The flames were initially pushed by gusts that topped 30 mph (48 kph), but firefighters were aided by calmer winds overnight, said department spokesperson Andrew Dowd.
“We’ve made a lot of progress against this fire with those improved weather conditions,” Dowd said. Crews hoped to make further progress before winds increased again, he said.
There was zero containment. The cause is under investigation.
Evacuation orders and warnings were still in place for several neighborhoods in Simi Valley, a city of more than 125,000 people.
Meanwhile, firefighters were battling a 23-square-mile (59-square-kilometer) blaze on Santa Rosa Island, off the Southern California coast. The fire destroyed a cabin and an equipment shed and forced the evacuation of 11 National Park Service employees.
Santa Rosa, a popular destination for camping and hiking, is home to island foxes, spotted skunks and elephant seals.