textproduct: Cheyenne

KEY MESSAGES

- Monsoon moisture has returned to the area, increasing storm chances through the weekend and bringing temperatures down to near or below seasonal averages.

- Scattered to numerous thunderstorms are expected Wednesday. Strong winds, large hail, and localized flash flooding are the primary concerns.

- Numerous to widespread showers and thunderstorms are expected Thursday with isolated strong to severe storms and flash flooding possible.

- A warming trend early next week may bring us back to near normal late summer warmth.

SHORT TERM /THROUGH FRIDAY NIGHT/

Issued at 245 PM MDT Wed Aug 12 2026

The long period of hot and dry weather has finally come to an end across the area as moisture returns today. The upper level ridge has shifted eastward into the central Plains, allowing for monsoon moisture to freely flow northward across the Four Corners states and into Wyoming. A 500-mb vort-max riding over the top of the ridge is currently visible on GOES satellite imagery moving across north central Wyoming, which is helping to provide a boost to today's storm chances. Mesoanalysis indicates that much of the area is still underneath a bit of CIN at this time, but storms are already kicking off in a similarly capped environment in northeast Wyoming likely due to the strong forcing. Convective development will spread southward into our area shortly, and then expect storms to grow upscale and spread northeastward through the evening hours. Other than the aforementioned capping, convective parameters are quite favorable for strong to severe thunderstorms today, especially east of the dryline which is currently retreating back to the Laramie Range. MLCAPE ranges from 1000 to 2000 J/kg with ample effective bulk shear around 35 to 40 knots. As such, the outlook for numerous showers/storms and isolated to scattered strong to severe storms remains on track for the afternoon and evening hours. Large hail and strong to damaging winds are the primary severe concerns, but as evidenced in northeast Wyoming earlier, an isolated tornado cannot be ruled out. Low-level rotation is most prominent in Niobrara county at this time. In addition, the moisture content of the atmosphere is extremely high today. The 1" contour for precipitable water extends all the way back to the Laramie Range, while our eastern zones are around 1.3 to 1.4". While storms should be moving along at a decent speed today, there is certainly potential for locally heavy rainfall, which may lead to isolated flash flooding. One particular concern is that the soils may be hydrophobic in areas due to the ongoing drought. While the severe hazard should conclude by around 9PM this evening, expect showers and a few thunderstorms to linger across the area all through the night and into Thursday morning.

Moisture will peak Thursday across the area. Expect precipitable water values to hover around the 97.5 to 99th percentile of climatology, with values over 1.0" for most of the area, and approaching 1.5" in western Nebraska. Another upper level shortwave will pull out of the southwest and move through the area late in the day on Thursday, providing some forcing for ascent ans also a mechanism to help organize convective development. The risk for severe weather looks a little lower tomorrow as lapse rates will be a more modest 6 to 7 C/km. Shear around 30 to 35 knots and skinny CAPE profiles will still allow for a few strong to severe thunderstorms, but the primary focus still looks like flash flooding. There may be multiple rounds of shower and thunderstorms Thursday, with coverage expected to be in the numerous to widespread categories. The one failure mode introducing forecast uncertainty is that we will have some decent capping present again tomorrow, especially in western Nebraska. A few hi-res models (e.g. 3km NAM) choke off all convection once it runs into the capping east of I-25. Of course, that specific model is generally too aggressive on capping, but it presents a plausible extreme scenario where capping does not break down at all. However, the other end of the range of plausible forecasts would support potent rainfall amounts. The ECMWF extreme forecast index for QPF is around 0.7 to 0.8 for most of the area, indicating fairly good consensus on locally heavy rainfall. A shift of tails around 1 to 2 indicates that a few ensemble members exceed the climatological maximum. So, all together, there is medium confidence in numerous thunderstorms with locally heavy rainfall potential. Extreme rainfall events cannot be ruled out, but there is also a failure mode in which storms end up much less widespread than currently forecast. With plentiful cloud cover, expect cooler temperatures across the area, mostly in the mid to upper 70s!

Little will change by Friday, but we will be past the peak of the monsoon surge, and should start to see precipitable water starting to gradually decrease. We will still be well above seasonal averages, around the climatological 90th percentile. We should have some more sunshine Friday morning due to subsidence behind Thursday's shortwave trough, which should allow for highs to climb perhaps around 5 to even 10 degrees warmer than Thursday, but still not really above seasonal averages. Friday's thunderstorm coverage will be a little less than Thursday, more in the scattered category, but locally heavy rainfall and flash flooding will remain a concern.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Issued at 245 PM MDT Wed Aug 12 2026

As the long term period progresses, the ridge starts flattening out as a low approaches from the PNW. Shortwaves move northward toward our region which brings widespread chances for showers and thunderstorms to our area. This moisture is still coming into the area as monsoonal moisture. Precipitable water values stay around the 90th percentile range, slowly tapering off through this long term period. With this plentiful moisture, there is higher confidence in precipitation each day through this period. Precipitation chances are greatest in the afternoon to evening hours through the CWA. 700mb temperatures are around 10-15C bringing us seasonal daytime highs and overnight lows.

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

Issued at 529 PM MDT Wed Aug 12 2026

West southwest flow aloft will continue, while a shortwave trough aloft moves to northwest South Dakota this evening. Meanwhile, at the surface, low pressure will deepen across eastern Colorado with a few low level convergence axises developing across southeast Wyoming and western Nebraska.

Isolated to scattered thunderstorms will continue across southeast Wyoming into this evening and develop and propagate eastward across western Nebraska this evening. Some of the storms across the eastern tier of southeast Wyoming counties and the Nebraska Panhandle, will become strong to severe this evening. Thunderstorms are expected to end across the Nebraska Panhandle around 03Z and exit to the east of the Panhandle.

Wyoming TAFS...Based on the recent NAM and the consistent runs of the HRRR, notably the 22Z run, high confidence in isolated to scattered thunderstorms continuing through early evening, with some strong storms mainly east of I-25, producing gusty and erratic winds to 40 knots along with brief MVFR visibilities. Widespread MVFR ceilings and visibilities due to fog will develop from 08Z to 15Z along and east of Interstate 25. Typically gusty diurnal winds to near 30 knots will occur until sunset. Confidence is high enough to include VCTS in the TAFS after 19Z to 20Z Thursday.

Nebraska TAFS...Using the latest NAM and the consistent past few runs of the HRRR through 22Z, high confidence in isolated to scattered thunderstorms strengthening and propagating eastward across the panhandle well into the evening hours. Some of the storms will become strong to severe this evening, producing erratic winds gusting up to 45 knots, with a few storms producing winds gusts in excess of 50 knots along with some hail of one inch diameter or larger, along with locally heavy rain. The strongest storms will produce localized MVFR ceilings and visibilities this evening. Widespread IFR and MVFR ceilings due to low clouds and fog will form from roughly 06Z to 15Z. Typically gusty diurnal winds to near 25 knots expected until sunset. VFR prevails Thursday afternoon.

CYS WATCHES/WARNINGS/ADVISORIES

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