textproduct: Western and Central Wyoming

KEY MESSAGES

- A mainly dry day for most today, although showers will increase from the southwest late today and this evening, especially in western and southern Wyoming.

- Cooler and unsettled Sunday night through Monday evening with a scattering of showers and thunderstorms, with the most numerous in western and southern Wyoming once again.

- Mainly dry weather returns Tuesday and lasts through at least Friday with temperatures averaging near to above normal.

DISCUSSION

Issued at 144 AM MDT Sun Sep 27 2026

Things are relatively quiet here at the humble Riverton weather abode early this Sunday morning. We will have some active weather in the forecast, but also a decently long period of fairly nice early autumn weather, so there is a little bit for everyone, depending on your taste.

The active weather will be front loaded. Most of the day should be fairly quiet, although a few showers will be possible, With the greatest number in the afternoon and in and near the mountains. However, a vast majority of the area should remain rain free through the daylight hours. The main cause of the active weather, a shortwave that will be rotating around an upper level low over California, will begin making a closet approach tonight, increasing the coverage of showers and thunderstorms. With the southwest flow ahead of the wave, the main area of showers and isolated thunderstorms will be across southern and western Wyoming, with drier conditions East of the Divide where downsloping flow may keep a lot of the showers away. As for temperatures, they will remain above normal, but more cloud cover will keep them a few degrees cooler than on Saturday.

The most widespread showers and thunderstorms still look to be on Monday. The shortwave will be passing through Wyoming at that point. There will also some upper level support from the left front quadrant of a 90 knot jet streak that will be passing across southeastern portions of the area. As for threats, locally heavy rain is possible. Precipitable water values will be from 125 to 150 percent of normal with the additional upper level forcing from the jet streak. However, instability may be limited as there could be a decent amount of sky cover around the area. In addition, there will be steering flow so storms will be of the moving variety. In addition, most locations have been mainly dry since last Sunday, so there is room in the soil. So, although the threat of flooding is not zero, it is low. As for overall amounts, most of southern and western Wyoming has at least a 1 in 2 chance of a tenth of an inch of QPF through Tuesday morning. Most of western Wyoming, especially the favored south and southwesterly upslope flow locations, have at least a 1 in 2 chance of a quarter of an inch of QPF. The highest amounts would be in the highest elevations of the southwestern slopes, like the western Wind River Range and the southern Absarokas where locally over a half an inch or rain could fall. And, with low wet bulb zero levels, small hail may also be possible with the stronger storms. Most showers decrease after sunset with only isolated to scattered leftover showers after midnight.

And we have to talk about the possibility of high elevation snow. The 700 millibar temperatures do fall as low as 2 celsius later Sunday night and Monday morning, this could bring snow levels down to around 9500 to 10000 feet. It would not surprise me to see some snowflakes on the higher passes like Beartooth or Togwotee. However, any accumulation would be above 11000 feet and even here it looks light. The only location with a greater than 1 in 2 chance of over an inch or snow is the highest elevations of the Wind River Range where impacts would be slim to none.

We then enter the mainly dry period of the forecast. At this point, we are around 90 percent sure that we will have a decent period of nice autumn weather. The one wild card is that will happen with the combination of the upper level low moving eastward from California when it interacts with the moisture from the remnants of Hurricane Polo after it makes landfall in Mexico. Almost all the ensemble members keep the moisture south and east of the Wyoming, but a few try to bring the edge into southern Wyoming. We will watch this but odds are against it at this point. Temperatures should average above normal for the most part as ridging builds into the area. A brief is likely Wednesday night and Thursday as a weak cold front moves through the area. With the lack of moisture, it will likely only bring some clouds and some breezes. Temperatures then warm again for Friday and into Saturday. The next chance of showers would be later Saturday into Sunday with another trough and cold front approaching. There has been poor model consistency with this feature though, so confidence in the details this far out is very low.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 427 AM MDT Sun Sep 27 2026

Largely VFR conditions are expected through most of the period. A weak disturbance this morning may bring isolated showers until around 15Z in vicinity of KJAC, KCOD and possibly KRIW. Breezy conditions are expected this afternoon at most terminals, although a bit lighter than yesterday, generally 12 to 20 knots in the afternoon. Shower activity will increase from the southwest starting around 22Z and lifting north to all TAF sites by 12Z Monday. An isolated thunderstorms is also possible but with the chance 20 percent or less there is not enough confidence to include in the TAFs. Brief MVFR conditions are possible around any heavier showers.

RIW WATCHES/WARNINGS/ADVISORIES

None.