Storms dumped rain and increased the risk of mudslides in fire-ravaged communities.
Northern California was hit hardest Monday evening, while Southern California was bracing for the worst to arrive overnight.
Both regions have been plagued with destructive wildfires.
Property owners stacked sandbags in devastated northern wine country areas where deadly fires hit in October and well over an inch of rain had fallen Monday night.
To the south in Santa Barbara and Ventura counties, evacuations were called for over 20,000 people in neighborhoods below hillsides laid bare by the state’s largest wildfire in modern history. Mud and debris flows were a serious threat.
“Creeks that normally would be dry would turn into raging rivers of mud and debris and large rocks and trees,” said Robert Lewin, director of the Santa Barbara County Office of Emergency Management. “These can be quite damaging. They’ll destroy roads, they’ll take out homes.”
In the foothills above seaside Carpinteria, about 220 students and 60 faculty members were riding out the storm at the college preparatory Cate School instead of evacuating as they did before, when the fire incinerated surrounding vegetation.
“We’re on a mesa in the foothills and we have two drainages on either side of us,” Headmaster Benjamin D. Williams IV said.
Williams expected those canyons would fill with runoff but decided the school’s campus would be safe after discussions with county officials. Access could be a problem, he said, but the school has plenty of food and supplies.
“We see worst case a couple of days,” he said. “We certainly have enough to satisfy everyone for the remainder of this week.”
Forecasters issued a flash flood watch for parts for Sonoma and Mendocino counties north of San Francisco, warning that the rains that were saturating the area Monday night could trigger mudslides in areas devastated by October wildfires.
The blazes leveled entire neighborhoods, killing 44 people and destroying more than 8,900 homes and other buildings.
“City crews are actively driving around looking for signs of any flooding, mudslides, things of that nature,” said Santa Rosa Assistant Fire Marshal Paul Lowenthal. “The fire damaged a significant amount of trees and although a lot of the trees have been cut down and removed, there are still a lot of trees that could be a concern.”
Mandatory evacuations were also ordered for about 700 homes in former burn areas of Los Angeles County.
A yearslong drought eased in California last spring, but Northern California had a dry start to winter and hardly any measurable rain fell 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 on both ends of the state.
Now, the storm coming in from the Gulf of Alaska could dump up to 4 inches (10 centimeters) 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,” Anderson said.
In the foothills just northeast of Los Angeles, residents placed sandbags outside houses that survived a December fire that scorched more than 24 square miles (62 square kilometers), destroyed 60 homes and damaged 55 others.
The weather service also issued a winter weather advisory for portions of the Sierra Nevada above 7,000 feet (2,134 meters), forecasting about 4 to 7 inches (10 to 18 centimeters) of snow and up to 1 to 2 feet (30 to 61 centimeters) on higher peaks Tuesday.
It said travelers should prepare for difficult travel conditions, including gusty winds, low visibility and snow-covered roads.