"Any fires that develop will spread rapidly", the National Weather Service Monterey Bay said.
According to the warning, the North Bay mountains, East Bay hills and Santa Cruz mountains are all vulnerable.
A Red Flag Warning means that critical fire weather conditions are occurring.
The warning - issued for 8 p.m. Thursday, October 12 through 10 a.m. Saturday, October 14 - includes using extreme caution if operating flame- or spark-producing machinery, reporting smoke or suspicious activity to 9-1-1, and beiong prepared with evacuation route knowledge for the duration.
Winds out of the northeast gusted around the Bay Area at 30 to 50 miles per hour early Monday, and on some higher peaks they hit 70 miles per hour, said Duane Dykema, a forecaster with the weather service. Outdoor burning is not recommended.* Strong winds and low humidity expected late Friday night through Saturday evening. However, they will be strong enough to down trees and powerlines.
Relative humidity will range between 10-20 percent during the day, and 30-40 percent at night.
Several parts of the Bay Area have been included in an expanded Red Flag Warning for most of Thursday, alerting residents of the high fire danger due to forecasted low humidity and gusty off shore winds, National Weather Service officials said Wednesday.