After making landfall in Mexico earlier Sunday, Tropical Storm Hilary has crossed into California, where it is unleashing heavy rain and turning roads into gushing streams as officials warn of potentially deadly floods.

“We are not used to this level of precipitation, generally. Certainly not in the middle of summer,” San Diego Mayor Todd Gloria told CNN’s Jim Acosta Sunday afternoon, saying he was worried about potential power outages from the wind as well as flooding. “We’re not built for this kind of rainfall.”

Authorities across southern California pleaded with residents not to drive, warning of mudslides, road deterioration and dangerous debris flows and flooding, as some communities declared emergencies to respond to the storm. One California official has warned Hilary could be among the most devastating storms to hit the state in recent years.

The National Weather Service said parts of Los Angeles and Ventura counties were already experiencing “dangerous flooding” Sunday evening, writing on social media, “THIS IS LIFE THREATENING FLOODING!”

Cars were getting stuck in floodwaters and local authorities were conducting rescues, the service said.

Hilary’s core – in other words, its center – crossed into Southern California Sunday evening, but the state began feeling the storm’s effects much earlier in the day and rain totals have added up. Parts of Palm Springs saw more than 2 inches of water in just six hours Sunday – nearly half of what they average over an entire year, the weather service said.

And with the day not yet over, there were multiple other rainfall records broken Sunday, including in downtown Los Angeles, Burbank and Palmdale.

In Palm Springs, the city manager declared a local emergency due to “unprecedented rainfall and flooding,” with city officials saying there had already been one swift water rescue by Sunday afternoon.

Live updates: Tropical Storm Hilary to bring major flooding risk to California

Though the storm is expected to weaken, it will continue lashing the region with severe weather as it moves further into the US.

In Arizona, authorities issued evacuation orders in parts of Lake Mead National Park urging residents to seek higher ground ahead of potential floods. And Nevada’s governor declared a state of emergency Sunday as the storm drew closer.

More than than 1,000 flights within, into or out of the US have been canceled Sunday and more than 4,900 have been delayed. The three most-impacted airports are all in Hilary’s range: Harry Reid International Airport in Nevada, San Diego International Airport and Phoenix Sky Harbor International Airport, according to flightaware.com.

And as Hilary triggered flood warnings across Los Angeles, a magnitude 5.1 earthquake shook the area and other parts of Southern California Sunday afternoon, according to the United States Geological Survey.

Millions face flash flooding threat

More than 7 million people, including those in downtown Los Angeles, are under a flash flood warning through early Monday morning. Parts of Los Angeles and Ventura counties could see up to 1.5 inches of rain dumped per hour, the National Weather Service has said.

Schools in the San Diego Unified School District announced they would be postponing the first day of the school year to Tuesday. The Los Angeles Unified district, the country’s second largest school district, also said schools would be closed Monday.

Hilary weakened from a Category 1 hurricane to a tropical storm before it made landfall over the northern Baja California Peninsula early Sunday.

At least one death is already attributed to the storm. A person died when their vehicle was swept away near Santa Rosalía in Mexico, along the Baja California Peninsula, Mexican officials said in a news release Saturday.

California Gov. Gavin Newsom proclaimed a state of emergency Saturday for a large swath of Southern California to support hurricane response and recovery efforts.

In a Saturday news conference, Nancy Ward, director of the California Governor’s Office of Emergency Services, warned Hilary “could be one of the most devastating storms that we’ve had hit California in more than a decade.”

Storm is ‘unprecedented’ event, LA mayor says

Parts of California, Nevada and Arizona that are unaccustomed to rain could suddenly receive a year’s worth or more. And along the coast, large swells generated by Hilary are likely to create life-threatening surf and rip current conditions.

Death Valley saw triple its average August rainfall in just a few hours Sunday morning. Nearly a month’s worth of rain fell in one hour on Sunday. It normally receives an average of 0.21 inches of rain the entire month of August, but the Furnace Creek observation site reported 0.63 inches since Sunday morning.

Roads within Death Valley National Park were expected to eventually become “impassable,” the park said on Instagram, sharing photos that showed floodwaters flowing over roads.

The threat triggered California’s first ever tropical storm warning extending from the state’s southern border to just north of Los Angeles – presenting an “unprecedented weather event” to a city with “deep experience” responding to natural disasters like wildfires and earthquakes, Mayor Karen Bass said at a news conference.

“It is critical that Angelenos stay safe and stay home unless otherwise directed by safety officials,” Bass said. “If you do not need to be on the road, please don’t get in your car. Make sure your emergency kit and essential devices are on hand. And ensure that all of your devices are charged in the event of life-threatening emergency.”

Residents of the San Bernardino County communities of Oak Glen, Forest Falls, Mountain Home Village, Angelus Oaks, and Northeast Yucaipa were all ordered to evacuate Saturday.

Visitors and some residents of Catalina Island, part of California’s Channel Islands, were “strongly encouraged” to leave the island ahead of the storm in a news release from the City of Avalon.

Meanwhile, helicopters from the Los Angeles County Sheriff’s Office were flying over riverbed areas Saturday afternoon, making announcements in both English and Spanish to warn homeless people about the extreme weather.

Los Angeles opened three more emergency shelters Sunday and provided transportation to help get more people to safety before the storm – bringing the total number of emergency shelters opened to eight.

Concern for deserts and recent burn areas

California has been particularly focused on preparing residents in areas that typically receive the least rain, or that were most recently scorched by wildfires, authorities said.

“We’re keeping a very close eye on our desert regions, east of San Diego and Los Angeles. Some parts of these areas may receive double their yearly amount of water in just a single day,” said Brian Ferguson, the deputy director of the California Governor’s Office of Emergency Services.

Lingering burn scars from wildfires can create a steep, slick surface for water and debris to flow off. People who live downhill and downstream from burned areas are very susceptible to flash flooding and debris flows.

“Rainfall that would normally be absorbed will run off extremely quickly after a wildfire, as burned soil can be as water repellant as pavement,” the National Weather Service said.

In Orange County, a voluntary evacuation warning was issued for Silverado Canyon and Williams Canyon in the Bond Fire burn area due to possible debris flows along or near the burn scar.

Residents have been offered sandbags to fortify their property in counties across Southern California, where some of the natural buffers against flooding have been burned away.