textproduct: Cheyenne

KEY MESSAGES

- Cool and seasonable temperatures will continue through mid- week, with a warming trend expected for the second half of the week into the weekend.

- Breezy conditions will be possible across much of the region Tuesday and Wednesday, especially across the western Nebraska Panhandle.

- Increasing coverage of showers and thunderstorms through Thursday, then a brief drying trend for the weekend.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 215 AM MDT Tue Sep 22 2026

Only minor changes to the short range package as short range ensembles and synoptic model solutions diverge as we head into the middle of the week. Primary forecast concern is the plume of subtropical moisture currently impacting the southern Rocky Mountains over New Mexico and western Texas. 00z models showing this moisture plume and associated dynamic forcing close enough to southeast Wyoming and western Nebraska where we can't rule out some rainfall and thunderstorm impacts over the I-80 corridor and possibly further north towards the North Platte River Valley.

For Tuesday, relatively quiet weather will continue with pretty stable conditions and low CAPE. Low clouds and fog will linger across mainly western Nebraska through much of the day under increasing south to southeast winds. A poorly organized upper level trough is forecast to lift northeast out of the Great Basin region later today and into this evening. There should be enough instability and midlevel forcing to initiate rain shower activity this evening across mainly Carbon and Albany counties, but can't rule out some of this activity making it as far east as the Laramie Range and into the I-25 corridor. Increased POP between 15 to 30 percent for the lower elevations. Low end chance of thunderstorms, so kept Prob thunder pretty low and mainly confined to the areas with the best forcing...which is still pretty limited and looks better across northern Wyoming. High temperatures will rebound into the 70s today as the midlevel flow shifts into the southwest. Kept the coolest temperatures across western Nebraska due to low clouds and fog...which may persist into the mid afternoon hours. Some locations may not make it out of the low to mid 60s.

For Wednesday through early Thursday, most of the forecast area is in-between systems early on Wednesday as Tuesday's upper level trough lifts north into the northern Plains...and the main subtropical moisture push will be located over New Mexico and slowly lifting north. This is where models diverge significantly. The Canadian and ECMWF show the broad trough and deep moisture lifting northward into Wyoming, while the GFS shows more of a southwest to northeast trajectory...barely brushing the southern most zones and as far north as the I-80 corridor over southern Nebraska. This is significant because PWATs will be above the 99th percentile for late September with this moisture surge. All models show enough instability and forcing to produce bands of rain showers and at least scattered thunderstorm coverage given these moisture profiles. Kept POP around 15 to 25 percent on Wednesday, mainly across the I-80 corridor and in the mountains...but expect most of the area to remain dry due to some short-wave ridging in between disturbances. Late Wednesday night/Thursday looks to be the most likely time period to see widespread rainfall, but that greatly depends on the movement and trajectory of the subtropical moisture plume and primary upper level disturbance lifting northward. Lowered max and min temperatures into Thursday to reflect the trend of cloudy skies and scattered to numerous rain showers.

LONG TERM /THURSDAY THROUGH MONDAY/

Issued at 338 PM MDT Mon Sep 21 2026

Southeastern Wyoming and Western Nebraska will be under southwesterly flow aloft as a mid-level ridge begins to build across the Central United States. With an embedded shortwave trough propagating across the region Wednesday afternoon combined with another surge of monsoonal moisture (PWAT again ranging within the 80th and 95th percentile), widely scattered showers and thunderstorms will be possible, mainly south of I-80. However, no severe weather is anticipated due to weak instability (MLCAPE ~100- 200 J/Kg) thanks to poor mid-level lapse rates. Temperatures look to be around or slightly warmer than average, in the low to mid-70s east of the Laramie Range and mid to upper 60s to the west. A weak cold front is progged to move through SE Wyoming and the W Nebraska Panhandle overnight Wednesday into Thursday, which may serve as a focal point for more shower development into the evening hours.

On Thursday, an additional shortwave trough within the longwave ridge pattern may result in another round of afternoon and evening showers and thunderstorms, with slightly cooler daytime high temperatures behind the front. With an effective bulk wind difference of only 15-20 knots along with relatively weak instability (MLCAPE < 500 J/Kg), no organized or severe weather is expected, despite a post-frontal air mass characterized by dewpoints in the low to mid-50s across the Panhandle and PWAT exceeding the 90th percentile yet again. This is rather rare for late September, but we will certainly take any moisture we can get despite nothing widespread expected...

The flow regime becomes more zonal on Friday which turns off our supply for the anomalously rich monsoonal moisture seen the previous days (PWAT ~0.9"). This will decrease our chances for any meaningful precipitation and will also be the start of a warming and drying period into the weekend. We will have to monitor increasing fire weather concerns into the weekend as long range ensemble guidance suggests gusty, westerly winds mainly across SE Wyoming, combined with daytime RH values of 20-25%. Not particularly concerned at this time as we are still short of critical fire weather thresholds, but will have to be monitored as we get closer given plenty of dry fuel and further warming underneath the building ridge. High temperatures this weekend look to be 5-10 degrees above average, perhaps reaching 80 degrees in areas east of the I-25 corridor.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1025 AM MDT Tue Sep 22 2026

Complicated forecast leading to a variety of flight categories over the next 24 hours. Visible satellite trends indicate low clouds hanging on over the Nebraska terminals, with slight hints of some gradual clearing at KSNY where LIFR CIGs are currently in place. Anticipate the Nebraska terminals to remain under MVFR or IFR until 19-21Z before we gradually erode the stratus deck. Under the anomalously moist air mass and another moisture surge expected overnight into tomorrow morning, have introduced a TEMPO group for BR and MVFR CIGs for KSNY and KAIA, however KBFF may have to be included in the 00Z update.

Different hazards for the Wyoming terminals this afternoon as a weak shortwave trough ejects from the SW over our area, which will result in widely scattered showers and thunderstorms primarily after 22Z through 03Z. Not expecting much in the way of VIS decreases in/around showers due to large surface dewpoint depressions, however showers/storms will be capable of producing gusty and erratic winds up to 35 knots. PROB30 groups have been left in at this time based on high-resolution ensemble guidance. Another weak disturbance is expected tomorrow morning after 10Z with isolated coverage, so included VCSH for KCYS and KLAR to account for < 30% probabilities of direct terminal impacts.

CYS WATCHES/WARNINGS/ADVISORIES

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