textproduct: Western and Central Wyoming

KEY MESSAGES

- One more day of above normal temperatures across the area. Showers and thunderstorms become more numerous, especially tonight.

- The period from Sunday through early Monday looks like the wettest period, with locally heavy rain possible, especially for the western mountains.

- Cooler temperatures move in starting Sunday. Several inches of snow are likely (a greater than 1 in 2 chance) in the mountains above 9500 feet. The chance of snow in the lower elevations remains uncertain.

- Periods of showers will continue through the weekend, although details of rainfall amount and placement of heaviest rain remains uncertain, especially from Tuesday and beyond.

DISCUSSION

Issued at 250 AM MDT Sat Oct 10 2026

Being a member of Gen X, I was the first generation to watch Sesame Street when I was a wee lad many eons ago. And at the end, right before the closing credits, they always said the episode was sponsored by two letters. Well, this Area Forecast Discussion is sponsored by the letters T and U. T stands for transition, as we move from the warm and dry pattern into a cooler and wetter one. And U is for uncertainty, as are still a fair amount of questions with the forecast, especially in regards to the snowfall potential at the lower elevations and details of the forecast toward the middle and end of next week.

The T for transition will be today. We had a few showers yesterday, and radar shows more showers, generally of the light variety moving from Idaho and Utah with the first hits of moisture. The players in the forecast remain the same, a cold front dropping in from Montana, an upper level low and associated trough dropping in from the Pacific Northwest and Tropical Storm Rachel. The cold front should remain largely north of the border today, so we will likely have one more warm day today, maybe the warmest for the rest of the calendar year. Some warmer spots will likely see another high in the 90s. We will also have continued gusty wind in the favored southwest flow locations from Rock Springs to southern Johnson County. As for showers, moisture from Rachel will increase through the day, drawn northward by the trough. The best chance of showers and storms will be largely West of the Divide today as downsloping flow should limit shower East of the Divide through the daylight hours. There will still be some, but not as numerous as West of the Divide. And there will likely be some thunderstorms today, as there is decent CAPE for October, up to 300 J/Kg with lifted indices down to minus 2. Coverage of showers looks to increase tonight as deeper moisture moves northward and some jet energy moves into the area, increasing upper level divergence.

The wettest day still looks to be Sunday, as the deepest moisture from Rachel moves across the area. Reasoning today is fairly similar to yesterday. There will be locally heavy rain. Precipitable water values could be as high as 0.75 inches, which is close to 200 percent of the mean and could be a daily record for Sunday. The jet will also be nearby, keeping upper level divergence around. Again, the placement of the heaviest precipitation will be across western Wyoming, with decreasing chances and amounts further to the east. I will discuss potential amounts later in the discussion. I did give some consideration to flood watches, but decided against them for now for the following reasons. One, it has been very dry over the past couple of weeks, so there is decent amount of room in the soil to soak up some of the rain. Two, the cold front will be further south today, and instability will be limited. So, precipitation looks more stratiform than convective, which should limit precipitation rates somewhat. Three, some of the highest elevations may be seeing snow at this point, limiting runoff. I will discuss snowfall mounts later in the discussion as well. And four, most deterministic models and a majority of the ensemble members show the best QPF remaining west of Wyoming. The area I would be most concerned about would be the western slopes of the Wind River Range with the upslope flow, the Tetons as well as Yellowstone Park. We still have time to decide though as the heaviest rain wouldn't be until Sunday, with most guidance zeroing in between noon and late Sunday night, with some decrease after that as the deepest moisture from Rachel moves away to the north and east.

By Monday, the dominant feature will be the Upper Level which by this point will begin to turn eastward toward the Rockies. Most guidance shows lighter precipitation Monday afternoon, and most confined to areas West of the Divide by that time, with only lighter amounts East, Most will also be confined to southern Wyoming, with northern Wyoming largely dry.

