SAN FRANCISCO (AP) — As a huge storm approached California on Wednesday, officials began ordering evacuations in a high-risk coastal area where mudslides killed 23 people in 2018, while residents elsewhere in the state scrambled to find sandbags, and braced themselves for flooding and power outages.

California Governor Gavin Newsom declared a state of emergency to allow for a quick response and to aid in cleanup from another powerful storm just days earlier. Dozens of flights were cancelled at the San Francisco International Airport, and South San Francisco schools preemptively cancelled Thursday classes. As the storm intensified, state officials warned residents in Northern California to stay off the roads.

The first evacuations were ordered for those living in the burn scar areas of three recent wildfires in Santa Barbara County, where heavy rain is expected overnight, and could cause widespread flooding and unleash debris flows in several areas. Among them is the tony town of Montecito, home to many celebrities, including Oprah Winfrey and Prince Harry and his wife, Meghan Markle.

“We anticipate that this may be one of the most challenging and impactful series of storms to touch down in California in the last five years,” said Nancy Ward, the new director of the California Governor’s Office of Emergency Services.

Officials asked drivers to stay off the roads unless absolutely necessary — and to stay informed by signing up for updates from emergency officials about downed trees and power lines, and flooding. In Northern California, a 25-mile (40-kilometre) stretch of Highway 101 was closed between the towns of Trinidad and Orick due to several downed trees.

Before the storm arrives late Wednesday, Santa Barbara County Sheriff Bill Brown said people should evacuate the areas impacted by the Alisal Fire last year, the Cave Fire in 2019 and the devastating Thomas Fire in 2017, one of the largest in California history.

On January 9, 2018, massive torrents carrying huge boulders, mud and debris roared down coastal mountains, and through the town of Montecito to the shoreline, killing 23 people and destroying more than 100 homes. Among those killed were two children whose bodies were never found.

Montecito Fire Department Chief Kevin Taylor said Wednesday that homes near waterways are at the greatest risk.

“What we’re talking about here is a lot of water coming off the top of the hills, coming down into the creeks and streams and as it comes down, it gains momentum and that’s what the initial danger is,” he said.

Storms in the last 30 days have produced between eight to 13 inches of rain, soaking coastal hills in Santa Barbara County. The current storm is projected to drop up to 10 inches of rain in the area, Taylor said.

“This cumulative rain … is what causes our risk,” he said.

The storm, set to be in full force in Northern California by Wednesday evening, is one of three so-called atmospheric river storms in the last week to reach the drought-stricken state. Because the states’ major reservoirs are at a record low from a dry three-year period, they have plenty of room to fill with more water from the impending storm, officials said.

Still, trees are already stressed from years of limited rain. Now that the grounds are suddenly saturated and winds are heavy, trees are more likely to fall. That could cause widespread power outages or create flood hazards, said Karla Nemeth, director of the state’s Department of Water Resources.

“We are in the middle of a flood emergency and also in the middle of a drought emergency,” she said during an emergency briefing.

The storm comes days after a New Year’s Eve downpour led to the evacuations of people in rural Northern California communities and the rescue of several motorists from flooded roads. A few levees south of Sacramento were damaged.