Powerful winter storm threatens California as Christmas approaches

"There's going to a lot water on the roadways, and it's going be very dangerous traveling for the next couple days," National Weather Service meteorologist Mike Wofford said.

Most areas saw scattered showers Tuesday morning, and the system is expected to pick up in the afternoon and intensify into Christmas Eve. Some regions will see rain and winds taper off Wednesday before another storm moves in.

Much of the Sacramento Valley and the San Francisco Bay Area were under a flood watch and a high wind warning through Friday. Forecasters warned of heavy snow and gusty winds for parts of the Sierra Nevada starting Tuesday that will create "near white-out conditions" and make it "nearly impossible" to travel through the mountain passes.

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

Heavy rain and flash flooding that started Saturday in Northern California already led to water rescues and at least one death, local officials said.

Southern California typically gets half an inch to 1 inch (1.3 to 2.5 centimeters) of rain this time of year, but this week many areas could see between 4 and 8 inches (10 to 20 centimeters), Wofford said. It could be even more in the mountains.

Potential widespread flooding, rockslides and mudslides are highly likely, especially in areas burned by last January's deadly Palisades fire, he added. Parts of Los Angeles were under evacuation warnings starting Tuesday and police officers were knocking on doors of particularly vulnerable households to order them to leave, according to Los Angeles Mayor Karen Bass' office.

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

Local and state officials are gearing up to respond to emergencies through the week. The state has deployed resources and first responders to a number of counties along the coast and in Southern California ahead of the storms. The California National Guard is also on standby to assist.

"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 through the sky, transporting moisture from the tropics to northern latitudes.

Earlier this month, stubborn atmospheric rivers drenched Washington state with nearly 5 trillion gallons (19 trillion liters) of rain in a week, threatening record flood levels, meteorologists said. That rainfall was supercharged by warm weather and air, plus unusual weather conditions tracing back as far as a tropical cyclone in Indonesia.