textproduct: Cheyenne

KEY MESSAGES

- Scattered showers and isolated thunderstorms are expected tonight into Tuesday morning as a cold front moves in.

- Cooler temperatures will prevail for the rest of the week with increasing confidence is widespread rainfall Tuesday evening through Thursday morning.

- Accumulating snow is likely (80% chance) above 8500 ft in elevation Tuesday evening through Thursday morning.

- Near freezing temperatures are possible (20 to 40% chance east of I-25) Wednesday night. A growing season-ending freeze is increasingly likely Thursday night into Friday morning across the entire area.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 345 PM MDT Mon Oct 12 2026

Our warm and dry pattern is coming to an end (or rather, a break) as a sprawling upper level low approaches the area from the west, bringing with it the first real taste of fall. Aloft, GOES water vapor imagery shows a dry airmass moving over the High Plains this afternoon, while deeper moisture is pushing back into Carbon and Albany counties kicking up scattered shower and isolated thunderstorm activity. Meanwhile, a surface cold front is already working its way into the area. The boundary is moving across Laramie county at this hour, but the cool air appears to be lagging somewhat, currently around Wheatland to Alliance. Expect the front to surge southward over the next few hours. With breezy northeast winds in place, temperatures will drop quickly this evening behind the front. Dewpoints in the upper 30s to lower 40s behind the front will provide the floor for tonight's low temperatures, but expect plenty of low clouds and patchy fog to develop along and east of the Laramie range by Tuesday morning. The upper level low digging southward this evening will help to amplify the downstream ridge to our east, enhancing isentropic lift. Shower activity will increase in coverage over the next several hours as a result, with scattered to numerous showers continuing through the night. The mid-level front will stall around Cheyenne to Chadron overnight. Warmer and drier air to the southeast of this feature will limit shower activity there, but expect higher chances for rainfall overnight along and to the northwest. Precipitable water remains very high, around the 90th to even 99th percentile of climatology, so showers will be capable of producing locally heavy rainfall. 700-mb temperatures look to remain above 0C even on the cold side of the mid-level front, so snow levels will be pretty high tonight (11 kft early, dropping to 10 kft by Tuesday AM), confining snow to the higher elevations of the Snowy and Sierra Madre ranges. But do not be surprised to see some light snow accumulations along Battle and Snowy Range Passes Tuesday morning.

The axis of a subtle mid-level shortwave will pass through around daybreak Tuesday, turning 700-mb winds more westerly and thus shutting off the isentropic lift and overrunning forcing for ascent. Shower coverage should drop off quickly after this occurs, with just a few isolated to widely scattered showers continuing through the day on Tuesday. Temperatures will be noticeably cooler than today thanks to plentiful low cloud cover. Look for highs stuck in the mid 50s to lower 60s for most of the area (although this is actually very close to seasonal average temperatures). Expect a little bit of midday sunshine in between when the morning low clouds break and when the afternoon mid to upper level clouds increase. Models remain consistent showing a stronger surge in positive theta-e advection and isentropic lift developing mid afternoon Tuesday. While instability is paltry, there will be some present along with very strong vertical wind shear. Thus, a few isolated thunderstorms are expected Tuesday afternoon and evening too. Overnight Tuesday into early Wednesday, the powerful upper level low will gradually progress east and break itself away from the northern branch flow. This will allow southerly mid-level winds to continue if not possibly gain an easterly component, continuing strong isentropic lift for an extended duration. Expect numerous showers Tuesday evening to transition more to stratiform rainfall late in the night and into Wednesday morning. Snow levels will continue their gradual drop during this period, starting off around 10 kft Tuesday evening, and dropping to 9 kft by Wednesday morning. While that is the most likely level, snow mixing in as low as Laramie cannot be ruled out by Wednesday morning.

Abundant moisture, upslope flow, and continued isentropic lift will produce a chilly day across the area Wednesday. Most areas will stay stuck in the mid 40s to lower 50s with widespread fog along and well to the east of the crest of area mountain ranges. Low elevation rain and high elevation snow will continue through the day Wednesday, but rates may be intermittent. Since upslope lift will be a contributing factor, rates will likely decrease the further east of the Laramie range one goes. The biggest remaining question with this event is Wednesday night into Thursday morning. Ensembles remain split concerning how effectively the primary upper level low will manage to close off from the flow to the north. If the system closes off fully (e.g. the vast majority of ECMWF ensemble members), the storm would be able to wrap plentiful moisture and isentropic ascent around to its northwest flank, allowing good precipitation rates to continue through the night Wednesday night and into Thursday. Another cluster of ensemble members, made up of a majority of GEFS members shows the upper level low struggling to fully close off. As a result, mid-level winds never go easterly, and the wrap- around moisture and lift is significantly less impressive. In this scenario, precipitation amounts would be greatly reduced. Due to this uncertainty, PoPs were held at around 60 to 75% Wednesday night into Thursday and QPF was adjusted slightly downward. Snow levels will continue to drop Wednesday night into Thursday morning. Most likely snow levels end up around 7500 to 8000 ft, but numerous ensemble members (mainly from the cluster that fully closes off the storm system) have snow levels dropping as low as 6000 ft. Due to this potential, a slight chance for snow was introduced Wednesday night and Thursday morning to Cheyenne and Laramie.

