By Theresa Waldrop and Christina Maxouris, CNN

Two California utilities said they may cut power to more than 900,000 customers for days starting after midnight Wednesday morning because of dry winds that could cause fires.

San Jose Mayor Sam Liccardo on Monday urged residents and businesses to prepare to be without electricity for as much as a week, saying the outages could affect almost 2 million people.

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 National Weather Service issued a "red flag" warning for Wednesday and Thursday, warning of possible "extreme fire weather conditions" for the "northern Coastal Range and foothills, the Sacramento Valley, the northern San Joaquin Valley, and the Northern Sierra Nevada and foothills below 6000 feet."

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.

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.

PG&E has previously said it's "probable" that its equipment started the 2018 Camp Fire -- California's deadliest and most destructive blaze -- when a power line touched nearby trees.

An investigation by the California Department of Forestry and Fire Protection found the company responsible for the fire, which killed 85 people and destroyed thousands of structures.

The utility giant announced last month it had reached an $11 billion settlement with insurance companies for claims stemming from the devastating 2017 wildfires across Northern California and the 2018 Camp Fire.

That followed a $1 billion payment in damages to local governments in June for blazes linked to its power lines and other equipment.