The extreme conditions set to batter the state will be the remnants of Hurricane Hilary, which was due to strike Mexico's Baja peninsula on Saturday.

Forecasters have predicted "catastrophic flooding", with those in impacted areas - including the tourist island of Santa Catalina - advised to evacuate.

Downgraded to a category one storm, having peaked at category four with winds of 145mph on Friday, metrologists warned the storm is still treacherous.

Mexico's navy had deployed 3,000 troops and already helped hundreds of people flee their homes ahead of the hurricane's expected landfall about 200 miles south of the port city of Ensenada.

'Historic' storm to bring 'year's worth of rain'

In California, emergency services as far north as Los Angeles are helping locals brace for the weather, which the National Hurricane Center (NHC) has described as "historic".

The "catastrophic" tropical storm and flood warnings cover a wide swath of California's southern regions, from the Pacific coast to several mountain and foothill communities.

California governor Gavin Newsom proclaimed a state of emergency, and officials urged people to finish preparations before Sunday.

Officials have closed the famous Joshua Tree and Mojave national parks, three Major League Baseball games have been cancelled, and SpaceX has delayed a rocket launch from a base on the state's central coast.

Sandbags have been in cities and towns and police are working to get homeless people into shelters.

Elizabeth Adams, a meteorologist at the National Weather Service San Diego office, said up to three inches (7.62cm) of rain could fall an hour across Southern California's mountains and deserts, from late Sunday morning into the afternoon.

Around 10 inches (25cm) - a year's worth of rain for some areas - is also expected in California and southern Nevada.

Federal disaster supplies prepared

The storm could also bring a new all-time rainfall record to the neighbouring state of Nevada, experts have said.

President Joe Biden has urged anyone in the path of the storm "to take precautions", with federal disaster supplies already positioned should they be needed.