textproduct: Western and Central Wyoming

KEY MESSAGES

- Widespread gusty winds begin to decrease around sunset tonight, with gusts of 30 to 45 mph remaining over higher elevations of the Absarokas and eastern Wind River Mountains.

- A flash freeze is possible (50%-70% chance) late this evening across northern Johnson County. The greatest impacts, including along Interstate 90 through Buffalo, are expected from 8PM tonight through Friday morning.

- 6 to 10 inches of snow are forecast (70%-90% chance) along and north of Powder River Pass from tonight into Friday.

- Little to no precipitation chances expected Saturday through early next week, with temperatures 15 to 20 degrees above normal.

UPDATE

Issued at 235 PM MST Thu Dec 11 2025

Strong northwest flow aloft, evident on 21Z water vapor imagery, continues to filter moisture rich air into the Intermountain West. With this 60kt-100kt upper level jet over the northeast half of Wyoming and a tight surface pressure gradient, gusty winds will continue through sunset across the Wind Corridor. These gusty winds will wane after sunset as high pressure slowly builds into the region from the southwest and as mixing subsides. Thus, the High Wind Warning is on track to expire at 5PM tonight (Thursday).

A change made on this afternoon's forecast package was the addition of a Winter Weather Advisory for northern Johnson County and the Bighorn Mountains. A disturbance embedded within the northwest flow aloft is on track to provide support for a potent, arctic cold front to scrape the northern portions of Wyoming. Ahead of the front, rain will be falling across northern portions of Johnson County and snow over the northern half of the Bighorn Mountains. Snow amounts of 6 to 10 inches are forecast along and north of Powder River Pass from 5PM today (Thursday) through 5PM Friday. Isolated amounts above 12 inches are possible (40%-60% chance) over the highest elevations of the Bighorns.

As for northern Johnson County, snow amounts of 0.5 to 2 inches are forecast through Friday evening. However, the threat of a flash freeze is more of concern during the overnight hours. This is what pushed us over the edge to issue the Winter Weather Advisory. As mentioned previously, rain will be occurring ahead of the arctic cold front. The cold front is on track to make its way into northern Johnson County between 5PM - 7PM this evening. There is high confidence that temperatures will quickly drop from the upper 40s around 5PM to below freezing by 11PM. This will cause roadways and sidewalks that were wet from evening rain to quickly freeze and become slippery. Rain will quickly turn to snow after 11PM. The greatest impacts are expected to be from 8PM tonight until 7AM Friday. Otherwise, the remainder of the forecast remains on track with much above normal temperatures and little to no precipitation chances as high pressure builds into the region.

DISCUSSION

Issued at 210 AM MST Thu Dec 11 2025

If there is one thing that Wyoming is known for weather-wise, it is the wind. The wind has been rather fierce around here for the past couple of days, with a few gusts in the triple digits. Thank you to a tight pressure gradient and the right front quadrant of an 140 knot jet passing overhead. We still have a few gusts over 75 mph, mainly around Chief Joseph Highway. But the wind has backed off a few ticks in most areas early this morning. But this is by Wyoming standards, this would be very windy by most areas standards.

And that is the main forecast predicament we find ourselves this morning. The High Wind Warnings will expire at 5 am this morning. General trends are for the wind to continue decreasing, as the jet is moving further north and the pressure gradient is beginning to relax somewhat. The 700 millibar winds are finally beginning to decrease, with all locations seeing them fall under 50 knots by around noon today. Probabilistic guidance also shows this trend, with chances of wind gusts over 50 mph falling to less than 1 out of 4 for most populated areas by noon. The exceptions are the usual suspects, mainly the south side of Casper and near Cody. Some of the deterministic MOS guidance does have a brief period of high wind around Casper this morning before decreasing in the afternoon. There could be a few stray high wind gusts at some of the mesonet sites, but these tend to be on high rock bluffs away from populated areas and have little impact. We will likely let the warnings expire, but this will be a game time decision as we still hold the right to extend them.

The second enigma (a problem, not the German new-age band popular in the 90s), is what will happen tonight and tomorrow across northern Wyoming. A cold front will sag southward toward the area from Montana and likely bring some snow or rain turning to snow across this location. I do get nervous when I see a boundary with some jet energy here. Just about all guidance keeps the best forcing over Montana and away from Wyoming. As for snowfall amounts, Johnson County will be the most impacted as far as the lower elevations. Chances of advisory amounts of snow are low though. At Buffalo, there is only a 1 out of 6 chance of 3 inches or more with around a 1 out of 3 chance of 2 inches or more. As for the mountains, the most would likely fall in the western Bighorn Range. A few locations have a 1 in 2 chance of 6 inches of more, but these are the highest elevations away from the mountain passes. A small part of the northern Absarokas has a 1 in 2 chance of 6 inches of snow or more. However, this is also in a remote area and Beartooth Highway is closed for the season. So, no advisory here as well. The steadiest snow will fall from late tonight through Friday morning before gradually ending Friday afternoon.

There is one more notable thing, high temperatures today. This will especially be the case across central Wyoming. The MOS guidance has high temperatures in the 60s in Riverton and in Casper. If it occurs at the Riverton Airport, it would only be the fourth sixty degree high in December in almost 30 years, and the latest by nine days. In Casper, which has records back to 1939, it would only be the fourteenth time in station history. Most locations across the area will be 20 to as much as 30 degrees above normal for high temperatures. It will cool off on Friday behind the front, but most locations, except for where snow will be falling will remain above normal.

Following that, it looks like we will enter into a quieter period. Flat ridging will build across the area and bring mainly dry conditions through Monday night. The next system looks to approach the state on Tuesday or Wednesday from the west. With this pattern, it would mean snow across the western mountains with favorable upslope westerly flow with mainly dry conditions to the east with downsloping. We could have another high wind event though, as the pressure gradient looks rather tight and a powerful jet, possibly over 150 knots, might be in place over Wyoming. It is still a long way out, but the potential is here. With the westerly flow, temperatures look to remain above normal with no signs of arctic air masses for the next seven days.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 404 PM MST Thu Dec 11 2025

VFR conditions prevail through the period. Winds will continue to be the main hazard, which lessen some overnight. A cold front dips into northern Wyoming early in the period, with northerly wind shifts at KCOD, KWRL, and then KCPR. Precipitation will be limited to far northern Wyoming and the mountains, with about a 30 percent chance at KCOD, less for KWRL. For other sites, increased mid-to-high levels clouds occur, with no precipitation expected.

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 Weather Advisory until 7 AM MST Friday for WYZ008>010.

High Wind Warning until 5 PM MST this afternoon for WYZ019-020- 022-030.