Fire spokeswoman Karen Stasko yesterday said the fire in Lake and Napa counties was 33 per cent contained, up from 16 per cent the previous day. It has burned through 95sq km.
The blaze was yesterday threatening 50 structures, but no homes had been destroyed.
No injuries have been reported.
At least 150 people have left their homes since Monday. Some in the region, about 160km north of San Francisco, had only recently returned after fleeing an earlier blaze.
About 2000 firefighters took a stand against the flames yesterday, some of them lighting backfires along a road as plumes of black and white smoke rose into the sky.
"We made great progress and that allowed us to move those resources and personnel," said Daniel Berlant, a spokesman for California Department of Forestry and Fire Protection.
Berlant said a portion of the new fire's northern edge merged with the huge blaze.
The new fire in Lake and Napa counties began in dry timber and brush on Tuesday several kilometres from the community of Lower Lake.
It leapt from Lake County into Napa County, but no vineyards were threatened in the famous wine-growing region.
Empire Mini Storage manager Desiree McAlear said the business in Middletown had received numerous calls and visits from people who wanted to rent units in case they had to evacuate.
"They're absolutely scared and terrified," she said. "If the winds decide to blow this way, then we all need to take action. Right now, we have the luxury of waiting and being patient."
The causes of the fires remain under investigation.
Temperatures have been relatively mild, but the gusty winds and dry conditions have stoked the flames.
In Southern California, evacuation orders were lifted as crews continued to surround a small wildfire sparked by a burning motor home in rural Riverside County.
