Crews continued to make progress on the Cold Fire near Lake Berryessa, which was reported as 45 percent contained at 6:45 p.m. Saturday.
About 5,400 acres have burned, and the steep, rugged terrain has created challenges for firefighting because flames can move quickly uphill, according to Cal Fire. More than 1,400 personnel are assigned.
No structures have been threatened or damaged by the fire since it began Tuesday afternoon. The cause remains under investigation.
One lane of Highway 128 west of Winters has been closed, with a pilot vehicle leading traffic. The fire has burned north of the highway and east of Lake Berryessa.
The fire is named after Stebbins Cold Canyon, which is in the area of the fire, said Rick Vogt, a spokesman for the California Department of Forestry and Fire Protection.