textproduct: Grand Junction

KEY MESSAGES

- Warm and windy today, with isolated shower and storm chances.

- Shower and storm chances increase Sunday, and peak in coverage by Monday, bringing in widespread cool, wet, and unsettled weather.

- Daytime highs this weekend will be 5 to 10 degrees above normal, before a shift to cooler temperatures by Monday.

DISCUSSION

Issued at 1023 AM MDT Sat Sep 12 2026

ISOLATED TO SCATTERED STORMS THIS WEEKEND:

Modest moisture has begun to settle over the region this morning, prompting the sparse, weak showers and thunderstorms over Colorado's Western Slope. You may have also noticed a breezy morning thanks to decent mixing under the strong zonal flow, paired with a modest pressure gradient. These breezy conditions will continue to be around this afternoon as decent mixing continues to be favored. Also worth mentioning is shower activity will also contribute to windy conditions should gusty outflow winds develop. Shower and storm chances this afternoon will be relatively isolated in nature and so should impacts. By Sunday, storm coverage becomes more scattered as the next strong push of moisture begins to move in. PWATs begin to climb Sunday, with values upwards of 200% of normal, with the likely chance of exceeding the 75th percentile and approaching, if not surpassing the max values for mid September.

COOLER AND WETTER PATTERN MONDAY AND BEYOND:

By Monday, a deep trough begins to push into the PacNW. Due to cyclonic flow around a low, southwesterly flow will be favored over the southwestern CONUS. This means subtropical moisture will be pushed further inland, centering the strong plume of moisture over our CWA. PWATs continue to be forecasted over the 75th percentile for this time of year, which will be enough to prompt our next chance for widespread showers and storms.

Cloud cover is expected to be generally widespread come Monday, especially in the mornings. This has the ability to dictate storm potential each afternoon, which just might be enough to push storm mode in the stratiform direction. However, it is worth considering the dynamic forcing at play during this time period as well. Firstly, Sunday night into Monday will see a cold frontal passage which will include pockets of frontogenesis. Secondly, by Monday we are expected to be under the right entrance region of an upper level jet. This promotes areas of upper level divergence and surface level convergence. Both of these factors will go towards supporting discrete storm development (embedded stronger storms in stratiform rainfall). Whether these storms are isolated or widespread in nature, localized heavy rainfall is a possibility. Impacts from this rainfall will be enhanced in places that experienced multiple rounds of showers and storms the past weekend.

In terms of QPF, the NBM continues to favor the southwestern facing slopes, particularly along the San Juans, Grand Mesa, and West Elks. Playing around with probabilities for those southwestern slopes shows a 60-80% chance of exceeding 0.75" of rain at least for the Grand Mesa and San Juans. As for the northeastern slopes, lower rainfall totals are likely, but still indicating some spots to see 0.25" of rain. Although, if rain stratiforms out, these totals will likely decrease. It is important to note that there is still time for these factors to evolve before we can speculate on the likeliest scenario. Stay tuned for future updates, especially once more HiRes guidance becomes available.

Look for daytime highs to cool down Monday and beyond due to that aforementioned frontal passage. For the rest of the work week, active weather will continue to be present. The moisture pool will continue to lock in over the Western Slope, with the upper level jet continuing to stretch across the Intermountain West. But, without clear signs of additional forcing, impacts from this moisture could be even more conditional on morning cloud cover. At this time, this is not a factor we can get too in the weeds about. By Friday, models are in agreement we see a dissipation of moisture, but not a complete elimination as PWATs still appear to be above normal.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1148 AM MDT Sat Sep 12 2026

Some isolated showers and storms continue to move eastward this morning and could impact the KMTJ and KHDN terminals in the next hour or two. Once this wave clears, afternoon convection is expected to redevelop over the high terrain with KASE, KTEX, KHDN and KGUC so included PROB30 mention. Otherwise skies will be few to scattered with breezy afternoon winds with gusts 25-30 knots.

GJT WATCHES/WARNINGS/ADVISORIES

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