CARPINTERIA, Calif. — As California reeled from another bout of extremely wet weather on Monday night, most of Los Angeles County was under a flash flood warning — an unusual twist for a dry, sunny place where the concern is typically a lack of water.
The flash flood warning for southwestern Los Angeles County was issued before 7 p.m. and would be in effect until midnight, the National Weather Service said in an advisory. The warning affected more nearly 8 million of the more than 10 million residents in the county, the most populous in the United States.
The full extent of flooding in Los Angeles County was not immediately clear as of about 10:30 p.m. California’s Central Coast, where officials had ordered evacuations on Monday in a coastal enclave of Santa Barbara County, still appeared to be the hardest hit area of the state for the day.
But as heavy rain pummeled the L.A. area, the Weather Service said that downtown Los Angeles, Malibu, Hollywood and Beverly Hills were among the places that would experience flash flooding. Early images showed cars partially submerged by floods near downtown. And more than an inch of rain fell in a single hour in a part of Los Angeles County that had once been damaged by wildfires.
The Federal Aviation Administration also issued a so-called ground stop for Los Angeles International Airport shortly after 8 p.m., a move that slowed the pace of takeoffs and landings for about an hour amid high winds, said Victoria Spilabotte, a spokeswoman for the airport.
“That usually happens at airports across the country, but we don’t often have a ground stop, mostly because Los Angeles has pretty good weather year round,” she added. “So this type of storm is not typical for us.”
Earlier on Monday, the Weather Service said that it expected “no significant letup” to the waves of moisture — known as atmospheric rivers — that had been pummeling California for weeks. Heavy precipitation was expected across the entire state on Tuesday, and parts of Southern California could see up to seven inches of rain over the next few days, the agency said.
The flooding, and fears of danger, in Los Angeles on Monday night followed a frantic day in Santa Barbara County, where officials ordered thousands of residents to quickly evacuate the coastal enclave of Montecito amid fears of mudslides in an area where wildfires have made soils and vegetation less stable.
Up to a foot of rain was expected to soak Montecito’s already drenched hillsides. And the evacuation orders were issued five years to the day that a deadly torrent of mud and boulders rushed through neighborhoods in that mansion community, killing 23 people and turning it into a disaster area.
“We’re in the midst of a series of significant and powerful storms,” Sheriff Bill Brown of Santa Barbara County said in a briefing. “Currently, we’re experiencing a storm that is causing many problems and has the potential to cause major problems across our county, especially in the burn scar areas.”
Late Sunday, President Biden approved an emergency declaration for 17 counties in California, allowing for federal assistance from the Federal Emergency Management Agency and the Department of Homeland Security in relief and rescue efforts.
Elsewhere along the Central Coast, one person was killed by floodwater while trying to navigate a submerged road in San Luis Obispo County, north of Santa Barbara. A 5-year-old boy remained missing. Residents were evacuated from numerous communities because of flood risks as their streets turned into gushing streams.
At the Best Western Plus Carpinteria Inn, several miles southeast of Montecito, a steady stream of people clad in rain gear pulled up in SUVs packed with luggage and provisions. Some who had evacuated said they were surprised to be among those ordered to leave because their homes were not in burn scars, areas hit by wildfire that are made more susceptible to landslides.
In the 2018 storm that led to the devastating mudslide, officials had issued mandatory evacuation orders for about 7,000 residents in Montecito and voluntary ones for another 23,000, but many disregarded them because they had just returned home after being forced to leave during a wildfire.
Montecito is a popular haven for celebrities, including Prince Harry and his wife, Meghan, the Duchess of Sussex; Oprah Winfrey; and Ellen DeGeneres, who posted a video on Twitter of a raging creek behind her house that she said “never flows, ever.”
“We need to be nicer to Mother Nature because Mother Nature is not happy with us,” Ms. DeGeneres said.
Evacuation orders were also in place in neighboring Ventura County, including in the tiny community of La Conchita, the site of a 2005 landslide around the same time of year that killed 10 people.
In Santa Cruz County, about 70 miles south of San Francisco, more than 30,000 residents were evacuated as creeks and rivers topped their banks, threatened homes and washed away at least one bridge. Mudslides blocked two highways in the Santa Cruz Mountains that connect the region to the San Francisco Bay Area.
The flooding in the county besieged an area already reeling from some of the heaviest damage from recent storms. Just last week, the confluence of a storm surge, high tides and high surf collapsed piers and flooded hundreds of homes and businesses.
The storm’s impacts continued farther south along the state’s Central Coast, with evacuation orders along rivers in Watsonville and Monterey County.
Numerous roads were closed amid flash-flood warnings in San Luis Obispo County, where the 5-year-old boy remained missing after he and his mother escaped from a car that was starting to be swept away by floodwaters. His mother, who had been driving him to school around 8 a.m., was rescued by nearby residents, but the boy was carried away by waters coursing down a rising creek, said Tony Cipolla, public information officer with the San Luis Obispo County Sheriff’s Office.
Divers with the agency’s underwater search and rescue team scoured the nearby waters for hours, but had to call off the search around 3 p.m. when the rising waters and rapid current made it too dangerous, he said.
Rivers and creeks in the area were gushing like they hadn’t in decades, said Scott Jalbert, the county’s emergency services manager. “They’re pretty monstrous,” he said.
California Polytechnic State University in San Luis Obispo shut down for the day. The university reported that students, faculty and animals were being evacuated from agricultural facilities with a reservoir about to breach.
In the nearby town of Santa Margarita, Tamara Snow Nyren said that for all her preparations the night before — building a fort of sandbags all around her home — she was not ready for Monday’s flooding.
“My God, I look out the window to my alley, and I saw a river coming down my alley,” she said.
Jill Cowan reported from Carpinteria, Calif., Victoria Kim from San Francisco and Mike Ives from Seoul. Katya Cengel contributed reporting from Grover Beach, Calif.