The fire, which broke out around 9:45 a.m., was burning about a mile from the recent Canyon Fire that eventually blackened more than 2,600 acres and took more than a week to contain.
By nighttime, the fire had burned 6,000 acres and 24 structures.
Homes just southwest of the interchange of the 241 and 91 freeways appeared to be most threatened, aerial video from Sky5 showed.
The city of Santa Rosa and its 175,000 residents felt much of the damage, with strip malls, business parks, hotels and subdivisions swallowed up by the fire.
More than 100 people are being treated in hospitals for fire-related injuries and smoke inhalation.
The smoke over the park was coming from a fire churning through canyons in hilly neighbourhoods of Orange County, where residents of about 1,000 homes were forced to evacuate and a state of emergency was declared. Dio Compolongo, 22, said he operated "on total instinct" when he saw the blaze creeping close to his family's residence in the Anaheim Hills.
An overnight shelter was also opened at Katella High School, 2200 E. Wagner Ave., Anaheim.
OCFA and Anaheim Fire have units at the scene.
The Orange Unified School District has suspended its afternoon sports for Monday.
By Tuesday, "we're gonna be as stretched as we can be", said Steven Beech, an incident commander with the California Department of Forestry and Fire Protection, according to the LA Times.
"To California, we say though this declaration, we are with you, our prayers are with you and we will be with you every day until we put the fires out", he said.
-The eastbound 91 freeway is closed at Weir Canyon Road.