About 21,000 people have been evacuated from neighbourhoods beneath hillsides laid bare by California's largest wildfire and other recent blazes as a storm raises fears of flash floods and debris flows.
Robert Lewin, director of Santa Barbara County Office of Emergency Management, is urging residents of Summerland, Carpinteria and Montecito to leave by mid-Monday local time.
The hillside communities were evacuated last month as the massive Thomas Fire raged.
Evacuations also include homes near other burn areas dating to 2016.
Mr Lewin said flash floods can turn normally dry creeks into destructive rivers of mud and debris that can wash out roads and destroy homes.
The first significant storm of the season is walloping much of the state with heavy rain, snow and strong winds.
Forecasters say several inches of rain could fall overnight on areas scarred by the largest fire the state has seen.
Forecasters issued a flash flood watch for parts for Sonoma and Mendocino counties north of San Francisco and warned that heavy rainfall could trigger mudslides in those areas devastated by October wildfires.
The blazes levelled entire neighbourhoods, killing 44 people and destroying more than 8,900 homes and other buildings.
A years-long drought eased in California last spring, but the north of the state had a dry start to winter and hardly any measurable rain has fallen in the south over the past six months.
The extremely dry conditions and high winds last year led to some of the most destructive blazes at both ends of the state.
The storm coming in from the Gulf of Alaska could dump up to 4in of rain on northern California areas still recovering from fires before clearing up by Tuesday evening, National Weather Service meteorologist Steve Anderson said.
"Everything is soaking into the ground at this time, but if it gets very heavy, it could trigger a flash flood warning," he added.
The wet and windy system moving ashore could soak much of the state and drop several inches in parts of Santa Barbara and Ventura counties.
About an inch of rain is forecast for central Los Angeles, the most in nearly a year.
The National Weather Service also issued a winter weather advisory for portions of the Sierra Nevada above 7,000ft, forecasting about 4in to 7in of snow and 1ft to 2ft on higher peaks.
It said travellers should prepare for difficult travel conditions, including gusty winds, low visibility and slick and snow-covered roads.