Atmospheric River Drenches California, With More Storms on the Way

A powerful storm known as an atmospheric river swept over California on Thursday, soaking the state with rain and leaving a trail of damage that has become familiar to residents in recent years: fallen trees, flooded roads and snarled travel.

Though the storm was not expected to cause the kind of chaos that was sown by a succession of atmospheric rivers last winter, forecasters and officials urged Californians to stay alert and be ready for an even larger tempest arriving over the weekend.

“We are in full preparation mode,” said Jackie Ruiz, a spokeswoman for the Santa Barbara County Office of Emergency Management. “We’re definitely encouraging people to stay local, hunker down and if there’s no urgent need to be on the road, stay off the road.”

Communities in Santa Barbara County, tucked between the mountains and the sea, are especially vulnerable to flooding and mudslides. But meteorologists said that significant rainfall from the back-to-back storms was likely to affect almost the whole state.

The current storm hit Northern California first, flooding roads on Wednesday and prompting the closure of streets and schools in rural communities. At least one person had to be rescued from a car that was taking on water in Sonoma County, and forecasters warned of big, dangerous waves on the coast.

The storm was potent enough to push San Francisco’s cumulative rainfall total above normal for the first time this season, according Jan Null, an adjunct professor of meteorology at San Jose State University.

Residents in Southern California awoke on Thursday to flinty gray skies, gusting winds and downpours. Flash flood warnings were posted for part of Los Angeles County near beaches and ports and on hillsides.

Rose Schoenfeld, a meteorologist with the National Weather Service in the Los Angeles area, said that residents of Palos Verdes Estates should be particularly vigilant. That is the area where homes perched atop seaside bluffs began slipping into a canyon last July, in a landslide that was blamed on the saturation of the soil by last winter’s storms.

In Long Beach, flooding forced the closure of a crucial freeway, bringing traffic in the area — which can induce headaches even in sunny weather — nearly to a standstill.

Flooding also closed the Pacific Coast Highway in Huntington Beach. And firefighters in Orange County said that they had rescued someone from a swollen storm drainage channel.

“As you can see, with the heavy rain, the channels are going to fill up quickly, with the water moving at a dangerous speed,” the Orange County Fire Authority posted on social media, along with video of water rushing into a storm drain. “Please stay clear of bodies of water,” the post said.

The storm system that poured down rain in Southern California on Thursday was also bringing much-needed snow to the Sierra Nevada. The rain was expected to continue through the day on Thursday, but forecasters predicted that Friday and Saturday would be clearer.

So far this week, Californians have not seen the kinds of weather-generated disasters that struck last winter, though there was flooding in Ventura County in December and in San Diego in January.

The next storm, expected to arrive late Saturday and continue into Sunday, is likely to be more severe. But contrary to online rumors, nothing suggests that the storm will be anything like the widely feared — but less widely understood — “ARkStorm,” an exceedingly rare and powerful atmospheric event modeled by scientists at the United States Geological Survey that could inundate major West Coast cities.

Instead, forecasters said that Californians should expect a storm more like the ones last winter that broke a number of daily rainfall records and were the first significant precipitation in the state after several bone-dry years.

Of course, those storms caused major damage. But meteorologists said it was too soon to say with certainty how dangerous the incoming storm would be.

Daniel Swain, a climate scientist at the University of California, Los Angeles, who focuses on public communication of weather phenomena, said in an email that he was “becoming increasingly concerned regarding what has the potential to become a major regional flood event somewhere between Santa Barbara and San Diego counties” at the beginning of next week.

He said that some of the prediction models showed precipitation totals that could overwhelm flood-control infrastructure in the state’s largest metro areas, including Los Angeles.

“This will bear close watching over the next couple of days,” Dr. Swain said.

Shawn Hubler contributed reporting.