Vehicles cross over a flood control basin that has almost reached the street on Sunday in Palm Desert, Calif. Forecasters said Tropical Storm Hilary was the first tropical storm to hit Southern California in 84 years, bringing the potential for flash floods, mudslides, isolated tornadoes, high winds and power outages. The Associated Press

PALM DESERT, Calif. — Hilary, the first tropical storm to hit Southern California in 84 years, swept people into swollen rivers, toppled trees onto homes and flooded roadways as the massive system marched northward Monday, prompting flood watches and warnings in more than a half dozen states.

The National Hurricane Center in Miami said Hilary had lost much of its steam and only vestiges of the storm were heading over the Rocky Mountains, but warned that “continued life-threatening and locally catastrophic flooding” was expected over portions of the southwestern U.S., following record-breaking rainfall.

Hilary first slammed into Mexico’s arid Baja California Peninsula as a hurricane, causing one death and widespread flooding before becoming a tropical storm, one of several potentially catastrophic natural events affecting California on Sunday. Besides the tropical storm, which produced tornado warnings, there were wildfires and a moderate earthquake north of Los Angeles. So far, no deaths, serious injuries or extreme damages have been reported in the state, though officials warned that risks remain, especially in the mountainous regions where the wet hillsides could unleash mudslides.

In the San Bernardino Mountains, east of Los Angeles, crews were working to clear mud that has been blocking the homes of about 800 residents, said Cal Fire Battalion Chief Alison Hesterly.

Residents also pitched in. In the mountain community of Oak Glen, Brooke Horspool helped dig out a home surrounded by about 4 feet of mud to free a couple, including an older man with medical issues.

Amid the storm Sunday in Palm Desert, Terry Flanigan heard a huge crash and then got a text from a neighbor that a Eucalyptus tree, more than 100 feet tall, fell onto a condo across the street. She later learned it landed on the bed of her neighbor’s 11-year-old son, who luckily was in another room.

“It was very unnerving,” Flanigan said, adding that the family had gone to stay with relatives while removal crews came Monday morning to remove the branches. “Oh my gosh, what could have happened.”

Maura Taura felt a similar relief after a three-story-tall tree crashed down on her daughter’s two cars but missed the family’s house in the Sun Valley area of Los Angeles.

“Thank God my family is OK,” she said.

Hot water and hot air were both crucial factors that enabled Hilary’s rapid growth — steering it on an unusual but not quite unprecedented path that dumped 10 months of rain in just one day in some normally bone-dry places.