We will now discuss precipitation amounts, I will reference from Saturday night through Monday, when the most rain is expected to fall. The vast majority of the area has at least a 1 in 2 chance of a quarter of an inch or precipitation or more, with the smallest chance south and east of a Rock Springs to Casper line. Chances of an inch or more (greater than 1 in 4 chance) are largely confined the northwestern Wyoming as well as the Bighorns where there will be an upslope component. The areas with a greater than 2 inches of precipitation, which is where the best chance of excessive rainfall would be is in western Yellowstone Park, the Tetons and the Wind River Range. However, some of this precipitation over the higher elevations will likely fall in the form of snow, especially Sunday through Monday when the cooler air moves out of Montana. Additional rainfall will occur Monday night but amounts look lighter at this time.

And now we will focus on the frozen form of precipitation, snow. With warmer air remaining over the area today, any snow would be restricted to the highest elevations, above 11000 feet for the most part. Cooler air begins to ooze into the area Saturday night and especially Sunday, lowering snow levels. Levels bottom out late Sunday into Sunday night. Across Central Wyoming, with 700 millibar temperatures bottoming out between plus 1 to plus 3, this should keep snow levels above 9000 feet, above most passes, Togwotee Pass could see some wet snow, but any impacts should be minimal. As for highlights, we have held off for now. Looking at the NBM ensemble 72 hour snowfall through 6 am on Tuesday the areas that have a greater than 1 in 2 chance of six inches or more are largely restricted to portions of the Absarokas and the Wind River Range, largely above 10000 to 11000 feet. Portions of the Wind River Range will almost certainly get at least a foot of snow, and possibly over two feet of new snow. However, this would only be at the highest elevations where impacts would be very limited. We will let the day shift take another look. In the lower elevations, the main area of concern would be northern Park County, where 700 millibar temperatures will be the coolest. The question that remains is will there be enough dynamic cooling to cool temperatures at the surface enough. Ensembles only give a 1 in 5 chance of an inch or more here, but model performance in this area is poor and we still have a decent setup with the overrunning moisture and the jet nearby. It wouldn't be until Sunday or Sunday night so there is still time to take another look but this has to be watched.

And following this is where the U for uncertainty comes in. And it has to do with the progress of the upper level low, as guidance has vastly different solutions with it. Tuesday has some agreement with showery weather during the day and possibly more at night as deeper moisture moves toward the area. Following this is were there is a large amount of divergence. The GFS as well as some ensemble members pick up the upper level low and move it quickly across the state getting kicked by a trough moving across the US-Candian border, bringing a wet Wednesday then moving it out of the area for later in the week for a return to drier weather. The European has the upper level low missing the connection and getting blocked by a building ridge to the west, cutting the low off over the Great Basin and keeping potentially unsettled weather into the end of the week. Confidence is very low on which will happen at this time though. So, there it is a (very long) Area Forecast Discussion brought to you by the letters T and U.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 416 AM MDT Sat Oct 10 2026

Showers will spread across the area today. Before 21Z, showers should be largely confined to terminals West of the Continental Divide, with the nest chance in vicinity of KJAC, KBPI and KPNA where PROB30 groups continue. Thunderstorms are also possible this afternoon. The chance of showers should spread to terminals East of the Divide after 21Z, although the best chance for showers across the area will be after 04Z tonight as deeper moisture spreads northward into the area. MVFR conditions are possible in vicinity of any shower or thunderstorm, with the best chance after 04Z tonight as more numerous showers move northeastward across most of the area with the possible exception of KCPR.

Wind will increase again between 15Z and 18Z, with gusts to 25 to 35 knots possible at most terminals. The strongest wind will likely be in vicinity of KCPR where occasional gusts to 40 knots will be possible. Wind could be stronger in outflows around thunderstorms but these are very difficult to time at this time. Gradient wind should decrease after 01Z as the sunsets and the atmosphere gradually decouples.

RIW WATCHES/WARNINGS/ADVISORIES

None.