textproduct: Grand Junction

KEY MESSAGES

- Scattered to widespread precipitation showers and storms continue.

- Increased flooding concerns for rain-soaked terrain, especially within and adjacent to the southern San Juans.

- A Flood Watch is in effect for southeast Utah and southwest Colorado through 6AM Wednesday.

DISCUSSION

Issued at 958 PM MDT Mon Sep 28 2026

Dry slotting is beginning to intrude on the Four Corners at this hour. This is acting to curb precipitation some across the region. Light rain is still reaching the surface at many surface observation sites. Some stronger rainfall is still coming down across the San Juans. This is still an area of concern for rising river levels, as highlighted in the hydrological discussion below. Lower rain rates have kept flash flooding off the table, but some basins could continue to rise due to the persistent nature of the rainfall. Some areas have already seen in excess of an inch of rainfall.

On Tuesday a pinched off low will drop out of the Great Basin moving south of the Four Corners. This continues to pull dry air in behind it which is expected to suppress some of our rainfall in the northern half of the CWA on Tuesday. However, down south rich moisture wrapped into the low will continue to add to the flash flooding concern. Hi res models are keeping the stronger convection south of the Four Corners on Tuesday, so rainfall rates could take a hit downward.

The low lifts out of the Rockies onto the Plains Wednesday, but a strong wraparound moisture plume is expected to produce another surge of rainfall for southern CO through the afternoon. Some colder air wrapped into the low looks to produce some accumulating snow on the San Juans and up the spine of the Divide Wednesday. Snow levels remain above pass level and much of it will melt out in the warmer afternoon temperatures. This should be the final gasp of the tropical moisture, as rainfall tapers to a close by Thursday morning. High pressure begins to build into the region on Thursday. Temperatures return to near normal then begin to climb through the weekend back into the 80s for the desert valleys and into the 60s for the mountains. Models are playing with some showers cropping up on the mountains Sunday and Monday, but that remains a very tentative forecast at this point.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 528 AM MDT Tue Sep 29 2026

Scattered to widespread precipitation is expected over the next 24 to 36 hours as tropical moisture continues to overspread the area. Terrain obscuration will also be widespread leading to areas of ILS to MVFR in adjacent valleys and associated TAF sites. Lower cloud lingering behind the moisture could also lead to MVFR/IFR conditions as stratus and/or fog develops.

HYDROLOGY

Issued at 153 PM MDT Mon Sep 28 2026

The bulk of tropical moisture will be streaming across eastern Utah and western Colorado today. Integrated water vapor transport (IVT) maximizes this evening across the San Juans, so anticipate precipitation amounts to wane as our moisture fetch declines, too. River responses will be highly dependent on precipitation totals, and peak flows will lag behind periods of moderate rainfall. Forecast guidance shows peak arrival times between midnight tonight to Tuesday morning, depending on river segments and proximity to the watershed's headwaters.

The Navajo River, Rio Blanco and San Juan River are forecast to inch towards bankfull conditions and near action stage by early Tuesday morning, with 2 to 2.5" of precipitation expected in their upper basins. However, if locally higher amounts are observed (i.e. exceeding the 90th percentile forecasts of 4.5 to 5.5 inches) then minor flooding is possible. Elsewhere, rivers and creeks will also see a notable rise, but probabilities of exceeding flood stage are lower. Nonetheless, a significant rise in several streams, rivers and creeks is expected in response to today's tropical surge, and minor flood poses a threat, namely across our southern counties.

The northern extent of moisture maintains some ambiguity as the systems merge over the next 48 hours. However, after a brief lull on Tuesday, wrap around moisture on the northern periphery of the low could support another wave of precipitation across the southern half of the CWA on Wednesday. As a result, river forecasts reveal a secondary peak Wednesday evening, albeit, less than its predecessor.

A Flood Watch remains in effect across southeast Utah and southwest Colorado, where the higher precipitation totals are favored. However, keep an eye on evolving conditions and conditions upstream. Flooding continues to be a concern, especially with rivers rising overnight.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Flood Watch through late tonight for COZ017>023. UT...Flood Watch through late tonight for UTZ022-028-029.