textproduct: Western and Central Wyoming

KEY MESSAGES

- Showers and thunderstorms are forecast through sunset across Johnson and Natrona Counties, with heavy rain and isolated flooding possible. A few showers and thunderstorms are possible (30% chance) across the Absarokas, Wind River Mountains, and Wind River Basin between 2PM and 8PM.

- Elsewhere across the state, drier conditions under partly sunny skies are expected throughout the day today. Expect widespread highs in the 60s.

- 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.

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 gradually drying as the low tracks to the east. We will still have the chance of 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 European, 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 workweek 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 mid or even the upper 80s from Thursday through Saturday.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1143 AM MDT Sun Sep 20 2026

IFR/LIFR conditions that have persisted at KJAC this morning are finally giving way to improving weather. KWRL should see an improvement to VFR conditions by mid-afternoon Sunday. Meanwhile, low pressure over Natrona County will allow for low- end VFR and occasional MVFR conditions through Sunday afternoon. Otherwise, other terminals to be VFR with the southwest terminals seeing generally only scattered cloud cover. The low spins slowly eastward today, finally reaching the eastern Wyoming border around midnight. Scattered showers and a few thunderstorms rotate southward on the backside of the low, providing for PROB30 convective showers later Sunday afternoon and early evening. KCPR has the best chance of seeing prevailing light rain as the low slowly pinwheels east. Mid-cloud decks persist east of the Continental Divide through the evening with gradually rising ceilings and dissipating cloud cover late tonight. Given what occurred at KJAC Sunday morning, have included TEMPO for at least MVFR fog Monday morning given continued moist boundary layer in the western valleys. Surface wind 10-20kts occurs at KCPR Sunday afternoon on the backside of the low, while most other terminals see speeds less than 12kts. Foothills frequently obscured until 20Z/Sunday with occasional mountain top obscurations through 06Z/Monday. Occasional foothill obscurations in the western valleys 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

Flood Watch until 6 PM MDT this evening for WYZ007>011.