textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers across Johnson and Natrona Counties, the Bighorn Mountains and Wind River Basin continue to wane and will end by sunset this evening. Temperatures will be a bit chilly tonight, with lows in the 30s and middle 40s.

- Areas of fog are possible Monday morning across much of central and western Wyoming, especially in low-lying areas.

- Apart from shower and thunderstorm chances (20% to 60%) on Tuesday afternoon and evening, this work week is forecast to be dry with above normal temperatures returning by Thursday.

UPDATE

Issued at 1258 PM MDT Sun Sep 20 2026

The rainy and stormy weather that we have been experiencing the past few days is coming to an end for most as the upper level low drifts into the High Plains. As shared in the morning discussion, abundant moisture will continue to wrap around the upper low into the far eastern Bighorn Basin, the Bighorn Mountains, and Johnson County this afternoon. These areas can expect showers and thunderstorms through sunset, with instances of heavy rainfall and localized flash flooding remaining possible. Thus, the Flash Flood Watch will remain in effect until 6PM this evening. Portions of Natrona County could also see instances of heavy rainfall and localized flooding, but the chances are much less due to drier air shifting into the area. Additionally, a few diurnal and terrain-driven showers and thunderstorms are expected across and around the Absarokas and Wind River Mountains between 1PM and 8PM.

The only addition to the forecast this afternoon is the potential for fog on Monday morning. Skies are forecast to be mostly clear tonight, so sufficient radiational cooling is expected. These clear skies, combined with the recent widespread moisture, creates favorable conditions for fog on Monday morning. Low lying areas such as basins and valleys will be the most susceptible locations. However, if we dry out enough this afternoon from the sunshine, less areas would see fog. Regardless, anticipate potential impacts for the Monday morning commute so be sure to stay vigilant tomorrow morning and take it slow.

DISCUSSION

Issued at 237 AM MDT Sun Sep 20 2026

Rain continues across portions of the area, mainly central Wyoming, as low pressure continues to move through the area. It is now fairly close to our office just after midnight with moisture wrapping back into the Wind River Basin. Some areas had a lot of rain yesterday afternoon and evening, with some street flooding in Lander. Most of the convective rain is done now, and is now mainly of the stratiform variety. The main area of rain will continue to move slowly east through the night and into tomorrow with gradual drying as the low tracks to the east. We will still have the chance for some flooding today as well, especially later in the day if there is some destabilization of the atmosphere to get some thunderstorms to fire. However, the impacted area will be more restricted. Precipitable water values remain over 150 percent of normal along and east of the Bighorns. So, we will maintain the Flood Watch here for the rest of the day. Across the western zones, with the deeper moisture moving away, we will cancel the Watch with issuance this morning. There will be enough moisture for isolated showers and storms to fire back into the basins east of the Continental Divide and the adjacent mountains, but coverage looks rather sparse (generally less than a 1 out of 3 chance). Showers should gradually end this evening across the eastern zones with all areas dry by midnight. Some locations in Johnson and Natrona Counties could pick up 0.75 to 1.50 inches of additional rain today, especially where storms persist. However, not all areas will see heavy rain.

Once we get past this evening, it looks like we will enter a quieter pattern. Monday looks like a nice last day of astronomical summer for 95 percent of the area with near normal temperatures and a sunny to partly cloudy sky. There may be just enough moisture leftover for an isolated, and this means isolated with a capital I, shower or thunderstorm in and near the mountains east of the Divide. The chance is at most 1 out of 6 though, and nothing strong is expected. The only fly in the ointment is an upper-level low moving northeastward from the Desert Southwest that will open into a trough and pass through Tuesday afternoon through Wednesday morning. The GFS has trended toward the ECMWF, so the chance of showers and maybe a few thunderstorms looks better. This system is a fast mover so no heavy rain or flooding is expected out of it at this time. Any precipitation from this system should clear the area by sunset on Wednesday. Following that, ridging should build across the area and bring dry weather for the rest of the work week and possibly into the start of next weekend. And with generally southwest flow across the area, temperatures should average above normal, in some cases well above normal. Some of the warmer spots may see highs in the middle or even the upper 80s from Thursday through Saturday.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 405 PM MDT Sun Sep 20 2026

VFR conditions continue through 06Z/Monday as a slow-moving low pressure system finally moves into South Dakota and Nebraska. Ongoing convection along and east of the Continental Divide will generally wind down between 02Z-04Z/Monday. A few PROB30 groups linger into Sunday evening as a result. As the low slowly pinwheels east overnight, persistent mid-cloud ceilings rise and cloud cover dissipates. The clearing sky may introduce areas of fog east of the Continental Divide and at KJAC. Given what occurred at KJAC Sunday morning, have included a TEMPO group for at least MVFR fog Monday morning given continued moist boundary layer. The heavy rain at KRIW and KLND should lead to late night fog in the Wind River Basin, but this may stay closer to the river valleys. Therefore, confidence is not as high at these two terminals and have not included late night impacts at either terminal. KWRL could also be in the same boat. All terminals to be VFR after 18Z/Monday. The gusty surface wind 10- 20kts at KCPR fades early in the period. All terminals to see wind speeds less than 10kts through the remainder of the forecast period. Occasional mountain top obscurations through 06Z/Monday, with occasional foothill obscurations in the western valleys and central basins after 10Z/Monday.

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

RIW WATCHES/WARNINGS/ADVISORIES

None.