textproduct: Cheyenne

KEY MESSAGES

- Another round of thunderstorms will bring the potential for localized flash flooding and isolated strong to severe thunderstorms this afternoon and evening. A Flood Watch is in effect from 2PM through 10PM this evening.

- Unsettled weather continues through at least Sunday, bringing cooler than average temperatures and daily chances for showers and thunderstorms.

- Isolated strong to severe storms and localized flash flooding will remain possible Sunday.

- Drier weather and temperatures close to seasonal averages will return early next week.

SHORT TERM /THROUGH SUNDAY NIGHT/

Issued at 157 AM MDT Sat Aug 15 2026

Monsoonal moisture continues to advect into the region again today with another round of slow moving, heavy rain producing thunderstorms this afternoon and evening. Upper-level southerly to southwesterly flow will continue to advect monsoonal moisture into the region all day Saturday with multiple 500mb vorticity maxima ejecting across the region. 700mb flow remains largely benign today, with generally westerly to southwesterly flow expected. A messy, surface high pressure system will attempt to develop west of the Rocky Mountains over western Colorado before strengthening into the afternoon hours. Surface flow will largely remain westerly to southwesterly as the high pressure remains weak and messy, but flow is expected to flip to the east, along and east of the Laramie Range, by the early afternoon hours and continue through much of the evening hours. With ample moisture available due to continued monsoonal moisture advection, the easterly flow will lead to very moist, upslope flow along and east of the Laramie Range. Combined with the surface upslope and 500mb vorticity maxima, weak 700mb warm air advection develops into later-afternoon hours. This will add another mechanism to get forcing across the region. Heavy rainfall is anticipated once again today, with the Weather Prediction Center placing portions of the region in a Slight Risk of excessive rainfall and a larger portion in a Marginal Risk of excessive rainfall. Therefore, localized flooding will be possible today with another round of slow moving thunderstorms. A Flood Watch has been issued from 2PM through 10PM this afternoon and evening for nearly all locations along and east of the Laramie Range, where Precipitable Water values remain at and above the 90th percentile and moist, upslope flow persists. In addition to the flooding risk today, the Storm Prediction Center has placed far southeast Wyoming and western Nebraska in a Slight Risk (Level 2 of 5) of severe weather this afternoon. Primary hazards look to be isolated, large hail and damaging wind gusts. Isolated pockets of MLCAPE and SBCAPE around 1000 J/kg will provide enough instability for stronger updrafts to develop this afternoon, especially across western Nebraska. The biggest question will be whether or not the early morning cloud cover anticipated across the region puts a damper on the severe potential this afternoon. Pockets of clearing skies will be possible this morning, especially in the southern Panhandle, but clouds will remain over much of the region due to the excess moisture present throughout the atmosphere.

Slightly drier conditions will start to try to return to the region on Sunday as upper-level flow turns more westerly, rather than southwesterly, cutting off some of the monsoonal moisture advection into the area. Upper-level zonal flow is expected to develop by Sunday afternoon, which should start to dry out the atmosphere. However, with the ample moisture present over the past several days, plenty of residual moisture will be present to get another round of showers and storms Sunday afternoon into the evening hours. A few 500mb vorticity maxima will advect across the region Sunday afternoon, providing some forcing for ascent. Slightly stronger 700mb warm air advection briefly develops Sunday afternoon, adding an additional forcing for ascent for showers and storms. Surface flow turns southerly Sunday afternoon through much of the overnight hours, leading to weak ascent across the Cheyenne Ridge, likely resulting in showers and storms developing in this area once again. Precipitable water values look to return towards more climatologically average values Sunday, decreasing the threat for isolated flooding across the region. However, with monsoonal moisture still in play, flooding potential continues for Sunday with another Slight Risk of Excessive Rainfall from the Weather Prediction Center. Another round of strong to severe storms will be possible Sunday, with a Marginal Risk of severe weather for much of the area and a small portion of the region under a Slight Risk of severe weather once again.

LONG TERM /MONDAY THROUGH FRIDAY/

Issued at 157 AM MDT Sat Aug 15 2026

Monday...A drier pattern ensues and is on tap as the flow aloft turns west northwest and the precipitable water values decrease, due to drying at low and mid levels and downslope southwest winds near the surface. Thus, a much drier day when compared to the weekend. A warming trend begins with more sunshine and warmer mid level temperatures near 17C at 700 mb.

Tuesday...Brisk northwest flow aloft prevails, and with a decent amount of low and mid level moisture availability as seen in the 12Z GFS run, expect isolated to widely scattered late day showers and thunderstorms, mainly east of Interstate 25 and the Laramie Range during the afternoon, with coverage everywhere in the evening. Similar maximum temperatures to Monday based on 700 mb temperatures.

Wednesday...The synoptic pattern looks more like September and early autumn with a deep shortwave trough aloft sweeping across the Dakotas into western Minnesota by evening, with a much cooler airmass moving southward across the northern Rockies, as well as the northern and central Great Plains, including the Dakotas and much of Nebraska behind a cold front. Maximum temperatures for our neck of the woods will be quite comfortable with readings in the mid 70s to lower 80s, perhaps even cooler than this if the Canadian cool surge lies further west. From experience, and from the model trends, this type of cold frontal passage is typically mostly dry for our forecast area, and for consistency sake, and matching up with our neighboring offices, we will continue with scattered showers and thunderstorms in the afternoon and evening for now, though suspect that later shifts will see far less POPS as models come to more congruency and consistency.

Thursday-Friday...Models suggest a return to bone dry and hotter conditions as the ridge aloft transitions across the northern and central Rockies and our counties. 700 mb temperatures rise from near 14C at 00Z/Friday to 18Z at 00Z/Saturday yielding maximum temperatures mainly in the 80s for Thursday, with some 5 to 10 degrees of warming for Friday. Another hot and dry period, stretching into the weekend,is on tap.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1142 AM MDT Sat Aug 15 2026

Thunderstorms and heavy rainfall will be the main aviation concern once again today. Storms will kick up in the next 1-2 hours in southeast Wyoming, impacting KLAR and KCYS in the afternoon, then progressing eastward into the Nebraska panhandle in the late afternoon and evening hours. Strong, gusty outflow winds and IFR conditions due to moderate to heavy rainfall will be the primary concerns. TEMPO groups were introduced where confidence in impacts is highest, while other terminals remain at PROB30 groups. Timing is still a little uncertain, so be alert for possible AMDs this afternoon and evening as confidence increases.

Look for largely variable winds overnight behind today's thunderstorm activity. Another round of low clouds and/or fog is possible over the High Plains, but confidence is fairly low. Model probabilities are around 20 to 30% for MVFR to IFR, but models have been underestimating the fog and low CIGs the last several mornings.

CYS WATCHES/WARNINGS/ADVISORIES

WY...Flood Watch until 10 PM MDT this evening for WYZ102-107-108- 117>119. NE...Flood Watch until 10 PM MDT this evening for NEZ002-003- 019>021-054-055-095-096.