textproduct: Western and Central Wyoming

KEY MESSAGES

- Snow returns to western portions tonight, as the start of a multi-day heavy snow event. Snowfall amounts of 18 to 36 inches are expected through Monday night.

- Winds of 30 to 40 mph can be expected Sunday and Sunday night, adding to the already difficult travel conditions over Teton and Togwotee Passes.

- Snow is expected in the western valleys at times, but will change over to rain by Sunday afternoon.

- Above normal, possibly record high, temperatures, and windy conditions Monday through Christmas Eve.

UPDATE

Issued at 1230 PM MST Sat Dec 20 2025

Winds have been relatively tame compared to the past few days, with the only higher gusts noted along I-80. Most areas should see limited wind activity through the rest of the day, with higher gusts limited to the wind corridor from Rock Springs to Casper. The atmospheric river tap of moisture begins to surge into the western mountains tonight, with a good 24 to 36 hours of snow for the western mountains. Totals have remained consistent, so have hoisted Winter Storm Warnings for the western ranges tonight through around sunrise Monday. The lower valleys will see snow tonight, with an inch or two for Star Valley and Jackson Hole through Sunday morning. These areas will switch over to rain by noontime Sunday, and will remain rain through the duration. With a total liquid precip over an inch at Jackson through Monday, some ponding is possible around the region, and temperatures near freezing Sunday night will bring some icy conditions to wet areas. Still looks like the mountains will wring out all the moisture from this atmospheric river, so areas east of the Divide, along with Sweetwater County, will not see precip from this episode.

DISCUSSION

Issued at 210 AM MST Sat Dec 20 2025

As of 07Z, the cold front has reached Casper and Riverton and is expected to reach the Divide in the next few hours. Temperatures at these locations have dropped below freezing, with bands of snow occurring over southern portions of the Bighorn Basin and Johnson County, Natrona County along the US-20/26 Corridor and off the southern end of the Wind River Mountains. Snow will also continue to fall over the Salt River/Wyoming Ranges and the Wind River Mountains through the morning hours with an additional 2 to 4 inches expected. Most of this will fall before sunrise. Rain showers are also expected along the I-80 Corridor shortly, pushing south of the Interstate by 10Z. Temperatures are currently in the upper 30s/lower 40s and are not expected to drop below freezing until 12Z. Winds continue to decrease behind the front, with gusts of 25 to 45 mph continuing across southern portions. These winds will gradually decrease by sunrise this morning, with the east slopes of the Wind River Mountains continuing to gust 45 to 55 mph. This will be a brief lull, as west-northwest winds increase to gusts of 25 to 45 mph by midday. These winds will occur for most areas, except eastern portions of the Bighorn Basin and northern Johnson County. Temperatures will be closer to seasonal levels, but still slightly above normal for this time of December.

Precipitation chances will increase over western portions after 06Z Sunday (11pm tonight), as a plume of Pacific moisture moves over the region. This will be part of an Atmospheric River moving onshore over northern CA today and be a multi-day event. Precipitation will likely start as snow in the Jackson/Star Valleys tonight, changing over to rain by the afternoon. Snow levels will range between 7000 and 8000 ft over the western mountains by 18Z Sunday, with moderate to heavy snow through the day and Sunday night. Precipitable water values will be close to record values with this event. This, coupled with favorable west-southwest upslope flow, will lead to snowfall amounts of 18 to 36 inches through Monday night. The bulk of this will occur through Sunday night, with snow becoming light during the day Monday. Wind gusts up to 30 mph will also occur with the snow, adding to the travel hazard over Teton and Togwotee Passes. One of the challenges with the forecast will be if temperatures drop below freezing in the western valleys Sunday night.

Elsewhere, it will be a windy day again Sunday with southwest winds gusting 25 to 35 mph across southern portions and the Wind Corridor. Exceptions will be portions of Natrona County and southern Fremont County, where gusts of 35 to 45 mph will be common. Locally higher gusts up to 60 mph will be possible over South Pass and Hwy 258 on the south side of Casper. There is a possibility for high wind Sunday night into Monday, as 700mb winds increase to 50kt over the Cody Foothills, South Pass and Natrona County. A cold front will move over areas east of the Divide Monday. No precipitation is expected either ahead or behind the front.

The impacts from the Atmospheric River will continue to wane Tuesday, as a ridge begins to build over the region. This will lead to another day of above normal temperatures (possibly record high temps), with highs in the 40s and 50s. Any precipitation will be confined to far northwestern portions, as the ridge shunts most of the moisture to the north and west. Dry, warm and windy conditions are likely Christmas Eve. Another storm system will impact the CWA on Christmas, with western portions having the best chance for a White Christmas. There is the potential that this will be another windy day, as the flow aloft will be southwesterly.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 406 PM MST Sat Dec 20 2025

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

An incoming weather system brings increasing clouds to start the period. A push of snow will begin to move into KJAC after 06Z/Sun. Conditions should quickly drop as low clouds and snow moves in. MVFR to LIFR conditions are expected from visibility drops due to snow and low ceilings. Once they start, these conditions should persist through about 17z/Sun, when snow begins to transition to a snow/rain mix, and then just rain as daytime heating increases. With this transition, conditions should improve some with visibilities increasing and cloud ceilings rising.

KRKS is least likely to see precipitation, with just isolated (30%) rain chances late Sunday morning into the afternoon. KBPI may also see rain chances drop in the afternoon, but currently have rain prevailing through the end of the period due to lower confidence.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

Gusty winds decrease shortly after sunset this evening. Winds coming off the Absaroka Mountains bring wind shear to KCOD through tonight. Clouds increase through the period as a weather system moves into western Wyoming. Precipitation should stay along and west of the Continental Divide, but a stray shower could move off the mountains and over KCOD or KLND (30% chance).

Winds are expected to increase and become gusty at KCPR around 16Z/Sun as southwesterly winds aloft increase. Low confidence in wind direction at KLND starting late Sunday morning, as its unclear if the westerly wind will keep the easterly push out. For now, have a prevailing northeast wind, with wind shear from the west for the wind aloft coming off the Wind River Mountains.

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 until 5 AM MST Monday for WYZ001-002-012- 014-015-024.