The 25,146-acre Jerusalem Fire in Lake County has burned a small area of the northernmost tip of Napa County near Knoxville, a Cal Fire spokesman said.
"It crossed into Napa County late Wednesday afternoon or early this morning," Cal Fire spokesman Steve Swindle said Thursday.
The Jerusalem Fire was 78 percent contained as of Saturday morning, and full containment is expected Monday, Cal Fire said.
The bulk of the Jerusalem Fire is circling around and burning into the 69,438-acre Rocky Fire to the north in Lake County, Swindle said. The Rocky Fire is 95 percent contained.
The temperature in the fire area is expected to reach triple digits this weekend, Swindle said.
Four residences and 13 outbuildings have been destroyed by the Jerusalem Fire, and 50 are threatened.
The cause of the fires is under investigation.