textproduct: Western and Central Wyoming

KEY MESSAGES

- Snow continues tonight into the day Monday with 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 tonight, 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.

UPDATE

Issued at 1234 PM MST Sun Dec 21 2025

Snow continues ramp up across the western mountains this afternoon. Snow accumulation rates have been 1 to 2 inches per hour, which will continue into the day Monday before tapering off Monday afternoon. Temperatures in the valleys have been resisting any attempt to rise above freezing this afternoon, with snow still occurring at KJAC, but rain at Alpine and Afton and just starting to switch over at Hoback Junction. If rain doesn't switch over by mid-afternoon, we would need an Advisory for the greater Jackson Hole, as snow accumulations would be considerably greater than currently forecast. Remaining Warnings remain in effect with no changes, other than noting that while the Wind River Range will see 20 to 40 inches, South Pass itself would only receive 2 to 4 inches.

Remaining forecast remains on track for tomorrow, with another breezy day, snow for the western mountains, and temperatures east of the Divide in the mid to upper 50s, likely easily breaking record high temperatures...

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 /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 410 PM MST Sun Dec 21 2025

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

A weather system impacting western Wyoming will bring MVFR to IFR flight conditions to KJAC, with lesser chances (20-30%) of IFR conditions at KPNA and KBPI. Light rain will fall most of the TAF period at KJAC with the most likely probable time of a heavier rain shower between 03Z and 09Z. The occurrence of light rain or a rain/snow mix is more uncertain at KPNA and KBPI; however, MVFR ceilings are still forecast. Continued mountain obscuration can be expected. KRKS will likely (90%) remain precipitation free with VFR flight conditions.

With strong west-southwest flow aloft, wind shear and mountain wave activity is most likely to occur at KJAC but cannot be ruled out at KBPI (30% confidence). Gusty winds are forecast throughout much of the the TAF period at all west of the Divide terminals.

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

Confidence is high (80-90%) that VFR flight conditions will persist the entire TAF period for all east of the Divide terminals. KCPR will see the gustiest winds this TAF period as west-southwest flow aloft strengthens. The flow aloft is forecast to be the strongest between 00Z and 18Z Monday. Winds become elevated or gusty at all other east of the Divide terminals between 03Z to 04Z Monday. With the strong winds aloft, wind shear and mountain wave activity is most likely to occur at KCOD and KLND. There is a 20-30% chance in a stray rain shower coming off the mountains around KLND and KCOD through 02Z. Confidence is higher (30%) at KLND so this chance continues to be reflected in a PROB30 group. Given the lower confidence at KCOD, opted to keep any mention of precipitation out of the TAF at this time. Occasional mountain obscurations are possible near KCOD and KLND.

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.