textproduct: Western and Central Wyoming

KEY MESSAGES

- Snow will spread into the north mountains tonight through Wednesday, with the highest amounts in the Absarokas.

- Strong to high wind is likely (greater than a 9 or 10 chance) in much of central and southern Wyoming starting tonight, with the strongest wind in Sweetwater County.

- Above normal temperatures will continue for the next few days.

DISCUSSION

Issued at 306 AM MST Mon Dec 8 2025

Sometimes I think the weather is like a rubber band. Basically, you can have a long stretch of quiet weather, but each day builds up tension in the band. Eventually, it ends up snapping. And this is the case over the next few days. Much of November was very quiet and tranquil weather wise. And we are now in one of the more active periods we have had in a while. And a rather complex one, with most threats including wind and snow. In a bit of a change in format, we will hit each one of these individually followed by a general outlook of aspects of the forecast that are less impactful as well as the weather past Wednesday night.

SNOW DISCUSSION...We continue to have some mainly light snow falling over the western mountains as I write this around 2 am. It is beginning to let up in the Tetons, so we will likely expire the advisory around 5 am. There should then be a lull in the snow for around 12 to 24 hours before the next round of snow moves in from north to south as a boundary sags southward and a potent 140 knot jet sags southward toward the area, enhancing upper level divergence. The highest confidence for heavy snow continue to be in the Absarokas where there will be some orographic enhancement to go along with the jet energy. Many locations here have greater than a 9 in 10 chance of a foot of new snow with some of the southern portions of the range having nearly a 1 in 2 chance of 2 feet or more. This, combined with some strong wind, looks to be the most impacted area. So, we have issued a Winter Storm Warning here starting tonight and continuing through Wednesday afternoon. Southern portions of Yellowstone Park also have a greater than 3 out of 5 chance of a foot of new snow, but lesser amounts elsewhere. As a result, we had more questions about a warning here, but with the strong wind we went ahead and issued and also to match up with our neighbors to the north. Northern portions of the Park will likely only see advisory amounts though.

Further south there are more questions, as the dynamics are not as favorable here with the jet being further away. Nevertheless, a decent portion of the Tetons, Wind Rivers and Salt and Wyoming Range have at least a 1 in 2 chance of a foot of new snow. I'm not completely convinced yet though. Since the bulk of the snow here would not fall until at least Tuesday, we have elected to go with watches and let future shifts decide between warnings, advisories or even nothing. As for the Bighorns, there is at least a 3 out 4 chance of greater than 6 inches of new snow, but the chance of a foot is generally less than 1 out of 3. Any impactful snow would hold off until Tuesday afternoon or evening, so we still have time to decide on any highlights here.

As for the lower elevations, impacts look minimal. Warmer air will try to push into the area, especially on Wednesday. With 700 millibar temperatures expected to rise as high as minus 2 by Wednesday, snow levels may rise as high as 7500 feet, keeping the valleys a rain / snow mix and even all rain at times. Chances of 3 inches of snow or more are basically zero because of this. As for the lower elevations East of the Divide, there could be some light snow in the Big Horn Basin and Johnson County. But, with the warmer air moving in, especially on Wednesday, any accumulation would be negligible.

One caveat is that there is still some spread in the details of timing of the periods of heaviest snow. The deterministic runs have a lot of differences as well, with the European much wetter than the GFS for example.

WIND DISCUSSION...We are already starting to see some gusty wind develop in places like Casper this morning. There has been a bit of a change in that the jet has moved a bit further north, and this brought some of the strongest wind gusts down a bit. However, we still have a good set up for strong wind much of southern and central Wyoming in the right front quadrant of the jet, a tight pressure gradient and strong winds at 700 millibars. The strongest 700 millibar winds are now down to around 60 to 65 knots as opposed to the 80 knots yesterday, guidance does tend to be a bit bold a few days out. Nevertheless, we still have a very good chance of high wind across much of the area. Much of the southwestern wind corridor from Kemmerer to Rock Springs to Casper has at least a 3 in 4 chance of wind gusts past 55 mph sometime between Monday night and Wednesday night. With this, we have upgraded the High Wind Watch to warning for all of the zones starting at 11 pm tonight. The most prolonged high wind will be across southern Lincoln and Sweetwater County, this includes Interstate 80 as well. In Natrona County, Tuesday looks like the most impactful day with a greater than 4 out of 5 chance of wind gusts past 55 mph. Chances drop a bit on Wednesday, but not enough confidence to drop it for this day.

There are two other areas of concern. One is in northern Johnson County, which could see a brief period of high wind Wednesday afternoon as flow turns northwest, with some areas having a greater than 1 in 2 chance of wind gusts past 50 mph. The other is the Upper Wind River Basin, which has a similar set up. Again, this would likely not be until Tuesday, so there is still time to decide. Gusty winds are possible elsewhere, but changes of high wind are 1 out of 4 or less.

REST OF THE FORECAST...It will be fairly mild outside of the mountains, with some areas seeing highs into the 40s and even the 50s, especially on Wednesday as flow becomes southwesterly. We are also watching another piece of energy that could bring some precipitation to northern Wyoming later Thursday into Friday. There is a large spread in guidance though so confidence remains rather low. Looking ahead to next weekend, at this point to looks largely dry with a gusty wind and above normal temperatures, but confidence remains rather low.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 417 AM MST Mon Dec 8 2025

Snow and low clouds starting to lift around KJAC currently, with continued improvements through the morning. Main aviation issue today will be gusty winds across the state. Most areas will be gusting around 20kts, but KRKS and KCPR will have frequent gusts above 30kts. Winds will stay elevated overnight with the approach of the next system, and increasing at all sites. The next weather system will begin to spread some light snow into KJAC again after sunset, with better chances waiting until well after 06Z/09

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

Winter Storm Warning from 5 PM this afternoon to 5 PM MST Wednesday for WYZ001-002.

Winter Weather Advisory until 5 AM MST early this morning for WYZ012.

Winter Storm Watch from late tonight through Wednesday afternoon for WYZ012-014-015-024.

High Wind Warning from 11 PM this evening to 5 AM MST Thursday for WYZ019-020-022-027>030.