Firefighters battling wildfires in California's wine country face the prospect of new outbreaks when dry, windy conditions return to an area where blazes have killed at least 17 people and destroyed 2000 homes and businesses.
Gusts of up to 80km/h and 10 per cent humidity are forecast for later on Wednesday and into Thursday for the region, where 17 fires have forced 20,000 people to flee their homes in one of the deadliest wildfire outbreaks in California history.
"The potential for new fires that could grow exponentially as these fires did in such a short time period is there," said Lynne Tolmachoff, spokeswoman for California Department of Forestry and Fire Protection.
The weather had given firefighters a reprieve on Tuesday as cooler temperatures, lower winds and coastal fog let them make headway against the fires that had burned 46,500ha.
New evacuation orders were posted overnight across Sonoma County, north of San Francisco.
About 155 people were missing in Sonoma County, although 45 others had been found and some of those unaccounted for may be due to confusion surrounding evacuations.
The city of Santa Rosa was particularly hard hit by the so-called Tubbs fire, which damaged a Hilton hotel and destroyed a mobile home park, among other damage.
At least 11 people have been killed by that fire alone, officials said.
It is the deadliest California wildfire since 2003, when the Cedar fire killed 15 people in San Diego, and the sixth deadliest since records began, according to state records.
In Napa County, the dead included 100-year-old Charles Rippey and his wife, Sarah, 98, according to the county sheriff's office. The couple were married for 75 years, a CBS affiliate in San Francisco reported, citing their son, Mike.
Napa Valley Vintners, a trade group, said it was too early to assess the economic impact on Northern California's celebrated wine country. At least four wineries suffered "total or very significant losses" and at least nine reported damage, the group said.
California Governor Jerry Brown declared a state of emergency in several northern counties, as well as in Orange County in Southern California, where a fire destroyed 15 structures and damaged 12, the Anaheim Fire and Rescue Department said.