Tropical Storm Hilary made its historic arrival in California on Sunday with forecasters warning of possibly catastrophic flooding after the system pummelled Mexico’s Baja California peninsula with heavy rains.

One person died in Mexico amid reports of flash flooding in the peninsula, where some roads were swept away and images on social media showed raging torrents gushing down city streets that been turned into rivers.

California Governor Gavin Newsom declared a state of emergency for much of Southern California, with flash flood warnings in effect throughout a region that is more accustomed to drought.

In Palm Springs, a desert getaway in Riverside County about 100 miles (160 km) east of Los Angeles, the city that typically gets around 4.6 inches (12 cm) of rain in an entire year could receive 6-10 inches from this one storm.

Newsom, on a tour of Southern California, said Palm Springs was dry when he left on Sunday but an hour later it had received “the most significant rainfall over a 60-minute period any time in the history of Palm Springs.” The streets were soon flooded.

“That’s how quickly this system is moving. Take nothing for granted,” Newsom told a news briefing in Los Angeles after he said he updated US President Joe Biden, who ordered federal agencies to move personnel and supplies into the region.

By 5 p.m. PDT (0000 GMT), Hilary was 25 miles (40 km) south-southwest of Palm Springs, packing winds of 50 mph (85 kph) and moving to the north, the weather service said.

Hundreds of flights in San Diego, Las Vegas and Los Angeles were cancelled, and professional sporting matches were rescheduled. The Los Angeles Unified School District and San Diego Unified School District, the two largest school districts in the state, cancelled classes for Monday. Dangerous surf pounded the beaches in Southern California.

San Diego County, just north of the Mexican border, received its first-ever recorded tropical storm. Tornadoes were possible in parts of California, Arizona, Nevada and Utah, the National Weather Service said, though only one had been reported so far, in a remote part of Arizona.

Amid all the storm preparation, a 5.1-magnitude earthquake rocked the Ventura County city of Ojai, about 80 miles northwest of Los Angles. There were no immediate reports of injuries.

Floodwaters raced through the concrete banks of the Los Angeles River, which normally contains barely a trickle. In Ocotillo, a desert town about 90 miles east of San Diego, rock slides brought boulders down on Interstate 8, causing traffic delays on the highway to Arizona.

Such a storm is exceedingly rare for Southern California. Nora hit a remote part of Imperial County to the east in 1997, and in 1939, before storms were named, another came ashore in Long Beach to the north, in Los Angeles County.

Mountain and desert areas could get 5 to 10 inches of (12 to 25 cm) rain, forecasters said.

“In some places in the desert, that’s a year’s worth,” Alex Tardy, a senior meteorologist for the National Oceanic and Atmospheric Administration in San Diego, told a news briefing. “The normal rainfall in Southern California and San Diego is nothing in August. So, a very unusual event is unfolding here.”

Hilary made landfall earlier on Sunday in the northern part of Mexico’s Baja California peninsula, where nearly 1,900 people were evacuated to shelters, according to the country’s army.

The storm was especially dangerous in low-income areas where many homes fail to meet building codes.

“We’ve always been aware that it’s a risky area. A lot of water runs (nearby) but what are we going to do? It’s the only place we have to live,” said Yolanda Contreras, living in a flood-prone area of Rosarito, about 15 miles south of the US-Mexican border.

Around the coastal town of Mulege, on the eastern side of the Baja California peninsula, one person died after his family was swept away while crossing a stream on Saturday. Phone lines and electricity were cut in several of the surrounding villages after lamp posts fell, the Mexican army said.

Mexico’s Navy also evacuated about 850 people from five islands in the path of Hilary, which was previously designated a Category 4 hurricane before it weakened.