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, bringing periods of showers and higher elevation 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 the chance is less than 1 out of 4, at least through 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 Ranges, 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 Bighorn 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) have 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 gusty 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 /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 457 AM MDT Sat Oct 31 2026

A weather system will move through Wyoming today and bring some impacts, especially in the morning. Areas of fog have formed in vicinity of KCPR, with occasional LIFR ceilings until around 15Z to 16Z when increasing southwest wind should dissipate the fog. Low ceilings, at times IFR to LIFR along with patchy fog will also be possible in vicinity of KCOD until 16Z. Occasional VFR to IFR conditions will be around in vicinity of KBPI, KPNA and KJAC as areas of showers move through mainly in the morning. Most showers should end by 18Z as the deepest moisture moves away to the north, although the chance will linger in vicinity of KJAC most of the day. There is around a 15 percent chance of thunder in vicinity of KJAC between 21Z Saturday and 01Z Sunday but not enough confidence to include in the TAFs. There will be occasional to frequent mountain obscurations through this morning before some improvement after 18Z. Another weather system will move in from the west and will bring more showers, but the large majority of the showers from this system should hold off until after 12Z Sunday.

RIW WATCHES/WARNINGS/ADVISORIES

None.