The blaze was reported around 10 a.m. Tuesday in the area of Geysers and Pine Mountain roads. Windy and hot weather continued to fuel the flames overnight, Cal Fire spokesman Michael Selmi said.
A red flag warning to declare fire weather was issued around 2 p.m. Monday for the North Bay and East Bay hills and is expected to remain in effect until 4 p.m. Thursday, according to the National Weather Service.
Winds of 25 mph and 35 mph with gusts up to 50 mph are hindering control of the flames, Selmi said.
"That's definitely fire weather," Selmi said.
No structures have burned or are threatened, and no injuries have been reported, Selmi said.
The cause remains under investigation.