Calfire has dispatched six air tankers and at least 200 firefighters to fight the flames.
The fire broke out in the southern part of Clear Lake, north of a fire burning near Lake Berryessa.
Officials say at least one structure caught on fire on Rocky Creek Road in Lower Lake.
PHOTOS: Smoke from Lower Lake area wildfire seen for miles
PHOTOS: Crews battle Rocky Fire in three Northern California counties
Thousands have been evacuated from the path of the Rocky Fire, a massive wildfire that started burning near Clear Lake on Wednesday, July 29, 2015. The explosive fire is now burning in three Northern California counties -- Lake County, Colusa County, and Yuba County.
Residents are being asked to stay away from the area, so emergency personnel can concentrate on fighting the fire and evacuate residents.
So far, no injuries have been reported.