textproduct: Cheyenne

KEY MESSAGES

- A wet pattern is expected through the end of the week, with almost daily chances for showers and thunderstorms.

- There is a Slight Chance of Excessive Rainfall and flooding issues Thursday as a stronger than average late-season monsoon with tropical moisture moves into the region.

SHORT TERM /THROUGH THURSDAY NIGHT/

Issued at 239 PM MDT Wed Sep 16 2026

The next 36 hours will be quite active with multiple rounds of storms expected to impact the area from a strong monsoon push. Currently, much of the CWA sits in the warm sector behind a warm front that moved across the area earlier. Moisture advection from southerly flow and daytime heating from little to no cloud cover, could cause storms to develop later this afternoon, mainly for areas east of the Laramie Range. Hi-Res guidance keeps backing off on storm potential, lessening the instability across the area. Nonetheless, cannot rule out an isolated thunderstorm or two ahead of the expected approaching cold front. Further west, showers and storms continue on radar with more precipitation entering Carbon County from Utah and northwest Colorado. Later this evening, a surface cold front will move through the CWA, stalling around the Wyoming/Colorado border. This will temporarily pause precipitation across the area as forcing weakens from the stalled front.

More widespread precipitation chances are favored for Thursday. An upper-level shortwave will move into Wyoming during the day, with a surface low developing in northeast Colorado. This shortwave will not only carry the typical monsoon moisture plume from southwest flow, but will also have the remnant moisture of the tropical low that formed in the Gulf of California. This will raise our PWATs to climatological max according to NAEFS percentiles. PWATs will range anywhere from an inch to 1.5 inches over the forecast area. Even with record high PWATs forecast, QPF amounts remain rather unimpressive. Most models and ensemble means only show between 0.3 and 0.7 inches across southeast Wyoming and western Nebraska. However, it is important to note, that especially for locations east of the Laramie Range, there is a lot of spread between members. This signals lower confidence in possible precipitation amounts. It is also likely that many of the models are underdoing precipitation amounts given the high PWATs so would nudge towards the higher range of mean QPF. Based off of Hi-Res guidance, precipitation will likely move into the CWA Thursday morning. The more stable environment will lead to more stratiform precipitation to start. Stratiform precipitation will also be most likely along and west of the Laramie Range. As the day progresses and destabilization occurs from daytime heating and the stalled front, convective storms will become increasingly likely later in the afternoon and evening, especially east of the Laramie Range. Hi-Res guidance supports thunderstorm development as well as model soundings. Soundings show a tall, skinny CAPE profile, indicating a heavy rain threat. Cloud layer winds will actually be quite fast so any flooding that occurs will not be caused by slow moving storms, but rather training storms over an area. Although the majority of locations will not see large amounts of QPF, a few isolated locations could see over 2 inches of rain and flash flooding from thunderstorms. The flooding threat will likely be greatest east of the Laramie Range, and later in the afternoon/evening when thunderstorms are expected. The heavy rain threat will continue until about midnight when storms finally move out of the area. For now, decided to hold off on a Flood Watch, mainly due to low confidence from ensemble member spread. However, consistency from run to run in Hi-Res models could increase this confidence and lead to the issuance of a Watch.

LONG TERM /FRIDAY THROUGH TUESDAY/

Issued at 239 PM MDT Wed Sep 16 2026

Friday will be another active and potentially wet day, as we remain under southwesterly flow that will provide us with another push of monsoonal moisture from Baja California. As a result, PWAT values will be above the 90th percentile with a vigorous closed 500 mb low located over Eastern Oregon and Western Idaho. The question will be, will we have enough mid and upper-level forcing for ascent to translate most (or any?) of this moisture to the surface as precipitation? Ensemble guidance shows a series of low-amplitude shortwave troughs embedded in the SW flow regime combined with weak isentropic ascent/warm advection primarily over SE Wyoming, which would favor another round of showers and thunderstorms Friday afternoon and evening. Any showers and thunderstorms will be capable of producing heavy rainfall and even some localized flooding, hence WPC placing much of SE Wyoming and the extreme W Nebraska Panhandle in a marginal risk for excessive rainfall given the anomalously moist air mass. High resolution guidance also hints at somewhat of a stationary front draped across the I-80 corridor late Friday afternoon, which may serve as an additional focal point for showers and thunderstorms. Not expecting storm motion to be an issue in regards to flooding, as cloud-layer mean winds are progged to be 25- 30 knots which will keep storms moving fast enough. However, any backbuilding convection will pose a flooding threat, as cloud-layer mean wind vectors are oriented at roughly a 25 to 45 degree angle relative to the stationary front (so not parallel, but also not perpendicular to the boundary). While not an ideal flash flooding setup, some degree of backbuilding may occur especially if convection remains anchored to the aforementioned stationary front/boundary given the very moist airmass (PWAT 0.9"-1.1") for this time of year. Highs on Friday will depend on cloud cover and precipitation coverage. Current LREF guidance has a median high temperature for Cheyenne in the mid 70s, however I wouldn't be surprised to see this trend cooler over time, especially depending on convective debris and coverage of overnight showers/storms on Thursday into Friday morning, in additional to the exact placement of the stationary front.

Heading into Saturday, ensemble guidance has backed off on shunting off our supply of monsoonal moisture as the closed low to our northwest begins to lift to the north and east. While not as impressive as the previous couple of days, PWAT values are still expected to range between the 85th and 95th percentiles relative to climatology, keeping additional shower and thunderstorm chances in the forecast. The bulk of the forcing for ascent looks to be from late Saturday evening into the overnight hours, with an additional chance for some much needed rainfall. As it stands, ensemble guidance has 55-65% chances of all areas east of the I-25 corridor seeing at least 0.1" of rain from midnight to noon on Sunday, with the highest values across the Nebraska Panhandle. However the spread remains rather large, with the 25th percentile QPF being essentially 0.01"-0.05", which wouldn't exactly do much to help our drought conditions. Forecast soundings across the Panhandle show skinny CAPE profiles throughout the troposphere, PWAT values around the 70th percentile, and warm cloud layers up to ~3 km, so any showers and storms are expected to be rather efficient rainfall producers. Unsurprisingly, high temperatures on Sunday will be rather cool (although not terribly far below average) in the 60s for SE Wyoming, and low 70s in the Panhandle.

Heading into the work week, a warming trend is expected as flow is more out of the WNW, resulting in drier conditions as another ridge builds to our west over the Great Basin. Expect temperatures to be around or slightly above seasonal values on Monday and Tuesday in the upper 60s to mid 70s (warmest temps across the Panhandle), and little to no chances for precipitation.

On Wednesday, ensemble guidance hints at the ridge building and shifting east, placing SE Wyoming and W Nebraska in more WSW flow aloft, allowing for some return of monsoonal moisture. At this time, any monsoonal moisture surge doesn't look nearly as impressive as the one in the short/medium range, but low precipitation chances look to return come mid next week.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1142 AM MDT Wed Sep 16 2026

Complicated forecast over the next 24 hours, with numerous rounds of showers and thunderstorms expected, particularly for the Wyoming terminals today, and all terminals beginning around sunrise on Thursday. The main hazards will be brief reductions in visibility in the vicinity of any showers/storms, with cigs briefly reaching MVFR criteria, down to around FL030. Confidence is high enough at this time based off HREF guidance to include TEMPO groups for -SHRA at all the Wyoming terminal. KLAR and KRWL will experience peak gusts of 20-25 knots out of the southwest this afternoon ahead of a cold front, which will gradually shift the winds to more SE for all terminals this evening.

CYS WATCHES/WARNINGS/ADVISORIES

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