textproduct: Grand Junction

KEY MESSAGES

- 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 1038 PM MDT Tue Aug 11 2026

A trough moving onto the West Coast has displaced the high pressure dome eastward. This has allowed flow aloft over the Desert Southwest and Southern Great Basin to shift to more southerly- southwesterly, which is advecting in monsoonal moisture. PWATs on this evening's 00z KGJT sounding are roughly the same as at 00z yesterday, however the profile is saturated to a lower level in the atmosphere. This increased level of saturation was evident in more precipitation reaching the ground today, with wetting rains observed across and south of the San Juans. By Wednesday, saturation down to the surface is expected, with widespread showers and thunderstorms in the afternoon, and wetting rain across the higher elevations. This will lead to an increased threat for flash flooding and burn scar debris flows as we move into Wednesday and Thursday.

Per ensemble forecasts, moisture, and therefore shower and storm coverage, is expected to peak on Thursday, with both Wednesday and Friday seeing widespread showers and storms as well. This means that Thursday has the highest threat for heavy rains, flash flooding, and debris flows. However, there comes the question of morning cloud cover. Hi-Res guidance is favoring a very monsoonal pattern of overnight convection the next several nights, with cloud cover lingering into the morning hours. Should this cloud cover be particularly dense and long-lasting, it will impact how much instability can develop over eastern Utah and western Colorado, and therefore how much precipitation will be convective versus stratiform. This will also impact timing of storm development, with lingering cloud cover displacing initiation later into the afternoon. This will be something to keep an eye on, especially when moisture is maximized on Thursday. The other question is that of a source of lift. Terrain is a big one, with afternoon showers and storms expected to form first over the ridges. However, there are also subtle waves rotating around the edge of the high that could give some extra lift over the next several days, as well as boundaries left over from the previous day's convection. There is less certainty in the impact from the shortwaves and the lingering boundaries, which does throw some uncertainty over where convection will develop once it's moved off the ridges. So while details are still a bit murky, confidence is high that we're moving into a wetter period for the second half of this work week, and even into the weekend.

Temperatures will cool through the back half of the week as showers, storms, and cloud cover rule the day. Near-normal highs on Wednesday will cool to close to ten degrees below normal by Friday. Overnight lows will also cool to near-normal levels, which will be a welcome relief.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 528 PM MDT Tue Aug 11 2026

Scattered showers with isolated thunderstorms currently over the southern and central Colorado mountains, and isolated showers and thunderstorms over the higher terrain of northwestern Colorado and northeastern Utah will diminish after 02Z with only light showers lingering past 06Z. Main threats are lightning and gusts to 35 kts. Look for showers to begin developing after 16Z becoming more widespread over the higher terrain with scattered thunderstorms after 18Z. Mountains TAF sites may see brief periods below breakpoints with passing storms. Otherwise, winds will generally be light across the region with VFR conditions.

GJT WATCHES/WARNINGS/ADVISORIES

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