A flash flood warning was issued by the National Weather Service in Flagstaff at 3:06 p.m. on Saturday, Sept. 27. This warning applies to Coconino County.
The National Weather Service warns: “Campers and hikers should hustle to higher ground immediately! Stay away from creeks and washes! Foot bridges will be washed out in Supai Campground. Some campsites will be flooded. Campers may be stranded in portions of the campground.”
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What are weather service meteorologists saying?
At 3:06 p.m., the National Weather Service issued a statement:
“The National Weather Service in Flagstaff has issued a Flash Flood Warning for: The Havasu Creek basin in, Coconino County in north central Arizona, This includes the Colorado River between river miles 156 and 158. Until 6 p.m. MST. At 3:06 p.m. MST, local law enforcement reported flash flooding near Supai from heavy rain over the Havasu Creek basin. Up to 0.5 inches of rain have fallen. Additional rainfall amounts up to 0.25 inches are possible in the warned area. Flash flooding is already occurring.
HAZARD: Life-threatening flash flooding. Thunderstorms producing flash flooding in and around the Havasu Creek basin.
SOURCE: Law enforcement reported.
IMPACT: Life-threatening flash flooding of areas in and around the Havasu Creek basin. Some locations that will experience flash flooding include: Grand Canyon National Park, Little Navajo Falls, Mooney Falls, Beaver Falls, Hualapai Hilltop, Supai and Havasupai Reservation. This includes the Colorado River between river miles 156 and 158. This includes the following swimming holes: All swimming holes on Havasu Creek near Supai. This includes the following creeks, washes and rivers: Heather Wash, Colorado River, Pasture Wash, Havasu Creek, Black Tank Wash and Little Coyote Canyon.”
What is a flash flood?
Arizona’s desert landscape is highly prone to flash flooding. A flash flood is a flood caused by heavy or excessive rainfall in a short period of time, generally less than six hours, according to the weather service.
A flash flood can be sudden and violent and take just minutes to develop. Even if skies are clear overhead, rainfall upstream can funnel into dry riverbeds and create dangerous flows miles away.
How much water is needed to cause a flash flood?
It doesn’t take much water to trigger a flooding event in the desert: An inch of rain or less in an hour is often enough to cause a flood. How quickly and where rain falls determines flooding more than rain totals.
Desert soils struggle to absorb water, especially after extended dry spells. The surface can become almost waterproof, causing runoff.
Topography also plays a role, as steep canyons, mountains and dry washes are susceptible to floods. Areas impacted by wildfires are more at risk, as the flames can burn the top layer of the soil, preventing absorption and increasing runoff.
What’s the difference between a flash flood watch and a flash flood warning?
A flash flood watch means conditions are favorable for flash flooding to develop, but it isn’t happening yet. In Arizona, this often happens when monsoon storms or heavy upstream rainfall could send sudden surges of water through washes, canyons or urban areas.
During a flood watch, it’s important to monitor local forecasts and alerts, plan evacuation routes in case flooding begins, move vehicles out of low-lying areas and avoid recreation in areas that could fill with water quickly.
A flash flood warning is more urgent, meaning flooding is impending or already occurring. Warnings are typically issued when rainfall is recorded at levels that can suddenly overwhelm desert drainage or when flooding has already been observed.
Tips for staying safe during a flood warning in Arizona
If advised to evacuate, do so immediately.
Do not walk through floodwater. Just 6 inches of floodwater can sweep you off your feet if it is moving swiftly.
Turn around, don’t drown: Never attempt to drive through a flooded street; instead, turn around and find an alternative route. Just 12 inches of water can carry away smaller vehicles, and 2 feet can carry away SUVs and trucks. Most flood-related deaths are caused by people driving through water.
Watch out for fire hazards and move to higher ground.
Pay attention to weather notifications and road closures.
Do not allow children to play in flowing water. Waters can hide rocks, trees and debris.
This weather report was generated automatically using information from the National Weather Service and a story written and reviewed by an editor.
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This article originally appeared on Arizona Republic: Flash flood warning issued for northern Arizona, Grand Canyon