The Sandy Fire was reported on Monday morning in hills above Simi Valley, about 50km northwest of Los Angeles.
By mid-afternoon it had consumed more than 200 hectares of dry brush and damaged at least one home, according to the Ventura County Fire Department.
The flames were pushed by morning gusts that topped 48km/h but were subsiding later in the day, said according to fire department spokesman Scott Dettorre.
"As the sun sets, those winds will calm down even more," Dettorre said.
Evacuation orders and warnings were issued for several neighbourhoods in Simi Valley, a city of more than 125,000 people that was shrouded in smoke as helicopters performed water drops.
Dettorre didn't know exactly how many people were ordered to leave.
The Ronald Reagan Presidential Library and Museum, perched on a hillside a few kilometres away, said it was closed for the day because of the fire.
The cause is under investigation.
Meanwhile, firefighters were battling a 40sqkm blaze on Santa Rosa Island, the second-largest of the Channel Islands off the Southern California coast.
The fire destroyed a cabin and an equipment shed and forced the evacuation of 11 National Park Service employees.
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