The blaze destroyed at least 150 structures, and Cal Fire said 12,000 more are threatened. A full damage assessment still needed to be completed.
PHOTOS: Thomas Fire burns in Ventura County
By Wednesday night, areas in Ojai and Santa Paula, along with communities in the north coast of Ventura County, were placed under mandatory evacuations.
"I've never seen a fire like that before. It looks like it's coming back over here," Santa Paula resident Eduardo Gonzalez said.
The fire ignited Monday evening and steadily increased in acreage, jumping the 101 Freeway near Solimar Beach Tuesday night and charring 70 square miles.
Authorities were concerned late Wednesday about strong, 40-50 mph Santa Ana winds, which could be a "recipe for explosive fire growth" overnight, according to a meteorologist working with Ventura County fire officials.
Fire officials cautioned that a difficult night was ahead for firefighters amid fire scenes that remained "very active."
MORE: NASA satellite captures raging SoCal fires from space
More than 15,000 homes in Ventura County were under evacuation as the blaze closed in on downtown Ventura, where multiple structures were reportedly burned. Fire officials said 50,000 people were evacuated from their residences.
In Ojai, patients and staff at the hospital were ordered to shelter in place as the fire roared back to life by the evening hours as the winds picked up speed and strength.
#SantaAnaWinds will continue to elevate fire danger in Southern CA with expectant winds reaching 80 mph on Thursday. Any new fires will have extreme levels of fire growth potential. Prepare now & be ready to GO! Learn more about evacuation preparedness: https://t.co/hHTBtHlGh9 pic.twitter.com/Tm0X2Tui6r— CAL FIRE (@CAL_FIRE) December 6, 2017
There were some concerns also that the fire could move toward Moorpark.
The city of Ventura imposed a curfew on residents until the state of emergency is lifted. The curfew, effective 10 p.m. to 5 a.m. prohibits people from being out in public places of the city, including streets and parks. The curfew was established to help protect people as well as limit looting in evacuation areas, officials said.
MORE: Latest Southern California forecast
On Tuesday, strong winds whipped through the area and made the firefight dangerous and difficult. There was a lull in the wind Wednesday morning, but stronger Santa Anas were expected to pick up later in the evening and continue into Thursday morning.
Communities from La Conchita on the border of Santa Barbara County all the way through Santa Paula in Ventura County were under mandatory evacuations.