California’s authorities and weather experts are strongly advising against holiday travel as fierce winter storms are set to unleash torrential rains, strong winds, and mountain snow across the state.

These storms, which began their approach late Tuesday night, are anticipated to strengthen by Christmas Eve. With millions of travelers expected on the move, conditions could become perilous, if not impassable, as multiple atmospheric rivers are predicted to sweep through, according to the National Weather Service.

“We urge anyone considering road travel for the Christmas holidays to rethink their plans,” stated Ariel Cohen, a meteorologist with the National Weather Service in Los Angeles, during a briefing on Tuesday.

Southern California is bracing for potentially its wettest Christmas in recent history, with warnings of flash floods, mudslides, and debris flows, particularly in areas affected by wildfires last January. Los Angeles County officials are proactively alerting about 380 at-risk households to evacuate.

The Sacramento Valley and the San Francisco Bay Area are under flood watches and high wind warnings through Friday. Forecasters predict heavy snow and strong gusts could lead to “near white-out conditions” in parts of the Sierra Nevada, making mountain travel “nearly impossible.”

Additionally, there is a threat of severe thunderstorms and a slight possibility of tornadoes along the northern coastline.

Heavy rain and flash flooding already led to water rescues and at least one death in Northern California, local officials said. Shasta County Sheriff Michael L. Johnson on Monday declared a state of emergency to prepare for more rain and allow the state to help with hazard mitigation and search and rescue operations.

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), National Weather Service meteorologist Mike Wofford said. It could be even more in the mountains. Gusts could reach 60 to 80 mph (96.5 to 127.8 kph) in parts of the central coast.

Officials expect multiple road closures and airport delays during the storms. Downed trees and power lines are also possible. Parts of Los Angeles are under evacuation warnings this week.

The county put up K-rails, a type of barrier, around the burn scar to help catch sliding debris during rainstorms. Residents could also pick up free sandbags to protect their homes, said Kathryn Barger, a Los Angeles County supervisor representing Altadena.

Many people in burn scar areas decided not to leave after receiving the evacuation notification, Los Angeles Police Department Chief Jim McDonnell said. He urged them to reconsider.

“The threat posed by this storm is real and imminent,” he said.

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. The California National Guard is also on standby to assist.

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.

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Associated Press writers Sophie Austin in Sacramento, California, and Jessica Hill in Las Vegas contributed to this report.