Tropical Storm Hilary inundated streets across Mexico’s arid Baja California Peninunsula 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 that rarely get such torrential rain.

Forecasters said Hilary was the first tropical storm to hit Southern California in 84 years, bringing flash floods, mudslides, high winds, power outages and the potential for isolated tornadoes.

Hilary made landfall along the Mexican coast in a sparsely populated area about 150 miles (250 kilometers) south of Ensenada, then moved through mudslide-prone Tijuana, threatening the improvised homes that cling to hillsides just south of the U.S. border.

Read: Powerful Hurricane Hilary heads for Mexico's Baja. Rare tropical storm watch issued for California

At least 9 million people were under flash-flood watches and warnings as heavy rain fell across normally sunny Southern California ahead of the brunt of the storm. Desert areas were especially susceptible along with hillsides with wildfire burn scars, forecasters warned.

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

The National Weather Service's Los Angeles office reported in the evening via X, formerly known as Twitter, that “very heavy rain” was continuing in much of Los Angeles and Ventura counties. The highest rainfall totals so far were 6.15 inches (15.62 centimeters) at Leona Valley and 5.94 inches (15.1 centimeters) at Lewis Ranch, the agency added, saying there was significant flooding and urging people to stay off the roads.

The Los Angeles Unified School District, the nation's second largest school system, said all campuses would be closed on Monday.

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“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. San Diego schools postponed the first day of classes from Monday to Tuesday.

Southern California got another surprise in the afternoon as an earthquake with a preliminary magnitude of 5.1 hit near Ojai, about 80 miles (130 km) 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 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.

By Sunday evening, Hilary had moved over San Diego and was headed north into inland desert areas. Around midday, it had maximum sustained winds of 60 mph (97 kph).

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Hurricane Center Director Michael Brennan said that while Hilary had weakened from a Category 4 hurricane, it’s the water, not the wind, that people should watch out for most — some areas could get as much rain in hours that they typically get in a year.

“You do not want to be out driving around, trying to cross flooded roads on vehicle or on foot," Brennan said during a briefing from Miami. “Rainfall flooding has been the biggest killer in tropical storms and hurricanes in the United States in the past 10 years, and you don’t want to become a statistic.”

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.

The Mexican cities of Ensenada and Tijuana closed all beaches and opened a half-dozen shelters 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.

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Brennan said rainfall could reach between 3 and 6 inches (7 centimeters and 15 centimeters) in many areas. Forecasters warned it could dump up to 10 inches (25 centimeters) — a year’s worth of rain — in some isolated areas.

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.

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.

Los Angeles authorities scrambled to get homeless people off the streets and into shelters, and officials ordered all state beaches in San Diego and Orange counties closed.

Across the region, municipalities ran out of free sandbags and grocery shelves emptied as people stockpiled supplies. California’s Joshua Tree National Park, Mojave National Preserve and Death Valley National Park were closed.

Death Valley's Furnace Creek Visitor Center received more than 1 inch (2.5 centimeters) of rain by 1:30 p.m., with up to 3 inches (7.6 centimeters) more possible overnight. “For comparison, Furnace Creek’s average annual rainfall is 2.2 inches (5.6 centimeters),” the park said in a statement, calling the rainfall “unprecedented.”

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 (97 kph) 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.

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.