textproduct: Cheyenne

KEY MESSAGES

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

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

- Seasonably cool temperatures with highs ranging from the middle 60s to middle 70s through Monday. Friday will be the warmest day with low to middle 80s possible across the lower elevations.

SHORT TERM /THROUGH FRIDAY NIGHT/

Issued at 250 AM MDT Thu Sep 17 2026

Active over the next several days with a late-season monsoon pattern setting up. Short term forecast through Friday night looks on track with excessive rainfall and flooding remaining the primary concern this afternoon through late this evening. High res CAMs are starting to come into better agreement regarding which area(s) have the best chance of seeing persistent heavy rainfall, in addition to a low-end severe weather threat across the eastern plains.

Current IR Satellite loop shows the potent shortwave trough, of tropical origin, lifting north across Arizona at this hour. Surface analysis shows the cold front has pretty much pushed through the area and is now becoming stationary near the mountains. Low to midlevel forcing appears to be in play later today, with heavy showers and thunderstorms possibly beginning as early as sunrise over the I-80 corridor as the shortwave trough axis lifts north into the forecast area. Still expected periods of very heavy rainfall, mostly across the eastern plains...but can't rule it out over Carbon and Albany counties either. Model soundings continue to show 600-1200 j/kg of CAPE with PWATs between 1.25 to 1.50 inches, and as high as 1.75 inches based on current model soundings. In addition, there is also a risk for some locally strong gusty winds as convective lines propagate southeast and then potentially redevelop up-shear. SPC has introduced a Marginal Risk for strong to severe thunderstorms, mainly for a strong wind threat. Large hail is not expected at this time due to the tropical nature of this event and long-skinny CAPE profiles on model soundings.

High res CAMs are finally coming into better agreement showing most of the training storms and persistent rainfall over far east-central Wyoming, South Dakota, and portions of the northern and central Nebraska Panhandle this afternoon through late this evening. Again, can't rule out heavy rainfall across other areas, but this appears the most likely region (generally north of US Highway 26) to see a flood threat with the most likely area(s) centered in southern South Dakota as the surface low pressure system intensifies and stalls across central Nebraska. Since our northeast zones are on the fringe of the heaviest rainfall, decided to not issue a Flood Watch at this time even though confidence has increased a bit. Current QPF is still a little low to justify a Flood Watch based on current flash flood guidance with limited areal coverage. Would also like to see a bit more jet dynamics as the area of max diffluence peaks over South Dakota and far northern Nebraska this evening. With all this in mind, things may change and we may end up issuing a Watch at a later time. Otherwise, kept heavy rain wording in the forecast and will message accordingly this morning. Expect high temperatures in the 70s to low 80s to occur late in the morning or early in the afternoon before temperatures fall into the 60s with the onset of precipitation.

For Friday, all models show the slow moving upper level trough, currently across northern California, slowly nudging eastward into the northern Great Basin Region and western Wyoming. With low to midlevel moisture remaining over the area and a potent midlevel shortwave ejecting northeast ahead of the main trough, expect another day of scattered to numerous showers and thunderstorms. This will be mostly over southeast Wyoming since the boundary layer in western Nebraska will be pretty worked-over from the previous night's activity. Although we lose some of our CAPE, heavy rainfall looks likely again on Friday with a Marginal Risk of Excessive rainfall. It will be a little warmer with highs back into the low to mid 80s below 6000 feet, and closer to the middle 70s for Carbon, Albany, and western Laramie Counties.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Issued at 240 PM MDT Wed Sep 16 2026

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 /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 617 AM MDT Thu Sep 17 2026

Remnants of a tropical disturbance from the Gulf of California will push across the area later today and tonight. This disturbance and a stalled cold front will help the development of showers and thunderstorms this afternoon through late this evening. Very heavy rainfall, frequent lightning, and strong gusty winds are possible until 06z Friday.

HAZARDS/WEATHER TRENDS: Difficult Aviation forecast setting up across southeast Wyoming and especially western Nebraska with models not agreeing on timing, coverage, and severity of convection today. Overall, current thinking is VFR conditions will prevail for all terminals through the afternoon hours. Shower activity this morning will lift north across the area, but other than some brief MVFR VIS...not expecting widespread impacts. By mid to late afternoon (20z to 23z), models show numerous showers and scattered thunderstorms developing over the mountains of southeast Wyoming and pushing northeast into the high plains. Periods of IFR conditions are possible with this activity...and there may be multiple bands that move across the plains. Added prevailing TSRA and TEMPO groups where we are most confidence, otherwise...kept PROB30 groups with the lower confidence terminals. After 03z, expect IFR CIGS in low stratus across western Nebraska. Increasing confidence with these CIGS due to expected heavy rainfall and light winds, so added IFR to KCDR, KAIA, and possibly KBFF later in the evening.

CYS WATCHES/WARNINGS/ADVISORIES

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