Parts of Southern California and the US Southwest on Monday faced the threat of flash flooding along with landslides and mudslides after storm Hilary unleashed record-breaking downpours overnight.

Some 17 million Americans were under flood and high-wind advisories, watches and warnings as remnants of the storm moved north, dumping heavy rain from the California-Mexico border up through Las Vegas and into parts of the Northwest, the National Weather Service said.

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.

A steady rain fell on Monday morning in many parts of the region, where record-breaking downpours had already fallen. Hilary was downgraded to a post-tropical cyclone overnight.

Some mountain and desert areas could see rain accumulation totals reaching 5 to 10 inches from the storm, as much as the deserts typically see in a year, forecasters said.

Los Angeles, the second-most populous city in the US, experienced flooded streets and downed trees and power lines in some neighbourhoods, officials said on Monday, urging residents to stay vigilant.