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Thayne, Wyoming Weather Forecast Discussion

644
FXUS65 KRIW 201702
AFDRIW

Area Forecast Discussion National Weather Service Riverton WY 1102 AM MDT Sat Sep 20 2025

.KEY MESSAGES...

- A warm and mainly dry start to the weekend, with any showers restricted to southwestern Wyoming.

- Turning much cooler and wetter Sunday night and Monday as a cold front moves across the area.

- Mainly dry and warm weather returns for the middle and end of next week.

&&

.DISCUSSION... Issued at 257 AM MDT Sat Sep 20 2025

We are now entering the last summer weekend of 2025, and it will be a warm one to close it out. The basic reasoning of the forecast remains the same as the previous couple of days. For most locations, it will be a dry and warm start to the weekend, with temperatures averaging 5 to 10 degrees above normal. There will be a bit more convection around today though. Radar does show a few showers in northern Utah and these are moving into southern Lincoln County, but nothing appears to the reaching the ground at this time. We made few changes to the placement today, with showers mainly near the Unitas and up to around Afton, and even here chances are low, only around 1 out of 4 at the most. Deeper moisture and an approaching shortwave will keep showers and a few storms going into the night, with coverage expanding to much of the western mountains by sunrise Sunday. The bulk of the showers should hold off until Sunday though.

And speaking of Sunday, a cold front will move into western Wyoming and bring more widespread showers and thunderstorms for this time. Most guidance has slowed a bit though in the eastern progression of the front. As a result, most areas east of the Divide should be largely dry through the daylight hours. And, with increasing breezes as the pressure gradient tightens ahead of the front, and better mixing, this could be the warmest day of the weekend in places like Buffalo and Casper. There could be enough of a breeze for elevated fire weather as well, although humidity and wind does not look sufficient (only a 1 out of 5 chance) for critical fire weather at this time. Although some showers are possible Sunday evening east of the Divide, the bulk of the precipitation should hold off until Monday.

We have some of the same concerns for early next week this morning as we had yesterday. We have a high confidence (greater than 9 out of 10) that it will be a much cooler day than Sunday (possibly as much as 30 degrees cooler in northern Johnson County). We have high confidence in a breezy day, with the strongest wind in the favored cold advection / northwest post- frontal locations like Johnson County where some 40 mph wind gusts are likely. The continued uncertainty is with the progression of the upper level low behind the front and placement of the heaviest precipitation. There continues to be a spread of around 18 hours as to when the low will move away to the southeast, anywhere from later Monday night to Tuesday afternoon, depending on what guidance you look at. And there is little agreement on placement of the heaviest precipitation as well. Generally, it looks to be in northern Wyoming but exact locations are still up in the air. And finally, we still have the likelihood of some snow in the higher elevations of the northern mountains (generally above 9000 to 9500 feet). This should not pose many travel concerns, but the highest locations have at least a 1 in 2 chance of a couple of inches of new snow.

Following the departure of the low, things look relatively tranquil as ridging builds back into the area, bringing above normal temperatures and mainly dry conditions through much of the middle and end of next week.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1101 AM MDT Sat Sep 20 2025

VFR conditions prevail through the period. Isolated showers and thunderstorms will be around southwestern Wyoming this afternoon. KBPI and KRKS would have the best chances to see nearby showers.

Additional showers are possible tonight, coming in from the southwest around 12z. These could impact KBPI, KJAC, and KPNA (lesser chances at KRKS) between 15z and 18z. Mid-day instability will allow for shower and thunderstorm chances late Sunday morning and through Sunday afternoon.

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.

&&

$$

DISCUSSION...Hattings AVIATION...Wittmann

NWS RIW Office Area Forecast Discussion

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