A series of powerful winter storms threaten to hit California with relentless rain, strong winds and mountain snow as vacationers hit the road for Christmas.

Millions of people across the state are expected to hit the road. They will likely encounter dangerous, even impossible, conditions with several atmospheric rivers forecast to cross the state, the National Weather Service warned. Forecasters said Southern California could have its wettest Christmas in several years and warned of possible mudslides and debris in areas affected by last January's wildfires.

“There's going to be a lot of water on the roads, and travel will be very dangerous in the coming days,” said Mike Wofford of the National Weather Service.

Most areas received scattered rain Tuesday morning, and the system is expected to intensify in the afternoon and into Christmas Eve. Rain and wind will decrease in some regions on Wednesday, before the arrival of another storm.

Much of the Sacramento Valley and San Francisco Bay area were under a flood watch and high wind warning until Friday. Forecasters warned of heavy snow and gusty winds in parts of the Sierra Nevada starting Tuesday, which will create “virtually zero visibility conditions” that will make it “almost impossible” to travel through the mountain passes.

There is also a risk of severe thunderstorms and a small chance of tornadoes along the state's northern coast.

Heavy rain and flash flooding that began Saturday in Northern California has already forced water rescues and at least one person has lost their life, local officials said.

Southern California typically receives between half an inch and one inch (1.3 to 2.5 centimeters) of rain this time of year, but this week many areas could see four to eight inches (10 to 20 centimeters), Wofford said. The figure could be even higher in the mountains.

Widespread flooding and mudslides are very likely, especially in areas affected by the Palisades fire last January, he added. Parts of Los Angeles were under evacuation warnings as of Tuesday, with police officers going door-to-door at particularly vulnerable homes ordering them to evacuate, Los Angeles Mayor Karen Bass's office said.

Forecasters urged people to stay home or make alternative travel plans.

Local and state officials are preparing to respond to emergencies throughout the week. The state has deployed resources and first responders to several counties along the coast and in Southern California ahead of the storms. The California National Guard is also on standby to provide assistance.

“When severe weather threatens our communities, we don't wait to react. We get ahead of it,” Gov. Gavin Newsom said in a statement.

An atmospheric river is a long, narrow band of water vapor that forms over an ocean and flows across the sky, carrying moisture northward from the tropics.

Persistent atmospheric rivers drenched Washington state with nearly five trillion gallons (19 trillion liters) of rain. in a week earlier this month, threatening to set new flood records, forecasters said. Those rains were intensified by warm weather and air, plus unusual weather conditions that can be traced back to a tropical cyclone in Indonesia.

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This story was translated from English by an AP editor with the help of a generative artificial intelligence tool.