A top-level “Red Flag Warning” has been issued for huge areas near San Francisco as meteorologists warn dry weather, high winds and low humidity “could lead to a dramatic increase in wildfire activity”.
San Francisco itself has a population of around 864,000, but the surrounding suburbs and Bay Area are home to seven million.
A total of 17 people are confirmed to have died in the fires which started on Sunday and more than 180 are still unaccounted for.
Officials from the California Department of Forestry and Fire Protection (Cal Fire) have warned residents in large areas around San Francisco, San Jose and Santa Rosa to expect the wildfires to worsen.
A statement said: “During heightened fire danger, Cal Fire will place additional firefighters on duty, staff more fire engines and keep more equipment on 24 hours a day to be able to respond to any new fires.
“Cal Fire urges Californians to be extremely cautious, especially during periods of high fire danger.”
President Donald Trump has made extra resources available after signing a major disaster declaration, though firefighters are struggling to contain the flames.
More than 20,000 people were evacuated from their homes last night, and authorities ordered more residents from their properties in the Sonoma, Napa and Mendocino counties today.
CNN has reported authorities are encouraging others to pack “ready-to-go bags” containing essentials in case they have to flee at short notice.
Most of the fires started on Sunday and spread quickly due to dry conditions and 50mph gusts of wind.
Similar strength winds are expected today and no rain is forecast over California for the rest of the week.
More than 122,000 acres – nearly 200 square miles – have been devastated by flames, including large parts of California’s famous wine country.
The worst of the fires are still in the northern area of the state in Sonoma, Napa and Mendocino counties.
In the city of Santa Rosa, entire neighbourhoods have been decimated by the Tubbs fire, the largest to have broken out so far.
Director of Cal Fire, Ken Pimlott, said it was too early to determine how the fires started but told reporters the chances of them being caused by lightning was “fairly minimal”.
He added: “All these fires are under investigation.”