Hilary, the first tropical storm to hit southern California in 84 years, swept people into swollen rivers, toppled trees on to homes and flooded roadways as the massive system marched northwards on Monday, prompting flood watches and warnings in more than a half dozen US states.
The National Hurricane Centre said Hilary had lost much of its steam and only vestiges of the storm were heading over the Rocky Mountains, but warned that “continued life-threatening and locally catastrophic flooding” was xpected over portions of the south-western US following record-breaking rainfall.
Flood and high-wind warnings were in place for about 17 million Americans as remnants of the storm moved north, dumping heavy rains from the California-Mexico border up through Las Vegas and into parts of the north-west, the National Weather Service said.
Hilary has weakened to a post-tropical cyclone since hitting southern California on Sunday with maximum sustained winds of 55 kph, the NHC said in a bulletin. The storm's centre is expected to quickly move across Nevada and dissipate.
“The storm could bring a year's worth of rainfall to a portion of the US that usually experiences dry conditions. It could produce rainfall amounts of 5-10cm with isolated storm total amounts to 30cm.
''Continued flash and urban flooding, locally catastrophic, is expected,” the NHC said.
“Despite its post-tropical classification, the system is still expected to produce heavy rainfall, significant flooding and gusty winds as it races northwards across the western United States today.”
Los Angeles, the second-most populous city in the US, experienced flooded streets, with trees and power lines downed in some neighbourhoods, officials said on Monday, urging residents to stay vigilant.
Los Angeles Mayor Karen Bass warned residents to take the storm seriously.
“My concern is that people will be a little dismissive and go out when we need people to stay at home, to stay safe,” she said at a briefing on Sunday.
Flash flood warnings have been issued for parts of the county, the NWS reported.
Downtown Los Angeles accumulated 6.04cm in rainfall on Sunday, a daily record.
One of the hardest hit communities has been Palm Springs, California, where video footage posted on social media showed flooded streets and debris flows.
Mayor Grace Garner told CNN the city's 911 emergency system was knocked out by the storm.
“Right now we have flooding on all of our roads. There's no way in or out of Palm Springs, and that's the case for the majority of the Coachella Valley. We're all stuck,” she said during an interview on the network.
A steady rain fell on Monday morning in many parts of the region, where record-breaking downpours had already been recorded.
Portions of Nevada also remained under flash flood advisories, the NWS reported.
California Governor Gavin Newsom had already declared a state of emergency for most of southern California in preparation for the rainfall, and President Joe Biden directed federal workers to assist the region.
Meanwhile, the Los Angeles Unified School District, the nation's second-largest school system 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.
One woman had to be assisted out of her vehicle after driving into the floodwaters. Witnesses were seen helping the woman climb out through the window of her sinking car.
The SUV was fully submerged in the waters by the time first responders had arrived.
The woman, who identified herself as Mary Ann Beth on social media, said he was “quite ashamed” that she had made it on California local news.
“But I am also very thankful for the gentlemen who came in to save me from my sinking car … I have learnt a very valuable lesson today, Hurricane Hilary in Southern California is no joke,” she said in a post on Twitter.
And in San Diego, firefighters rescued nine people who were trapped in a riverbed.
Hilary was the first tropical storm to hit southern California in 84 years.
The storm made landfall in the Baja California peninsula in Mexico on Sunday.
No fatalities or significant injuries were reported in the US.
One man was killed in Mexico when his family was swept away while crossing a stream on Saturday, Mexican officials said.
Reuters contributed to this report