Lastly, we'll take a look at total QPF expectations for the entire event combined. Ensemble systems are generally closely clustered to other members of the same system, but are still divergent between different ensemble systems. The official forecast is generally close to the GEFS median, which is the system in roughly the middle of the grand ensemble. The GEFS AI system median is a bit lower, and the ECMWF ensemble is a bit higher. NBM guidance still appears to be showing up on the higher end of the ensemble envelope, so QPF was blended to a small contribution of the 10th percentile to bring amounts more in line with the LREF median. This results in storm total QPF around 0.3 to 0.6" east of a Douglas to Kimball line, and 0.5" and 1.0" along and west of I-25. Accumulating snow is increasingly likely above 8500 ft in elevation. The most likely scenario at this time would produce Winter Weather Advisory amounts in the Snowy Range (80% probability of 6" or more), but we are a little too far out to issue any products at this time.

LONG TERM /THURSDAY THROUGH MONDAY/

Issued at 345 PM MDT Mon Oct 12 2026

The long term will be starting off quite interesting on the back side of of the cutoff low that will bring widespread precipitation to the region and cooler air as well. Thursday morning we'll see widespread precipitation still ongoing, with mostly rain for our lower elevation zones, rain/snow to pure snow for our higher elevations, and likely snow in the mountains. Of note though will be morning temperatures and their effect on our mid to lower elevation locations. While we're not expecting the first widespread winter storm of the season quite this early, some ensembles and deterministic guidance are starting to hint that we may get temperatures cold enough spilling just off of the high terrain that we could see some near surface wintry mix, possibly off the Laramie Summit and even coming down into the Cheyenne area. Even if this does occur though, being the first cold spell of the year combined with warm ground temperatures, accumulations are not likely or will melt very quickly, but the chance is there. Nonetheless, morning snow will likely be ongoing at the Laramie Summit and for both the Sierra-Madres and Snowies, but QPF will be quickly dwindling and accumulations will be lessening by the early afternoon. Otherwise expect a breezy day, particularly east of the I-25 corridor this time, with temperatures struggling to make it out of the 40's to mid 50's thanks to the lingering cold air and cloud cover.

That should begin to change into Friday morning as clouds will begin moving out of the region and high pressure ridging starts to overtake as we move into the weekend. At first though this will have a cooling effect overnight, and the current forecast calls for all zones to reach freezing or below. Many zones could see hard freeze conditions as well, and it's becoming more and more likely that freeze products will be necessary. The NBM indicates a 30-100% probability of morning lows below freezing, with the "warmest" locations and lowest probabilities around Wheatland and the northern Nebraska Panhandle. Temperatures begin to rebound into the afternoon, with highs in the 50's to near 60 for the day. Following this we'll continue to see a warming trend with temperatures back around 10 degrees above normal as the 60's and 70's return under stagnant high pressure ridging. A cutoff upper level low along the Baja Peninsula in Mexico should help to stave off even stronger ridging from building in, but it will continue to promote the return of above normal temperatures nonetheless. Pressure gradients may strengthen enough on Saturday to bring a high wind event for our western zones, with in house guidance already starting to spark a 40% probability for the Bordeaux area, and around a 20-30% probability for our other wind prone locations. This may be the most noteworthy forecast element as we move into next weekend, with an otherwise quiet and pleasant forecast on tap.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 511 PM MDT Mon Oct 12 2026

Complex weather is anticipated for the 00Z TAF issuance with a whole slew of flight categories anticipated across the region. Gusty winds are ongoing at most terminals at this time and will continue for the next 2 to 4 hours before decreasing into the overnight hours. Isolated showers will continue to move across the CWA this evening with gusty and erratic winds likely with the showers. Going into the overnight hours, fog and low- ceilings are anticipated to develop at KCYS, KBFF, KCDR, KSNY, and KAIA with ceilings likely dropping into the IFR category between 04Z and 08Z. These low ceilings are currently expected to persist once they develop and may not clear out until late morning into the early afternoon hours, especially at KSNY and KAIA.

CYS WATCHES/WARNINGS/ADVISORIES

WY...None. NE...None.