textproduct: Grand Junction

KEY MESSAGES

- Terrain based showers and thunderstorms will continue Tuesday. Gusty outflow winds, lightning and dry lightning will remain threats.

- Moisture increases through the week with an uptick in storm coverage and intensity expected Wednesday onwards.

- Localized heavy rainfall may prompt flash flooding and debris flows, especially around burn scars.

- Temperatures cool down now through the weekend.

DISCUSSION

Issued at 1233 PM MDT Tue Aug 11 2026

Today, near-surface conditions are still on the drier side in spite of moisture creeping into the region. Today's 18Z sounding indicates an environment similar to this time yesterday, albeit with less instability. Like yesterday, storm development is expected, though virga showers will likely dominate with sparse areas of measurable rain (primarily over the high terrain) and little chance for wetting rain. On Wednesday, the broad high centered overhead skirts eastwards, shifting upper-level flow from westerly to southerly/southwesterly. In this shift, mild moisture from the Gulf and the Baja Peninsula rises northwards through the Desert Southwest towards our region. More notably though is the potent moisture plume to our west, which is currently rising into the Great Basin. The shifting high will pull that moisture eastwards over our CWA, which should be the final push needed to saturate our low-levels and bring about another round of widespread showers and thunderstorms.

The strongest portions of the moisture plume slide north of us through Wyoming, but ensemble PWAT's are forecasted at 150-180 percent of normal across portions of the CWA nonetheless. This should be sufficient to prompt widespread rainfall Wednesday onwards, including some stronger cells capable of producing localized heavy rainfall. The threat for flash flooding and debris flows appears to be maximized on Thursday, but a marginal threat remains in place Wednesday, Friday, and possibly through the weekend as well. The manifestation of this threat will depend on when and where we see substantial lift and/or instability. The lift part of that equation is where uncertainty is greatest. Deterministic models are hinting at some shortwave features transiting the ridge through the mid- and late-week period, though there is very little agreement on when these will occur, if they manifest at all. Models are better-aligned in regards to instability though. Per yesterday's 00Z sounding, mixed-layer CAPE neared 1000 J/kg, and deterministic models are signaling that this sort of instability, if not stronger, will continue through the work week. This will also mean potential for sub-severe hail and winds. As the low-level atmosphere continues to moisten up today and Wednesday, gusty winds will be favored.

In spite of the model agreement, some doubt in the development of strong instability is being cast due to the anticipated scattered cloud cover during mornings. This is often one of the biggest caveats we must consider during monsoonal pushes, and it will be a key consideration over the coming days. Should morning cloud cover be ample, look for thunderstorms to generally be short-lived and more stratiform-type storms to quickly develop. Conversely, if clearer morning skies present themselves, stronger discrete storms will be favored. At this time, it is too difficult to speculate which mode will come to fruition on which days, so stay up-to-date with future forecast updates for additional information. Either way, wet weather is on its way.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1155 AM MDT Tue Aug 11 2026

Showers and thunderstorms will form on the terrain this afternoon. Breakpoint conditions will likely be only temporary at the terminals, as these showers decay over the valleys. Gusty outflow winds will be the primary threat to aviators during these showers. VFR conditions will return this evening. Some high based ceilings will begin to develop ahead of a bigger push of monsoonal flow late tonight and early Wednesday. Ceilings will likely remain well above breakpoint through the overnight hours, with light winds.

GJT WATCHES/WARNINGS/ADVISORIES

CO...None. UT...None.