textproduct: Western and Central Wyoming

KEY MESSAGES

- A weather system will move across the area and bring some showers, most numerous in western Wyoming and during the morning hours.

- A more substantial system crosses Wyoming Sunday into Monday, bring periods of showers and higher elevations snow. The most impacted areas will be western Wyoming Sunday and Sunday evening, and East of the Divide later Sunday night into Monday.

- Strong wind is likely (greater than a 2 in 3 chance) on Monday to East of the Divide, with gusts over 50 mph possible in Johnson County and the northern Bighorn Basin.

DISCUSSION

Issued at 120 AM MDT Sat Oct 31 2026

Well, the day has arrived. Some people think it is the best holiday of the year, and in my opinion it is one of the best. A chance to be someone or something else, and of course, copious amounts of candy. The question is whether or not the weather will be spooky or if the weather will give us a trick or a treat on this last day of October and beyond. Well, you will find out in the discussion, that is why you are here after all.

We still have the two weather systems that will impact the area over the next three days or so. The first is now moving northward through Utah and Colorado and will cross the area today, lifting northward into Montana by later this afternoon. This is the weaker of the two systems. Reasoning remains fairly similar to yesterday. It is that the most numerous showers will occur over western Wyoming (as high as a 4 out of 5 chance) with tapering chances further east. A shower can't be ruled out anywhere, but the smallest chance would be in Johnson and Natrona Counties where the chance is only 1 out of 10. The most numerous showers would occur through early afternoon, tapering off later in the afternoon with a lull expected to last most of the night. So, it looks like the weather has given us a treat when it comes to trick or treating earlier in the evening and for any adult gatherings later at night. The main chance of a shower this evening would again be in western Wyoming, but chance is less than 1 out of 4, at least through at least midnight. As for amounts, the highest amounts would be focused in areas prone to southwest upslope flow, like the Western Wind River Range. The chance of two tenths or greater would be confined to areas West of the Divide. Snow levels will remain above 8000 feet through this time as well, so impacts to travel would be minimal.

The main event still looks to be Sunday into Monday. The main players continue to be a long wave trough that will cross Wyoming Sunday night into Monday with associated upper level low that will pass into Colorado and then move northeast, inducing some lee cyclogenesis over Colorado. This will also phase with a stream of Pacific moisture as it moves into the Rockies. It still looks like the deepest moisture and heaviest precipitation will remain south of the area over Utah, Colorado and the desert Southwest, but most areas should see some precipitation from it. And, this will be a tale of two different upslope / downslope patterns. The steadiest and heaviest precipitation on Sunday into Sunday evening will occur in western Wyoming and ares favored by west to northwesterly upslope, mainly to Tetons, Salt and Wyoming Range and the western Wind River Range. Then from later Sunday night into Monday, as the low passes to the east, the heaviest will shift East of the Divide to places favored by northerly upslope as moisture wraps around the developing low. This favors places like from south of Riverton to Lander and eastward to Casper, the southern Bighorn Basin from Thermopolis to south of Worland as well as the Cody Foothills and the Big Horn Range. With the north flow, places like the eastern Bighorn Basin, northern Wind River Basin and much of the Green River Basin would see the least with downsloping flow.

Then there are two more questions, how much total precipitation and potential, snow and how much could fall. We will start with precipitation. Most areas except the southern Green River Basin have at least a 1 in 2 chance of a quarter of an inch or QPF between 6 am Sunday and 6 am Tuesday. The aforementioned favored areas that were mentioned above are the areas what have at least a 1 in 2 chance of over three quarters of an inch of QPF in that same timeframe. The greater than 1 in 2 chance of inch or more is largely restricted to the highest elevations though. Precipitation should taper off by sunset Monday, with all areas dry by Tuesday morning.

Now for the more juicy or should i say, cooler question, what about snow? The air with this system is of Pacific origin, and as a result not the coldest in the world. The coolest 700 millibar temperatures look to drop to is minus 4, which would keep snow levels at roughly 6500 feet or higher. So, most population centers and roads outside of the mountain passes would see mainly rain or only wet snow that would not cause many problems. Most locations above 7500 feet should see some accumulation though. As for advisory amounts of snow (6 to 12 inches of snow in the mountains), the chance of this is largely above 8500 feet. However, this does include Teton and Togwotee Passes in western Wyoming as well as Granite and Powder River Passes in the Bighorn Range. We will likely (a greater than 3 out of 5 chance of some advisories in the mountain ranges). However, with main impact Sunday afternoon to Monday morning, we will hold off for now and would issue them this afternoon or at this time tomorrow night. As for the chance of greater than a foot, the best chance would be in the Bighorn Range where there would be a longer period of upslope and wrap around moisture. There are decent areas of a greater than 1 in 2 chance of greater than a foot of snow but these are mainly above 10500 feet and above pass level. So, we favor advisories at this time.

And there is one more concern for Monday. As the low deepens over the plains, the pressure gradient will tighten and bring some gust y to strong wind. Some of the latest guidance is showing at least 35 knots of northerly flow at 700 millibar flow across the eastern two thirds of the area. There are even some 50 knot barbs across portions of Johnson County on Monday afternoon. The NBM ensemble shows a greater than 1 in 2 chance wind gusts over 55 mph for this timeframe so we may need a high wind watch for Monday as well.

Quieter weather looks to move in for Tuesday and Wednesday with gradually warming temperatures. A weak system may bring some showers on Thursday but it does not look like a big deal at this time. Temperatures should average near to above normal from Tuesday to the end of the workweek on Friday.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 948 PM MDT Fri Oct 30 2026

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

A weather system will bring rain and plentiful cloud cover to the area through the TAF period. Precipitation type should remain mostly rain at terminals, with snow in the mountains. Some snow may mix in at KJAC, KBPI, and KPNA late tonight into early Saturday morning. Conditions will drop to MVFR at times. There may be some isolated thunder Saturday afternoon, with more scattered convective showers in the area. Wind will remain quite light (less than 8 knots) at all terminals. Frequent mountain obscuration, with some improvement Saturday evening.

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

There will be minor impacts from a weather system moving through the area tonight into tomorrow. Overall it will be mostly dry with this weather system. Clouds will drop to low VFR to near MVFR CIGS Saturday morning. Most terminals have a PROB30 of light rain at some point tonight into Saturday morning, with the exception of KCPR where the chance will be too low (20% late Saturday morning). Terminals will have light wind (below 8 knots), with the exception of KCPR. KCPR will have wind increase late Saturday morning to around 15-20 knots, with gusts around 25-30 knots through the afternoon before weakening around 00Z Sunday. Frequent mountain obscuration through Saturday morning with improvement Saturday afternoon.

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

None.