textproduct: Grand Junction

KEY MESSAGES

- Showers and thunderstorms redevelop over the higher terrain Wednesday afternoon and evening. Main concerns will be gusty outflow winds and lightning.

- A warming trend will continue. Triple digits will remain confined to the Four Corners and low desert valleys.

- Monsoonal flow begins to appear in model outputs this weekend, along with cooler, more seasonal temperatures.

DISCUSSION

Issued at 1026 PM MDT Tue Aug 18 2026

A frontal boundary is dropping south through eastern Utah and western Colorado tonight into Wednesday, triggering some scattered showers and thunderstorms. An accompanying upper level jet is keeping this shower activity tracking along pretty quickly, and dry surface levels are limiting the amount of precipitation reaching the ground. The main concerns continue to be gusty outflow winds and lightning. Showers and storms will taper off after midnight, but will redevelop across the higher terrain Wednesday afternoon and evening, again with the same concerns. The frontal boundary will drift east by Thursday as ridging builds in across the Four Corners. Enough moisture trapped under this ridge will keep chances for mountain showers and thunderstorms in the forecast, but storm motion will slow right down with the lack of steering flow. This means that, while surface levels remain dry, a slow moving storm over a sensitive area could eventually lead to some heavy rain and flash flooding concerns. Otherwise, temperatures continue to rise through the end of the work week, running around 10 degrees above normal by Friday. Triple digit heat will be possible across the desert valleys of eastern Utah and western Colorado.

An omega block sets up for the weekend, with the high centered just far enough east of the Divide for southwesterly flow to dominate over the Great Basin and Southwest. As a result, monsoonal moisture will begin advecting into the region. Discrepancies on things like timing, intensity, and extent of the moisture plume remain between deterministic and ensemble model solutions. However, the overall signal of above normal moisture returning to eastern Utah and western Colorado remains, increasing forecast confidence. Clouds, showers, and thunderstorms will become more widespread through the weekend, along with a return to more seasonable temperatures. Further speculation on impacts will depend on storm motion, the potential for training storms, and instability resulting from cloud cover (or the lack thereof), all of which are too early to speak on with confidence at this time. We'll have to wait on higher resolution guidance and better model consensus before these details become clearer.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1124 PM MDT Tue Aug 18 2026

A band of isolated to scattered showers with a few embedded thunderstorms are quickly moving east across the region generally along and north of I-70 favoring the higher terrain. Look for these showers to to sag south to about KMTJ and KGUC through the overnight as they diminish, ending by about 12Z. Isolated to scattered showers and thunderstorms pick up again after 16Z favoring the mountain areas. Primary threats are lightning and outflow gusts to 35 kts. Probabilities remain low, 20%, at TAF sites, but expect storms over the mountain around KEGE, KASE, KTEX and KDRO through the afternoon. Conditions should remain above ILS breakpoints. Otherwise look for VFR conditions with light winds becoming westerly after 18Z with gusts 20-25 kts.

GJT WATCHES/WARNINGS/ADVISORIES

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