The region northwest of Los Angeles has seen some of California’s most destructive fires over the years. The Mountain Fire swiftly grew from less than half a square mile (about 1.2 square kilometers) to more than 16 square miles (41 square kilometers) in little more than five hours on Wednesday, with gusts topping 61 mph (98 kph).

Calmer winds helped California firefighters battle the once out-of-control Mountain Fire on Friday as they continued to defend homes from the destructive blaze, officials said. By Friday evening, the fire had burned 20,630 acres, with firefighters ...