textproduct: Cheyenne

KEY MESSAGES

- There is a Marginal Risk (Level 1 of 5) of severe weather from Chadron south through Sidney. Large hail and damaging winds are the primary threats.

- There is a Marginal Risk (Level 1 of 5) of severe weather on Tuesday for most locations along and east of the Laramie Range. Large hail and damaging winds are the primary threats.

- Seasonable temperatures anticipated through mid-week.

- Warmer and drier weather will return to the area Thursday and Friday.

SHORT TERM /THROUGH TUESDAY NIGHT/

Issued at 149 AM MDT Mon Aug 17 2026

Upper-level ridging starts to return today with northwesterly to westerly flow developing aloft. Northwesterly flow will keep the door open for multiple 500mb vorticity maxima that are progged to eject across the region this afternoon. While the monsoonal moisture tap will be weak at best, residual moisture remains mainly in western Nebraska. Westerly 700mb flow is expected for much of the area this afternoon, acting to dry out some of the lower-levels. The terrain-induced dryline will finally be able to push eastward this afternoon, rather than remaining pinned against the high elevations. Therefore, the I-25 corridor looks to dry out quickly this afternoon, preventing any significant precipitation potential, despite the multiple vorticity maxima pushing through. Western Nebraska on the other hand will continue to see mositure. As the dryline pushes eastward, it will lead to forcing for western Nebraska. With residual moisture and support from 500mb vorticity maxima, showers and storms are anticipated to develop this afternoon. MLCAPE values increase this afternoon to around 1000 to 1500 J/kg in western Nebraska. Forecast soundings from the RAP suggest stout Inverted-V signatures west of the dryline across southeast Wyoming, with more favorable severe soundings in western Nebraska, though still will substantial DCAPE values. Therefore, some storms this afternoon could become severe and the Storm Prediction Center has highlighted Chadron south through Sidney in a Marginal Risk (Level 1 of 5) of severe weather this afternoon. Large hail and damaging winds are the primary concerns, with tornadoes not expected. Precipitable Water values remain fairly average across the region, so while heavy rain cannot be completely ruled out, it is unlikely due to the lost of the strong monsoonal moisture tap.

The upper-level ridge attempts to strengthen throughout the day Monday, but a strong trough moving south out of Canada will act to flatten the ridge for much of Tuesday afternoon. Largely zonal flow aloft returns. Multiple lobes of 500mb vorticity look to move across the region Tuesday afternoon into the overnight hours, due to increased vorticity from the proximity of the upper-level trough. Westerly flow is typically drier across the region, though some moisture will be retained. 700mb flow becomes stronger Tuesday afternoon behind a 700mb shortwave ejecting across the region early Tuesday morning. This stronger 700mb flow will lead to more shear across the region, with strong 700mb warm air advection allowing increased forcing. The terrain-induced dryline will retreat westward Tuesday afternoon, allowing stronger moisture advection along and east of the Laramie Range, setting up another day with increased precipitation chances. MLCAPE and SBCAPE look to be a smidge stronger for Tuesday, likely due to increasing moisture in the region. Shear will be much stronger than previous days as 700mb flow strengthens aloft. With stronger forcing, increased moisture, and increased shear in the region, storms will have a better chance of becoming severe on Tuesday, with large hail and damaging winds possible once more. The Storm Prediction Center has highlight all areas along and east of the Laramie Range in a Marginal Risk (Level 1 of 5) of severe weather. Excessive rainfall looks unlikely Tuesday as storms should be quicker moving and precipitable water values remaining near average for this time of year.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Issued at 149 AM MDT Mon Aug 17 2026

Wednesday will bring a fairly similar setup, but probably with better surface moisture due to models picking up on slightly more organized southerly flow across the High Plains. While it is still early, parameters look somewhat favorable for strong to severe thunderstorms both Tuesday and Wednesday, despite lower overall thunderstorm coverage compared to the monsoon surge our area has been under for the last several days. The ECMWF ensemble Extreme Forecast Index for combined CAPE-Shear is around 0.7 to 0.85 for much of the High Plains both Tuesday and Wednesday, indicating that a large proportion of ECMWF ensemble members have combined CAPE- shear values that are climatologically unusual to extreme. We will need to continue to watch this period in the coming days.

A more defined upper level shortwave trough will pass through the area Wednesday evening. Behind this feature, the upper level ridge over the southwest will begin to re-amplify to our west, initiating a drier, northwest flow pattern for the remainder of the week. Precipitable water won't be terrible during this weather pattern, so we should still see at least a few isolated PM showers and storms Thursday and Friday, but expect those two days to be considerably drier. Temperatures will also begin to climb once again. 700-mb temperatures climb back to near the climatological 90th percentile by Friday into Saturday, peaking around +16 to +18C across the area. Expect high temperatures to climb above seasonal averages Thursday, and then closer to 5 to 10 degrees above average Friday and Saturday. Since we'll be somewhat on the periphery of this heat wave, current forecasts do not challenge daily record highs, but this could still change in the coming days. Over the course of Thursday through Saturday, the upper level ridge should start to very slowly meander back to the east. Once this moves east of the Four Corners, the monsoon should surge back to the north, and we may see precipitation chances increasing again by Saturday.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 512 AM MDT Mon Aug 17 2026

Mostly clear skies early this morning with some patchy fog developing near KCDR and KAIA. Patchy fog may continue through 14 or 15Z, but confidence is low on this fog decreasing visibility out of VFR conditions. Partly cloudy skies develop by mid-morning as winds start to increase. Isolated showers and thunderstorms are possible today. Gusty and erratic winds are possible in and around any shower or storm that develops.

CYS WATCHES/WARNINGS/ADVISORIES

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