textproduct: Grand Junction

KEY MESSAGES

- Scattered to widespread precipitation showers and storms are expected through midweek.

- This rainfall increases flooding concerns, 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 1256 PM MDT Mon Sep 28 2026

WIDESPREAD PRECIPITATION THROUGH TUESDAY:

Moisture advection from tropical storm activity in the Eastern Pacific will bring anomalous moisture to our region through at least midweek. PWAT anomalies will be upwards of 200-250% of normal for a good chunk of the forecast area, especially south of I-70. This means widespread precipitation chances for much of the region. A closed upper-level low digs southward through California on today, and then across the Desert Southwest on Tuesday and Wednesday. A jet streak aloft and energy from a trough axis will work in tandem to create lift for continuous showers and storms for the rest of the day. As the low continues to track just south of the AZ/NM/UT/CO state lines on Tuesday, drier periods are possible with the best forcing skirting south of the forecast area. However, The potential for moderate to heavy precipitation will continue to pose a threat for the Southern San Juans and adjacent valleys.

The potential for prolonged periods of rainfall, including in areas that have recently received substantial rainfall, poses flooding concerns through Wednesday. As a result, a Flood Watch is in effect for southeast Utah and southwest Colorado through 6AM Wednesday. Please see the Hydrology Section below for more information on those flooding concerns.

MOISTURE BEGINS TO EXIT THE REGION WEDNESDAY:

By Wednesday evening, the elevated moisture and closed low will move east of the Rocky Mountains, leading to drier weather for much of the forecast area.

High pressure looks to drive the remainder of the seven-day forecast. As such, predominately dry conditions with temperatures running around 5 degrees above normal for early October can be expected late week through next weekend.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 542 PM MDT Mon Sep 28 2026

Showers and a few embedded thunderstorms are ongoing with broken to overcast skies being reported at most TAF sites. These conditions are expected to persist tonight before a break near sunrise. Showers are expected to redevelop over southern Colorado tomorrow morning. Winds will be generally light and terrain driven under these showers. Where heavier rainfall is present, expect flight conditions to vary between low end VFR and MVFR, due to low cigs and visibility constraints. ILS breakpoints will likely be met for some terminals and mountain obstructions will be common.

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 Tuesday night for COZ017>023. UT...Flood Watch through late Tuesday night for UTZ022-028-029.