should not be stationary. Precipitable water is also coming in about 15 to 20 percent less than yesterday, generally running 125 to 150 percent of normal versus 150 to 175 percent yesterday. The chance is not zero though. The best chance this morning would be in areas prone to southerly upslope, like the western Wind River Range. The chance is small though, generally less than 1 out of 5. There look to be two periods of showers. The initial one this morning with the wave lifting northward. The second will be this afternoon, as a trailing shortwave moves into western Wyoming. Most of this wave would remain West of the Divide, as downsloping west to southwesterly flow develops behind the wave. (Around a 1 in 4 chance west of the Divide, less than 1 out of 5 East of the Divide except near the mountains). A few of these showers will linger into the evening and possibly after midnight before ending before dawn on Wednesday as drier air punches in from the south and west.

And now for something completely different, an 180 degree flip as it would be. Drier air will move across most of the area, with precipitable water values dropping back to close to normal values. We can't rule out a stray high based shower or thunderstorm but these would be few and far between (less than a 1 in 6 chance). For a few days, we will be shutting off the monsoon machine as it goes in for maintenance. However, we are in the process of changing the fluids in the famous Wyoming Wind Machine, which will be fired up for the first time in a while starting Wednesday. It will only be running at half power though, not red lining it Wyoming will be close to the middle point between and upper level low over the Pacific Northwest and a strong ridge over the southwest. The result will be a tight height gradient. We will also have around an 80 knot jet nearby. The result will be a breezy to windy afternoon across much of the area. With 700 millibar winds only expected to be around 35 knots, high wind is not expected. Much of the area has at least a 3 out of 5 chance of wind gusts over 30 mph. There is at least a 1 in 2 chance of wind gusts past 40 mph in the favored southwest flow locations from South Pass through southern Johnson County. And humidity will be lower. However, it does not look quite low enough for critical fire weather at this time. The most likely spot would be Wind River Basin to the eastern Bighorn Basin where there is around a 1 in 2 chance of humidity falling to under 15 percent. It may not be for three hours though. So, for now we will hold off on any Fire Weather Watches. We will let the day crew take another look as they can issue watches and we could upgrade tomorrow night if need be. And with the downsloping flow, it will get quite warm. The warmer spots like the Bighorn Basin have at least a 1 in 2 chance of high temperatures reaching 90.

Thursday at this point looks like a dry day across the area, With the jet still nearby, it will still be breezy to windy. However, dew points will be around 5 to 10 degrees lower. This will lead to a more widespread area of relative humidity under 15 percent. Wind may not be quite as strong, but there is still a greater than 1 out of 2 chance of gusts over 25 much across much of the area. As a result, this looks like the best chance (greater than 3 out of 5) of critical fire weather across portions of the area. We will likely need some fire weather watches but it is a bit too far out at this point. Temperatures should remain above normal, but a few degrees cooler than Thursday.

Fire conditions should begin to improve somewhat on Friday as humidity begins to rise. There could still be some elevated fire to possibly critical fire weather though as the wind will remain rather gusty and relative humidity could approach 15 percent again. These concerns should end Saturday as the wind decreases and humidity rises further in advance of another plume of moisture from the southwest. The main players on this will be an upper level low off of the Pacific coast and the ridge of high pressure which will gradually retrograde from the southeastern United States to the southern High Plains. The wild card will be potential hurricane off of the Mexican Coast and it's eventual track. If the tropical system can move closer to the coast, there is chance that some of it's moisture could get caught in a moist conveyor belt and moved into the Great Basin and potentially Wyoming. There is a still a lot of uncertainty as to the track of the possible hurricane though. There is decent agreement in the delayed arrival of the moisture though. Yesterday at this time, it was late Saturday into Saturday night. It is now trending more toward Sunday now. And, with the ridge shifting back to the west, temperatures will remain above normal. It does not look like record heat, but temperatures will be above normal through at least Saturday, with some location having at least a 1 in 2 chance of a high temperature each day from Thursday through Saturday before more potential cloud cover on Sunday brings cooler conditions. So, we will have a wide variety of weather over the next several days, from potential heavy rain to critical fire weather to more summer like temperatures. It is what keeps the weather in Wyoming interesting.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1140 AM MDT Tue Sep 1 2026

VFR flight conditions are expected at most terminals through 18Z Wednesday. Isolated to scattered showers and thunderstorms are expected the first half of the TAF period at or in the area of KCOD, KRKS, KJAC, KBPI, and KPNA. Each of the aforementioned terminals have a 30 percent chance of a shower or thunderstorm impacting the site so maintained PROB30 groups. All other terminals have a 10 to 20 percent chance of a shower or thunderstorm. Gusty winds around 35 kts are possible in any shower or thunderstorm along with brief periods of MVFR flight conditions. Outside of any convection, breezy winds with gusts of 20 to 30kts are expected at most terminals through 02Z and 05Z. Winds remain relatively light overnight before increasing and becoming gusty Wednesday morning.

There is a 15 percent chance of low/MVFR clouds developing around KBPI and KPNA after 05Z Wednesday. This potential is reflected in a FEW010 group at this time given the low confidence.

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

FIRE WEATHER

Issued at 134 AM MDT Tue Sep 1 2026

No fire weather concerns today as relative humidity should remain above 20 percent. From Wednesday through Friday, a combination of humidity falling into the teens, above normal temperatures and wind gusting over 30 mph will bring the likelihood of elevated to critical fire weather each afternoon, with Thursday afternoon looking like the period of most concern with the lowest humidity.

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.