Pacific Gas & Electric and Southern California Edison on Tuesday raised the number of its customers who may be without power from 600,000 to almost 800,000, affecting 34 northern, central and coastal counties, starting with counties in the north just after midnight Wednesday morning.
Southern California Edison said it's considering cutting power to more than 100,000 customers in eight counties. The utilities haven't said how many people the outages will affect in all.
"Given the prolonged period during which the wind event will unfold, and the large number of power line miles that will need to be inspected before restoration, customers are being asked to prepare for an extended outage," PG&E said in a release.
"The conditions are ripe: dry fuel, high winds, warm event. Any spark can create a significant event," said Ray Riordan, director of the Office of Emergency Management in San Jose, at a press conference Tuesday.
The California Department of Forestry & Fire Protection said Wednesday its firefighters are "staffing up and preparing" and warned that "we need Californians to not be complacent," since "some of the most destructive and deadliest fires" happen between October through December.
The city of San Jose had been preparing for a similar event since June, the mayor said.
"Some of what we need to do to get ready will in fact take years," he said. "That is ensuring we have sufficient backup power generation, establishing microgrids where we can do so, things of that nature."
Utilities and authorities asked residents to get ready by preparing emergency kits with flashlights and fresh batteries, first aid supplies and cash, and to plan for medical needs and devices that will need power.
Liccardo urged residents to shelter in place, as street lights and signals will not be working if there's no power. He also encouraged households to create emergency plans and kits which include food and water for all family members and pets.
Areas close to wildlands are most at risk of a shutoff, San Jose Deputy City Manager Kip Harkness said, but homes within that distribution system will be affected, even if they're not close to wildland.
"That whole thing needs to be shut down as a unit," Harkness said.
Late last month, PG&E cut power to more than 48,000 northern California customers as a precaution in dry and windy conditions. It was the second consecutive public safety power shutoff in that week following outages that affected about 24,000 customers in three northern counties.
That followed a $1 billion payment in damages to local governments in June for blazes linked to its power lines and other equipment.
CNN's Cheri Mossburg and Carma Hassan contributed to this report.