California’s authorities and meteorologists have advised those planning to travel for the holidays to reconsider their journeys. A series of formidable winter storms are set to bring continuous rain, strong winds, and snowfall in mountainous regions, posing significant risks to travelers.

The storm system, which began rolling in late Tuesday night, is anticipated to escalate by Christmas Eve. With millions expected to traverse the state, the National Weather Service has cautioned that travel conditions could become perilous, if not entirely unfeasible, due to several atmospheric rivers predicted to sweep through the region.

“If you’re thinking of hitting the road for Christmas, it might be wise to think twice,” advised Ariel Cohen, a meteorologist with the National Weather Service in Los Angeles, at a news briefing on Tuesday.

Forecasts suggest Southern California may experience its wettest Christmas in years, with warnings of flash floods, mudslides, and debris flow, particularly in areas affected by last January’s wildfires. Los Angeles County officials have been proactively alerting approximately 380 households at high risk, urging them to evacuate.

The Sacramento Valley and the San Francisco Bay Area are currently under flood and high wind warnings through Friday. Meteorologists predict that heavy snow and strong winds will lead to “near white-out conditions” in parts of the Sierra Nevada on Wednesday, making travel through the mountain passes almost impossible.

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

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.