July 17, 2017
Flash floods filling forest canyons with high water – like the one that killed nine members of an extended family last weekend in Arizona – are more likely in desert habitats of New Mexico and Arizona. But the possibility of a similar incident occurring in Southern California is still very real, authorities said Monday.
Though locally most rain falls during the winter, the occasional summer thunderstorm can drift over the San Gabriel and San Bernardino mountains and release an inch or more of rain, especially in the Inland Empire where weather patterns are not tempered by the Pacific Ocean, meteorologists said. The National Weather Service forecasts a slight chance of afternoon and evening thunderstorms Tuesday and Wednesday across the mountains and Antelope Valley.
In San Bernardino County, the one natural hazard common to cities, steep mountains and vast deserts are flash floods, said Eric Sherwin, a spokesman for the San Bernardino County Fire Department.
“Just like we saw in Arizona in a desert region, the water doesn’t have anywhere to go, so it is going to find a path, usually dry creeks and washes,” Sherwin said.
Forest Falls and Lytle Creek in the San Bernardino National Forest are prone to flash floods from summer thunderstorms, he said. In the Angeles National Forest, the Mount Baldy area comes a close second, Sherwin said, followed by the Apple and Lucerne valleys, Needles and Big River. Sometimes a flash flood will force Caltrans to close U.S. Route 95 or the 40 Freeway, he said.
Flash floods that turn a slot canyon into a raging river are less likely to occur in the Santa Monica Mountains, which do not contain slot canyons, said Kate Kuykendall, spokeswoman for the National Park Service.
“We do have a number of steep and narrow canyons that would be dangerous in the case of heavy rainfall,” she said.
Hikers should avoid Solstice Canyon, Zuma Canyon and Carlisle Canyon “in the case of predicted heavy rains,” which typically happens in winter, she said.
Southwestern states of Nevada, Arizona, New Mexico and Colorado are prone to flash floods because of weather patterns and topography, experts said. Summers often bring monsoon season, when moisture is pushed up from the Gulf of California creating scattered thunderstorms, explained Bill Patzert, a climatologist with the Jet Propulsion Laboratory near Pasadena.
“Over the dry terrain there is nothing to absorb all that rapid rainfall, so it will focus into canyons and arroyos that are usually sleepy stream beds,” he said. “You can get rapid flash floods that often takes people by surprise.”
Patzert said that while Los Angeles County is on the fringe of the monsoonal effect, it is not likely to see thunderstorms but rather more humidity this week.
Five children were among the nine people killed during a flash flood Saturday at a swimming hole at Tonto National Forest, about 100 miles northeast of Phoenix. The thunderstorm hit about 8 miles upstream from where the family had gathered.
Often, victims don’t even know it’s raining. In the case of the family in Arizona, they probably did not hear about the flash flood warning issued 11/2 hours prior because they didn’t have a radio and were in an area with no cell service, authorities said.
Many people in the Phoenix area leave for forest swimming holes to escape triple-digit heat. Often, the same is true in San Bernardino and Riverside counties, as well as eastern L.A. County, Sherwin said.
Anyone hiking should listen to weather reports before they go. The Onyx Summit area near Big Bear often attracts thunderstorms in summer, he said. All that water flows into streams that lead to Forest Falls, a popular hiking spot.
In September 2015, Brett Alan Usher, 25, of Rancho Cucamonga was swept to his death by a flash flood in Forest Falls when he and a companion entered a swollen creek. The female companion survived.
With a flash flood, the force of water uproots trees and carries branches and rocks, Sherwin said, overpowering people, vehicles and everything else in its path. “Often, people make a big mistake of attempting to cross a flooded stream,” he said. “They should seek higher ground and wait it out.”
The most common rescues are people in vehicles, he said.
“They are usually locals who say, ‘I know my road,’?” he said. “But it only takes 12 inches of flowing water to move a vehicle.”
In the Santa Monica Mountains, landslides and debris flows can be a problem in fire-ravaged areas, Kuykendall said. In Arizona, a previous fire intensified the flash flood because the burn scar repelled water, fire officials said.
Staff writer Brian Rokos and Anita Snow of the Associated Press contributed to this report.