Heavy rains lashed California on Sunday as Tropical Storm Hilary raced in from Mexico, bringing warnings of potentially life-threatening flooding in the typically arid southwestern United States.

As of Sunday afternoon Hilary's core was in California packing maximum sustained winds of 95 kilometers per hour, after barreling up Mexico's Baja California Peninsula, the US National Hurricane Center (NHC) said of this tropical weather event which was very rare for southern California. The storm was moving at the brisk pace of 37 kph.

"Catastrophic and life-threatening flooding likely over Baja California and portions of the southwestern U.S. through Monday," it warned.

"This is an unprecedented weather event," Los Angeles Mayor Karen Bass warned.

More than 4,500 flights were delayed or canceled across the U.S. on Sunday evening as California continues to brace for the impact from Tropical Storm Hilary, CNN reported, adding that many of the flights originated from or were headed to destinations in California or nearby states.

Beaches were ordered closed and people rushed to stores to stock up on water and other essentials. Flash flood and even tornado warnings were issued for some areas.

Hilary earlier reached Category 4, the second-most powerful on the five-step Saffir-Simpson hurricane scale, but was downgraded to a tropical storm as it headed towards the densely populated Mexican border city of Tijuana.

Despite the weakening, U.S. Federal Emergency Management Agency (FEMA) administrator Deanne Criswell urged people to take the dangers seriously.

"Hurricane Hilary is going to be a serious impact and threat to southern California," she said on CNN.

California Governor Gavin Newsom declared a state of emergency for much of the state's southern area.

"Stay safe, California," he wrote on social media as Hilary approached. 

Authorities opened five storm shelters and deployed more than 7,500 personnel, including several hundred National Guard soldiers as well as swiftwater rescue teams, Newsom's office said.

In San Diego, people filled sandbags to prepare for possible flooding, while lifeguards warned people to stay out of the sea.

(With input from AFP)

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