Authorities declare a state of emergency to allow for a quick response and to aid in cleanup from another powerful storm just days earlier.
Officials have ordered evacuations in a high-risk coastal area of California, while residents elsewhere scrambled to find sandbags and braced themselves for flooding and power outages as a huge storm approached the western US state.
California Governor Gavin Newsom declared a state of emergency on Wednesday 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.
'Middle of a flood and drought emergency'
California officials asked drivers to stay off the roads unless absolutely necessary and stay informed by signing up for updates from emergency officials for notices on downed trees and power lines, and flooding.
"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, on Wednesday.
The storm arriving later on Wednesday is one of three so-called atmospheric river storms in the last week to reach the drought-stricken state.
Trees are already stressed due to three years with limited precipitation.
Now, suddenly saturated ground and heavy wind mean they're more likely to fall over, possibly knocking down power lines or creating 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.
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Flooding in urban areas
In the San Francisco Bay Area, 8,500 sandbags distributed by officials weren't enough to reach demand as forecasters warned of imminent flooding. The South San Francisco Unified School District announced classes for its 8,000 students would be cancelled on Thursday "out of an abundance of caution."
The rain led to the cancellation of more than 60 flights at the San Francisco International Airport and delays of another 70, said Doug Yakel, an airport spokesman.
Heavy downpours accompanied by winds with gusts of up to 96 kph were expected later on Wednesday and through Thursday and could cause flooding, downed trees and power outages, making driving conditions difficult, the National Weather Service said.
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, increasing flooding in the mostly rural area.
A December 27 storm brought powerful winds, rain and snow to the state, substantially lowering temperatures that topped 26.6 Celsius in some areas over Christmas. It was the first storm in a series of downpours forecast through next week.
"Excessive rainfall over already saturated soils will result in rapid rises on creeks, streams and rivers as well as flooding in urban areas," forecasters said in a report.
The storms in California still aren't enough to officially end the drought, now entering its fourth year. The US Drought Monitor showed that most of the state is in severe to extreme drought.
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