SAN DIEGO — Tropical Storm Hilary inundated streets across Mexico’s arid Baja California Peninsula with deadly floodwaters Sunday before moving over Southern California, where it swamped roads and downed trees, as concerns mounted that flash floods could strike in places as far north as Idaho.

Forecasters said Hilary was the first tropical storm to hit Southern California in 84 years, bringing floods, mudslides, high winds, power outages and the potential for isolated tornadoes. The storm already dumped more than 6 inches of rain in some mountain communities and threatened more than an average year’s worth of rain in inland desert areas.

Hilary made landfall along the Mexican coast in a sparsely populated area about 150 miles south of Ensenada Sunday, then moved through mudslide-prone Tijuana, threatening the improvised homes that cling to hillsides just south of the U.S. border. By Sunday evening, the storm had moved over San Diego and was headed north into inland desert areas.

As evening fell in California, the National Weather Service in Los Angeles warned of significant flooding risk throughout populous mountain areas along the coast northeast of Los Angeles.

“PLEASE … STAY OFF THE ROADS,” the agency posted on X, formerly known as Twitter.

Mud and boulders spilled onto highways, water gushed onto roadways and tree branches fell in neighborhoods from San Diego to Los Angeles. Dozens of cars were trapped in floodwaters in typically hot and dry Palm Desert and surrounding communities across the the Coachella Valley. Crews pumped floodwaters out of the emergency room at Eisenhower Medical Center in Rancho Mirage.

Other Western States Still in Storm Path

Hilary could wallop other Western states with once-in-a-century rains, with a good chance of it becoming the wettest known tropical cyclone to douse Nevada, Oregon and Idaho. Hilary was expected to remain a tropical storm into central Nevada early Monday before dissipating.

The Los Angeles Unified School District, the nation’s second largest school system, and said all campuses would be closed on Monday. San Diego schools postponed the first day of classes from Monday to Tuesday.

“There is no way we can compromise the safety of a single child or an employee, and our inability to survey buildings, our inability to determine access to schools makes it nearly impossible for us to open schools,” Superintendent Alberto Carvalho said at a media briefing.

Southern California got another surprise in the afternoon as an earthquake with a preliminary magnitude of 5.1 hit near Ojai, about 80 miles northwest of downtown Los Angeles, according to the U.S. Geological Survey. It was felt widely and was followed by smaller aftershocks. There were no immediate reports of major damage or injury, according to a dispatcher with the Ventura County Sheriff’s Office.

Hilary is just the latest major climate disaster to wreak havoc across the U.S., Canada and Mexico. Hawaii’s island of Maui is still reeling from a blaze that killed over 100 people and ravaged the historic town of Lahaina, making it the deadliest U.S. wildfire in more than a century. Firefighters in Canada are battling that nation’s worst fire season on record.

Beaches were closed across the Mexican cities of Ensenada and Tijuana while shelters were opened at sports complexes and government offices.

One person drowned Saturday in the Mexican town of Santa Rosalia when a vehicle was swept away in an overflowing stream. Rescue workers saved four other people, said Edith Aguilar Villavicencio, the mayor of Mulege township.

Mexican army troops fanned out across Mulege, where some of the worst damage occurred Saturday on the eastern side of the Baja Peninsula. Soldiers used bulldozers and dump trucks to help clear tons of boulders and earth clogging streets and roads that were turned into raging torrents a day earlier.

Power lines were toppled in many places, and emergency personnel were working to restore power and reach those cut off by the storm.

Newsom Declares State of Emergency

California Gov. Gavin Newsom declared a state of emergency. The Federal Emergency Management Agency said it has officials inside California’s emergency preparedness office and teams on standby with food, water and other help.

To the north in Nevada, Gov. Joe Lombardo declared a state of emergency and activated 100 National Guard troops to assist with problems from predicted flooding in western Clark and Nye counties and southern Esmeralda County. In Arizona, wind gusts neared 60 mph in Yuma County, where officials gave out thousands of sandbags.

“I urge everyone, everyone in the path of this storm, to take precautions and listen to the guidance of state and local officials,” President Joe Biden said. Biden said in a later statement that he was being briefed on the storm and was prepared to provide federal assistance.

The warnings from officials didn’t keep everyone indoors. On Sunday morning in coastal Carlsbad, just north of San Diego, 19-year-old Jack Johnson and his friends kept an eye on the huge waves, determined to surf them at some point Sunday.

“It’s really choppy out there, not really surfable yet, but I think we can find a good break somewhere later,” Johnson said. “I can’t remember a storm like this.”

The weather service said tornadoes were possible in eastern San Diego County.

Death Valley National Park could get more rainfall from the storm than the area sees in an average year, officials said.

Meanwhile, one of several budding storm systems in the Atlantic Ocean became Tropical Storm Emily on Sunday, according to the National Hurricane Center. It was far from land, moving west in the open ocean. Also, Tropical Storm Franklin formed in the eastern Caribbean. Tropical storm watches were issued for the southern coasts of Haiti and the Dominican Republic.

In Sept. 1939, a tropical storm that roared into California ripped apart train tracks, tore houses from their foundations and capsized many boats, killing nearly 100 people on land and at sea.