Tropical Storm Hilary has reached the US state of California after bringing fierce winds and flooding to Mexico's Pacific coast.
It lashed Mexico's Baja California peninsula with winds of 70 mph (119km/h). One man died there trying to cross a stream in his car.
A number of houses are now submerged in the town of Santa Rosalia.
Hilary is set to weaken as it moves north, but it is already California's first tropical storm in 84 years.
The last time a tropical storm made landfall in southern California was in Long Beach in 1939.
Experts say recent abnormal weather events that have plagued the US - and several areas across the globe - have been influenced by human-caused climate change.
The storm made landfall in the northern part of Mexico's Baja California peninsula at 11:00 local time (18:00 GMT) on Sunday, the US National Hurricane Center (NHC) said.
It warned that Hilary could cause "life-threatening" floods in both Mexico and the US.
In Mexico, 18,000 soldiers were placed on standby earlier to assist in rescue efforts.
The man who died in the state of Baja California Sur was in a car with his wife and children, local officials said - his family survived.
In Santa Rosalia, on the state's eastern coast, dramatic videos have emerged showing powerful torrents of muddy water cascading down the main street.
Across California, residents have been putting out sandbags, including in Long Beach and Palm Springs.
Eugenie Adler, a resident of Long Beach, told Reuters: "Flooding where people lose some property is one thing, but flooding where people die is another. And I'm afraid people might die."
Los Angeles mayor Karen Bass called it "an unprecedented weather event".
"But Los Angeles has deep experience responding to crises whether it be wildfire or earthquakes," she said. "The city is prepared."
Nearly 26 million people in the south-western US are under flood watch.
Hilary was downgraded to a Category 1 storm after weakening on Saturday, but officials kept up their warnings.
Tropical-storm-force winds extend outward up to 240 miles (390 km) from its centre, according to the NHC.
Nancy Ward, director of the California Governor's Office of Emergency Services, said Hilary could be one of the worst storms to hit the state in more than a decade.
"Make no mistake," she told a news conference on Saturday. "This is a very, very dangerous and significant storm."
Up to 10 in (25cm) of rain is expected in parts of Mexico, California and Nevada, according to the NHC. On Sunday, rain began to fall in southern California deserts.
There is a risk of tornadoes from mid-morning through Sunday evening in parts of the Colorado River Valley, Mojave Desert and Imperial Valley.
As the storm approached, Major League Baseball rescheduled three games in southern California, while SpaceX postponed the launch of a rocket from its base on the central California coast until at least Monday.
The National Park Service also closed Joshua Tree National Park and Mojave National Preserve, both in California, to prevent visitors from being stranded in the event of flooding.
Extreme weather has recently caused chaos across North America.
In the wake of the hottest month on record, July 2023, according to Nasa, the deadliest wildfire in modern US history spread across Hawaii on 8 August, killing at least 111 people.
The damage was escalated by hurricane winds passing through the area.
And in Canada, hundreds of wildfires are raging in the province of British Columbia, scorching homes and forcing tens of thousands of people to evacuate.
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