Flood watches are in effect for nearly seven million people across the Southwest, where rounds of heavy rain could trigger fast-moving flash floods with little to no warning.
Alerts covered parts of Arizona, Utah and Colorado on Monday, with many expected to remain in place into the evening. Areas under threat included Phoenix, the Grand Canyon, Zion National Park, Moab, Aspen, Vail and Durango.
The warnings follow deadly flooding at the Grand Canyon on Saturday afternoon, where one hiker was found dead and at least 14 others were reported missing after floodwaters swept through the area.
Forecasters said the risk has been heightened by deep monsoonal moisture, repeated thunderstorms and soil already soaked from previous rainfall.
In southern Arizona, some communities could see one to three inches of rain, while the most intense storms may drop more than an inch in a single hour, according to the National Weather Service.
Forecasters warned that runoff could transform dry washes and narrow slot canyons into ‘raging killer currents’ while overwhelming roads and low-water crossings.
Rivers, creeks, urban drainage systems and recent wildfire scars also face an increased risk of flooding.
Residents and tourists were told to monitor alerts and reconsider plans to hike, boat or paddleboard in slot canyons and normally dry washes.
Parts of Arizona, including the Grand Canyon (above), Utah and Colorado, were under flood watches on Monday
The flooding threat is being caused by deep monsoonal moisture fueling repeated, slow-moving thunderstorms.
Some storms could dump more than one inch of rain per hour, with isolated totals of one to three inches.
Soil already saturated by earlier storms cannot absorb much more water, causing rapid runoff into rivers, creeks, dry washes and slot canyons.
Steep terrain, low-water crossings, poor urban drainage and wildfire burn scars further increase the risk of sudden flash flooding.
Meteorologist Max Velocity said on X: ‘Dual upper-level disturbances lifting out of Mexico and California are fueling strong atmospheric lift across the Desert Southwest. Anomalous tropical moisture pooling across the region will fuel widespread thunderstorm development.
‘With wind shear organizing storms to repeatedly track over the exact same areas, rainfall rates could easily surpass one inch per hour. Sensitive features like slot canyons, dry washes, and recent-burn scars face life-threatening flash flood danger!’
In southern Arizona, a flood watch covered the Tohono O’odham Nation, including Sells, through Monday evening.
Storms could dump one to three inches of rain, while rainfall from Sunday has left dry washes, creeks and road dips vulnerable to rapid flooding.
The National Park Service said on Sunday that it is seeking information about more than 20 people who may be missing or unaccounted for in Bright Angel Canyon and the Phantom Ranch area
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Northern and central Arizona were also under alerts, including the Grand Canyon, Flagstaff, Sedona, Prescott, Payson, the Mogollon Rim and White Mountains. Saturated ground increased the danger in slot canyons, swimming holes and wildfire burn scars.
The National Park Service announced that the body of a 46-year-old man had been recovered near Crystal Rapids along the Colorado River.
The victim’s identity has not yet been released, while the Coconino County Medical Examiner’s Office investigates.
The other missing hikers are believed to have been trekking on routes near the Bright Angel Canyon and the Phantom Ranch area of the national park.
Authorities are asking the public to help identify people who were traveling near the North Kaibab Trail from the North Kaibab Trailhead to Phantom Ranch.
This includes anyone who was staying at Bright Angel Campground, or traveling or staying in the area known as ‘the Box’ north of Phantom Ranch.
A separate watch covered Phoenix and surrounding communities, including Scottsdale, Mesa, Chandler, Glendale, Apache Junction, Casa Grande and Globe.
Storms repeatedly passing over the same areas could produce another one to two inches of rain and rapid runoff.
Alerts also extended across western and southwestern Arizona, including Buckeye, Avondale, Tonopah, Gila Bend, Central La Paz County and the Sonoran Desert National Monument.
The debris flow out of Bright Angel Creek – including sediment, trees, bridges and structures – had formed a dam that raised the effective height of the Colorado River by about eight feet
The Northwest Plateau, including the western Grand Canyon, Tuweep and Colorado City, faced heavy rain capable of forcing streams over their banks and flooding roads.
In Utah, watches stretched from the central mountains to Zion, Bryce Canyon and Capitol Reef national parks.
Glen Canyon, Lake Powell, the Sanpete and Sevier valleys and Castle Country were also included. Saturated soil left those areas especially vulnerable, while flooding could strike dry washes, slot canyons and several recent burn scars.
Southeastern Utah alerts covered Moab, Arches, Canyonlands, Blanding, Bluff and Monticello.
The same storm system threatened much of western Colorado, including Grand Junction, Aspen, Vail, Montrose, Gunnison, Telluride, Ouray and Cortez.
Several rounds of heavy rain could trigger flooding around rivers, creeks and recent wildfire scars.
A separate watch covered the Animas and San Juan river basins in southwestern Colorado from Monday morning through the evening, including Durango, Bayfield, Ignacio and Pagosa Springs.
The NWS urged residents and tourists to monitor flash-flood warnings and be prepared to act immediately.