RANCHO MIRAGE (California), Aug 21 — Tropical Storm Hilary unleashed furious flash floods east and west of Los Angeles yesterday as the system made its historic arrival in California after barrelling through Mexico’s Baja California peninsula with deadly force.

One person died in Mexico amid reports of flash flooding in the peninsula, where some roads were swept away and images on social media showed raging torrents gushing down city streets that been turned into rivers.

California Governor Gavin Newsom declared a state of emergency for much of Southern California, with flash flood warnings in effect until at least 3am (6pm Malaysian time) on Monday in a region that is more accustomed to drought.

Mountain and desert areas could get 12 to 25cm rain, as much as the deserts typically see in a year, forecasters said.

The first tropical storm to pelt Los Angeles County since 1939 triggered serious flooding in the San Gabriel Mountains east of the city and coastal areas to the northwest in Ventura County.

San Bernardino County ordered evacuations of a number of towns in the mountains and valleys where social media images showed torrents of water, mud, rock and trees.

In Wrightwood, California, about 112km northeast of Los Angeles, the rain washed trees and mud down a hill in Sheep Canyon. Further east in Oak Glen — one of five San Bernardino County towns under evacuation orders — gushing floodwater threw trees, mud and rock into the air.

To the west in more populated Ventura County, the National Weather Service warned of life-threatening flooding where up to 5cm of rain fell within two hours. The weather service reported cars stuck in the community of Spanish Hills, where it said firefighters conducted swift water rescues.

Newsom, on a tour of Southern California, said Palm Springs, a desert getaway in Riverside County about 160km east of Los Angeles, was dry when he left on Sunday but an hour later it had received "the most significant rainfall over a 60-minute period any time in the history of Palm Springs.” The streets were soon flooded.

"That’s how quickly this system is moving. Take nothing for granted,” Newsom told a news briefing in Los Angeles after he said he updated US President Joe Biden, who ordered federal agencies to move personnel and supplies into the region.

‘Never seen anything like this’

The storm stunned people in the nearby town of Rancho Mirage, where water and debris rushed over closed roads and stranded at least one pickup truck that was stuck in water nearly to the top of its bed.

"It’s quite amazing. I’ve never seen anything like this,” said Sean Julian, 54, of Rancho Mirage. "I’m seeing a lot more trees down. And there’s a big tree that just fell over there, and I probably shouldn’t be out here.”

DJ Hilton of neighbouring Cathedral City said: "We’ve had storms before, but never anything quite this windy and rainy at the same time.”

At 8pm PDT (11am Malaysian time today), Hilary was 170km northwest of San Diego, packing winds of 75kph and moving to the north-northwest, the weather service said.

Hundreds of flights in San Diego, Las Vegas and Los Angeles were cancelled, and professional sporting matches rescheduled. The Los Angeles Unified School District and San Diego Unified School District, the two largest school districts in the state, cancelled classes for Monday. Dangerous surf pounded the beaches in Southern California.

Floodwaters raced through the concrete banks of the Los Angeles River, which normally contains barely a trickle. In Ocotillo, a desert town about 145km east of San Diego, rock slides brought boulders down on Interstate 8, causing traffic delays on the highway to Arizona.

Hilary made landfall earlier on Sunday in the northern part of Mexico’s Baja California peninsula, where nearly 1,900 people were evacuated to shelters, according to the country’s army.

The storm was especially dangerous in low-income areas where many homes fail to meet building codes.

"We’ve always been aware that it’s a risky area. A lot of water runs (nearby) but what are we going to do? It’s the only place we have to live,” said Yolanda Contreras, living in a flood-prone area of Rosarito, about 24km south of the US-Mexican border. — Reuters