textproduct: Cheyenne

KEY MESSAGES

- A few isolated showers and storms are anticipated Sunday afternoon.

- Widely scattered strong to severe thunderstorms are possible again on Monday, with large hail and strong winds as the primary hazard.

- Daily chances for showers and thunderstorms will continue through much of the week ahead with temperatures near average west of I-25 and cooler than average east of I-25.

SHORT TERM /THROUGH MONDAY NIGHT/

Issued at 130 PM MDT Sun Jun 21 2026

Thunderstorms are getting an early start today. GOES visible satellite imagery shows convective plumes developing already over portion of the High Plains. Analysis of the environment early this afternoon suggests that capping will not be a problem today. On the large scale, we are now on the backside of the shortwave trough with its upper level circulation apparent over the Dakotas. The surface trough behind this feature has mostly cleared the area. This is is showing up as a weak surface cool front and a wind shift. Surface northwest winds have spread across the entire area behind this boundary. Wind shear is fairly weak as a result, with a little bit of backing flow in the lowest 3-km or so mostly in the ballpark of 20 to 40 knots. Fairly potent instability has developed where low- level moisture remains abundant (north of a roughly Douglas to Sidney line). Isolated to widely scattered storms are already developing over the area, and should continue through the afternoon. Overall, today will be an earlier show, with most of the hazard expected to conclude by around 7PM or so. However, forcing for ascent will shift west this evening as the surface cool front pushes over the Laramie range and into Carbon/Albany counties. Expect scattered showers and a few thunderstorms to continue over our western zones late into the evening. Today's storms will have some limited potential to become strong to severe, most likely in the southern/eastern panhandle where vertical wind shear is the most prominent.

With strong moisture return overnight as the moisture and frontal boundary surge west of the Laramie range, we will have another round of patchy fog and low clouds developing over area Monday morning. The highest probabilities for fog are around the Pine Ridge and the northern Panhandle. Models are in fairly good agreement showing the low clouds/fog burning off fairly quickly Monday. While we should have some CIN present during the morning to early afternoon, it appears the chance for this continuing all day (as yesterday) is a bit lower. The surface high and associated moisture will retreat back to the Laramie range Monday morning as dry, westerly flow pushes back into Carbon and Albany counties. This will accentuate the dryline along the terrain boundary, and thus provide a focus for convective initiation around 1-3PM. Monday's environment looks fairly potent with ample instability and deep layer shear around 50 knots. Large hail and gusty winds will be the primary hazards through most of the day. In the late afternoon, models suggest that the low-level jet will start to ramp up, accelerating southerly flow over the High Plains. This will enhance low-level shear and may increase the tornado threat for a few hours. Hi-res models struggle with what to do with the explicit convection tomorrow, especially after about 6PM. An upper level low sitting to our north will provide a little bit of synoptic scale lift through the evening and overnight hours as a surface front pushes southward across the area. Overrunning lift in place of continuing elevated instability may allow for some nocturnal convection through much of the night. This should be mostly isolated, but we may not have clear radar for much of night. PoPs in the 20 to 40% range are maintained all through the night along and north of a roughly Douglas to Alliance line, which is the area closer to the forcing. Another round of patchy fog is anticipated Monday night into Tuesday morning over the plains.

LONG TERM /TUESDAY THROUGH SUNDAY/

Issued at 130 PM MDT Sun Jun 21 2026

The long-term period will begin under zonal flow aloft underneath a flat longwave ridge with numerous embedded vorticity maxima propagating across SE Wyoming into Western Nebraska. High temperatures on Tuesday will be cooler than the previous day thanks to a cold frontal passage during the morning hours which will shift our surface winds to NNE. Moist, post-frontal upslope flow will increase our surface dewpoints across SE Wyoming and the Nebraska Panhandle into the mid to upper 50s combined with 40-50 knot 500 mb flow and an associated shear-induced vorticity maximum moving across the area which will set the stage for convective initiation on Tuesday afternoon. Forecast soundings continue to indicate effective bulk shear values in excess of 45 knots, so there will be the potential for a few severe thunderstorms with the addition of 500- 1000 J/Kg of MLCAPE confined to areas east of the I-25 corridor where the greatest low-level moisture will exist. This lines up with the Day 3 SPC Convective Outlook which places Cheyenne and other locations along and south of the I-80 corridor in a marginal risk for severe weather. Despite sufficient deep-layer shear for supercells, low and mid-level flow is progged to be quite weak, lower than 10-15 knots, yielding little to no low-level curvature in the hodograph. This should limit the overall tornado threat with any storms, leaving large hail and damaging winds as the main threats with any supercells. With the cooler post-frontal air mass and lingering low-level moisture, forecast highs on Tuesday are expected to be near or slightly below climatology in the mid to upper 70s mainly east of the Laramie Range, with upper 70s to low 80s to the west.

The pattern remains active into Wednesday as another shortwave trough embedded in the zonal flow regime progresses across our area, providing another source for forcing for ascent. Surface winds turn to more SE, which will allow for additional theta-e advection and low-level moistening throughout the day. It is too far out to get into specifics, but the combination of a lifting mechanism, progged dewpoints in the mid 50s (resulting in surface-based instability), and effective bulk shear values greater than 50 knots, there will be a threat for severe weather on Wednesday as well. SPC has placed much of SE Wyoming and the Panhandle in a Day 4 risk for severe weather, so future forecasts will have to be monitored. High temperatures will again be slightly below climatology mainly in the low to mid 70s.

Much of the same for Thursday as embedded disturbances continue to move across our region and consistent signals from ensemble guidance that low-level moisture will remain in place east of the Laramie Range. Forecast soundings and medium-range ensembles continue to show sufficient deep-layer shear and flow aloft for organized convection, with primary hazards to be ironed out in the coming days. As we finish off the work week and head into the weekend, the synoptic-scale flow pattern will become southwesterly downstream of a rather high-amplitude trough centered over British Columbia and the Pacific Northwest. PWAT is progged to be at or above the 80th percentile on Friday, so there will be the possibility for an additional round of late day afternoon showers and storms. Severe potential looks low at this time given the lack of shear. A warming trend will continue into the weekend with mostly dry conditions and temperatures near or slightly above climatology.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

Issued at 519 PM MDT Sun Jun 21 2026

An active 24 hours ahead with numerous hazards, the first being lowering CIGs particularly for KBFF, KAIA, and KCDR mainly after 04Z as low-level moisture remains in place combined with light winds. CIGs may lower below 1 kft especially at KCDR where the heaviest rainfall fell today and fog appears to be most likely. Can expect IFR to perhaps LIFR in these areas of patchy fog before this surface moisture mixes out within a couple hours after sunrise.

Focus then shifts to the chance for a few severe thunderstorms tomorrow afternoon, primarily after 20Z, for all terminals except for KRWL and KLAR. The main hazards with these storms will be gusty and erratic winds up to 65 knots, large hail, and frequent lightning. Any terminal in the vicinity of a stronger storm may briefly see IFR conditions with visibilities down to 1SM in the regions of heavier rain and hail. Have low confidence (~30%) in thunderstorm coverage tomorrow, so included a PROB30 group for the Nebraska terminals rather than a TEMPO group due to this uncertainty.

CYS WATCHES/WARNINGS/ADVISORIES

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