'Atmospheric river' wallops California; what are these 'rivers in the sky'?

An "atmospheric river" brought torrential rain across much of California on Thursday, flooding roads and causing dangerous mudslides in areas already swollen from days of rain and still recovering from devastating wildfires last year.

Severe flooding washed away part of a two-lane mountain highway southeast of Los Angeles. About 75 feet of pavement completely collapsed along State Route 243 near the remote community of Idyllwild, state Department of Transportation photos showed. 

“We’re basically stranded right now,” said resident Gary Agner, adding that several other roads were closed because of flooding and debris. “I’m glad I went to the grocery store yesterday.”

One woman rescued from rising water in a low-lying area had a heart attack and died at a hospital, said Capt. Ryan Rolston with the Corona Fire Department. Three dogs pulled her and eight other people from rising water in a flood control channel where homeless people camp, Rolston said.

In Sausalito, north of San Francisco, a home smashed into another house after sliding down a hill. One woman was buried under a tree and mud for two hours before fire crews rescued her. Officials evacuated at least 50 homes in that neighborhood where a mudslide also barreled over cars and uprooted trees. 

A flooded creek triggered the evacuation of another 300 residents near Paradise, a town destroyed by the Camp Fire in November. 

Up to 3 inches of rain will fall along the coast from San Diego to the Oregon border, AccuWeather said. Some mountain slopes could see twice that; 12 inches was possible in lower elevations of the Sierra Nevada.

High in the Sierra, already overwhelmed with snow, up to 10 feet of snow could fall in some areas.

Since the storm began Tuesday, there have been at least 78 reports of debris flows or flooding in California, weather.com meteorologist Chris Dolce said.

The storm slammed the Northwest earlier this week, dumping 2-3 feet of snow in portions of Washington, Oregon, Idaho and Montana.

'Sky rivers'

These rivers in the sky are responsible for up to 65 percent of the western USA's extreme rain and snow events, a 2017 study said.

Made visible by clouds, these ribbons of water vapor known as atmospheric rivers extend thousands of miles from the tropics to the western USA. They provide the fuel for the massive rainstorms and subsequent floods along the U.S. West Coast, such as the one hammering California on Thursday.

This one stretched from Hawaii to California, the National Weather Service said, thus the nickname "Pineapple Express."

Storm blasts West

The system also brought high winds. Gusts reaching 60 mph are likely over parts of Southern California, AccuWeather said. Power was out to more than 130,000 homes and businesses across the state, poweroutage.us reported.

California, coming off years of drought, has been awash in rain in recent months. On Wednesday, San Francisco was hit with a "hefty 2.49" inches of rain, the 5th-wettest for any February day on record, the weather service said.

"There’s a lot of standing water and some flooded roadways," Caltran's Bay-area director Tony Tavares said. "Please drive carefully and never drive through flooded roadways."

Caltrans was reporting road closures in San Diego. A mandatory evacuation  was in effect southeast of Los Angeles in an area recovering from wildfires. 

Tim Suber said his hillside neighborhood in Lake Elsinore has been evacuated countless times since last summer’s wildfire.

“I’m not going this time,” Suber said after Riverside County sheriff’s deputies warned him that he could end up trapped if roads flood. “I’ve got 35 chickens and a daughter who won’t leave them behind. So we’re staying.”

The soaking rain also halted play at the Genesis Open, the professional golf tournament at Riviera Country Club in Pacific Palisades near Los Angeles. 

After smashing into California, the storm will continue to slide east across the nation, spreading snow and rain across the central Plains, Midwest and the Mid-Atlantic states Friday and Saturday.

The Weather Chanel has named it Winter Storm Nadia.

Contributing: The Associated Press, Kristin Lam, USA TODAY