textproduct: Grand Junction

KEY MESSAGES

- Showers and storms will continue this week each afternoon and evening, with coverage becoming more widespread by Tuesday and Wednesday.

- Primary threats include heavy rainfall, lightning, and outflow winds up to 30-40 mph. Small hail cannot be ruled out in stronger storms.

- Storms have the ability to produce flash flooding and debris flows in vulnerable terrain, including areas near and downstream of burn scars.

DISCUSSION

Issued at 1148 AM MDT Tue Jul 21 2026

STORMS THIS AFTERNOON/EVENING:

Little change in the synoptic pattern is expected today with high pressure parked to our northeast leading southeast flow aloft. However, a few mesoscale developments will prove pivotal to the forecast. Monsoonal moisture will continue to trickle in beneath the high this afternoon. Weak shortwave energy is expected engage the existing moisture pool and produce another round of showers and thunderstorms this afternoon and evening. Yesterday's storms produced rainfall rates of 2-3 inches per hour in places, enough for numerous Flash Flood Warnings on susceptible burn scars and flood- prone terrain. This gave us high enough confidence to issue a Flash Flood Warning today for portions of southeast Utah and western Colorado with additional dynamic forcing from the shortwaves. Please see the Hydrology Section below for more information.

Please continue to monitor the forecast and adjust your plans as needed. Storms can develop rapidly, bringing sudden hazards.

MIDWEEK OUTLOOK:

By midweek, the high-pressure system will shift slightly to the east, allowing a weak low pressure system to set up over the Four Corners region. While storms have recently been taking an unusual east-to-west path, the new pattern will likely steer them up from the south and perhaps the west by late week. This will also increase cloud cover across the region which will lower our confidence in storm strength, but increase the coverage of showers and storms across the region. We will be monitoring this aspect of the forecast closely. If there is enough clearing tonight, a Flash Flood Watch may be warranted again tomorrow.

Daytime highs will cool a handful of degrees tomorrow and return to seasonable or slightly below normal through Friday.

LOOKING AHEAD:

High pressure builds back in by the end of the week bringing warmer temperatures with southwest flow aloft. The amount of moisture available varies greatly between models by this point in the forecast, so forecast confidence decreases. However, there is no real push of dry air across the region, so we anticipate enough moisture for afternoon showers and storms favoring the mountains each day through early next week. The strength, coverage, and hazards associated with this convection remain uncertain at this time.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 541 PM MDT Tue Jul 21 2026

Showers and thunderstorms will continue to threaten aviation operations this evening. Gusty winds, hail, and torrential rains will lead to breakpoint conditions for terminals in southwest CO and southeast UT now through the evening hours. This activity will be more isolated north of the highway 50 corridor. Storms will decay into steady light rain, likely hanging on into the morning hours of Wednesday. This will likely keep some ceilings over a few terminals into daybreak. Widespread thunderstorms will develop on Wednesday afternoon again, with similar impacts as today.

HYDROLOGY

Issued at 1148 AM MDT Tue Jul 21 2026

A rich plume of monsoonal moisture has become established across portions of eastern Utah and western Colorado. Scattered thunderstorms capable of producing heavy rain are expected to continue this afternoon into late evening hours. Rainfall rates of 2- 3 inches per hour will be possible with the stronger storms. These rates could lead to flash flooding and debris flows especially over steep terrain and recent burn scars. Also, rain over the past several days has lead to saturated soils, which increase the chances for excessive runoff.

Southeast to east Utah, and nosing into southwest Colorado, show higher levels of surface based CAPE this afternoon, which will support deeper convection to develop. These areas are more likely to observe stronger thunderstorms, rich with the atmospheric moisture. For that reason, we have hoisted a Flash Flood Watch for said areas. In addition, CAM guidance illustrates another round of evening convection driving across western Colorado. Due to excessive rainfall observed within the last 24 to 72 hours, saturated soils over already susceptible terrain to flash flooding triggered an expansion of the Watch area to include the Grand Mesa, portions of the I-70 corridor and across the Roan Cliffs.

As a reminder: Keep an eye on our forecasts through the end of the week and stay tuned for updated hydrology alerts if visiting or if you reside below terrain vulnerable to flash flooding and/or debris flows. Dry creeks and arroyos can fill quickly and without warning from storms miles away.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Flood Watch until 3 AM MDT Wednesday for COZ003-007-009-017- 020-021. UT...Flood Watch until 3 AM MDT Wednesday for UTZ022-025-027>029.