textproduct: Western and Central Wyoming

KEY MESSAGES

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

- Winds of 35 to 45 mph can be expected Sunday and Sunday night, adding to the already difficult travel conditions over Teton and Togwotee Passes. These winds will also occur from southern Lincoln County to southern Johnson County. Locally higher wind gusts of 60 to 80 mph will occur near Casper/Casper Mountain and over South Pass/Red Canyon, respectively.

- Snow is expected in the western valleys at times this morning, but will change over to rain by the afternoon. Rainfall amounts of 0.80 to 1.25" will fall Monday afternoon in the Jackson/Star Valleys, Hoback Junction and Bondurant as well. Ponding of water, and even some icy conditions, will be possible tonight, as temperatures drop to near freezing.

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

DISCUSSION

Issued at 136 AM MST Sun Dec 21 2025

Light bands of snow over eastern ID are pushing into far western portions of the CWA, as well as the Upper Green River Basin, as of 07Z. This is the beginning of the incoming atmospheric river that was discussed in the previous discussion. Overall, little has changed with that forecast: Precipitation will start as snow in the Jackson/Star Valleys, with 1 to 2 inches expected before changing over to rain after 18Z today. This will be due to rising snow levels, ranging between 7000 and 8000 ft by 00Z Monday. Snowfall rates over the mountains will quickly increase this morning, with rates of 2"/hr expected through the afternoon. Periods of moderate to heavy wet snow will continue tonight into Monday morning, becoming light Monday afternoon. Will extend the Winter Storm Warnings to 00Z (5pm Monday) as a result. Precipitable water values will be close to record values with this event. This, coupled with favorable west-southwest upslope flow, will lead to this relatively brief period of heavy wet snow. Snowfall amounts of 18 to 36 inches, with locally higher amounts close to 40 inches, are expected through Monday afternoon. Rainfall amounts of 0.80" to 1.25" will fall during this timeframe in the Jackson/Star Valleys, Hoback Junction and Bondurant as well. Ponding of water, and even some icy conditions, will be possible tonight, as temperatures drop to near freezing. If this wasn't enough, southwest wind gusts of 35 to 45 mph will be in place over western portions (including the valleys) by 18Z creating areas of blowing snow in the mountains. This will add to an already hazardous situation over Teton and Togwotee Passes, as well as on the ski slopes. These winds will continue through the overnight hours tonight into Monday morning. Snow and wind will begin to decrease late Monday morning, with snow becoming light in the afternoon and wind gusts decreasing to 25 mph.

Elsewhere, it will be a windy day again today with southwest wind gusts of 40 to 55 mph from southern Lincoln County to southern Johnson County. Locally higher gusts of 60 (south side of Casper and Casper Mountain) to 80 (South Pass/Red Canyon) mph will occur. Some of these higher winds will also occur over the east slopes of the Wind River, Absaroka and Bighorn Mountains. Winds will decrease a bit overnight (30 to 45 mph) tonight and not be as widespread. They will increase again by midday Monday and be similar to today (gusts of 40 to 55 mph). There is also a signal these high winds will impact Lander some time tonight. The timing of this occurring is difficult due to the nature of a breaking mountain wave. Models are currently forecasting this to occur sometime between 03Z and 12Z Monday. Apart from the higher peaks of the Bighorns and the Upper Wind River Basin near Dubois, areas east of the Divide will stay dry from this event as the western mountains will keep any moisture from crossing the Divide. These windy conditions will also lead to above normal temperatures, with highs in the upper 40s to middle 50s this afternoon. Highs will approach the 60 degree mark again Monday, namely the Casper area, Riverton, and southern portions of the Bighorn Basin and Johnson County.

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 (record high temps likely), with highs in the 50s to near 60 degrees. Any precipitation will be confined to far northwestern portions, as the ridge shunts most of the moisture to the north and west. Dry and windy conditions are likely again Christmas Eve, with record high temperatures possible once again. Another storm system looks to be moving onshore over the PACNW on Christmas Eve/Christmas, possibly impacting western portions with a White Christmas.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 303 AM MST Sun Dec 21 2025

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

KJAC is expected to see MVFR to IFR impacts through most of the period, as well as at KBPI and KPNA much of the time.These conditions should last through the morning before snow gradually makes a transition to a rain/snow mix and then likely all rain. This should lead to a slight improvement in ceilings and visibility, though confidence remains low on totally breaking out of IFR, especially at KJAC. Mountain obscurations will be frequent through the period. KRKS is not expected to see precipitation impacts. Gusty winds will be seen with this system, especially in the afternoon. This will be most notable at KJAC, which will first see strong southwest wind above the surface before surface gusts reach 30 knots after 00Z Monday and continuing through Sunday night.

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

Precipitation will largely be confined west of the Divide. VFR conditions are expected through tonight, with wind beginning to pick up at KCPR by 16Z. KCOD and KLND will have the best chance of seeing at least brief precipitation impacts coming off of the mountains through the afternoon; KLND will carry a PROB30 group to account for this potential, but not enough confidence to include KCOD for now. Occasional mountain obscurations are possible in the Wind River Range and the Absarokas. There is also the potential for strong winds at KLND after 17Z as a mountain wave develops, though confidence remains low enough to prevail this wind. Wind shear at KLND is much more certain during this period. KCPR is expected to see southwesterly gusts to 40 knots after 00Z Monday.

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 PM MST Monday for WYZ001-002-012- 014-015-024.