The fire started Saturday afternoon and exploded in size overnight. Cal Fire was doing water drops from firefighting helicopters.
TIMELAPSE: Viewer captures flames consuming hillside in Napa County
"The day is heating up, a little breeze is kicking up and we're starting to see fire activity out there," said Cal Fire Battalion Chief Jonathan Cox.
200 homes remained threatened, and mandatory evacuation orders remained in place.
VIDEO: Flames burn out of control in Napa County
Snell Fire in Napa County 20 percent contained after burning 2,400 acres
"Hot, windy, crazy and chaotic," said resident Chris Anderegg.
Anderegg and his wife safely evacuated their home on Saturday.
"My wife is pregnant so I had to get her out of the house," Anderegg said. "The baby is due in February," he added.
RELATED: Napa County fire threatens 180 homes
No injuries have been reported and no structures have been lost.
Cal Fire says the cause of the fire is under investigation.
For the latest information about evacuations click here. An evacuation center has been established at Pope Valley Farm Center, located at 5800 Pope Valley Road in Pope Valley, Calif., the Napa County Sheriff's Office said. Another center has been opened at the Calistoga Fairgrounds at 1435 North Oak Street in Calistoga for impacted residents.
Get the latest stories and videos on wildfires burning across California, here.