More than 2,500 firefighters are battling the Glass fire near the resort community of Calistoga, which has already destroyed 248 structures.
California is poised to hit a fearsome milestone: 1.6 million hectares (4 million acres) burned this year by wildfires that have killed 31 people and incinerated hundreds of homes in what is already the worst fire season on record.
Fire crews at a blaze in the wine country north of San Francisco were on high alert Friday as forecasters warned of extreme fire danger into Saturday.
Powerful winds didn't materialise early Friday, allowing fire crews a chance to make gains. But winds up to 48 km/h (30 mph) were forecast to push through the hills of Napa and Sonoma counties as the Glass fire, which exploded in size earlier in the week, threatens more than 28,000 homes and other buildings.
“So far we have not seen the velocity of the winds that we were expecting,” Cal Fire Battalion Chief Mark Brunton said. “But there will be gusts and ... we do have hot embers and it won’t take much to take that and blow it into a very dry receptive fuel bed. That gives us cause for concern.”
Winds were blowing at higher elevations on the western side of the fire. Crews are expecting a long battle to keep flames from jumping containment lines and to prevent spot fires from leaping ahead to spark new blazes.
More crews and equipment were deployed in and around Calistoga, a town of 5,000 people known for hot springs, mud baths and wineries in the hills of Napa county, about 110 km (70 miles) north of San Francisco.
The area is also experiencing high temperatures and thick smoke that has fouled the air throughout the San Francisco Bay area.
Fire crews deploying water-dropping helicopters have made a defensive stand against flames raging in the foothills of the Napa Valley wine region.
More than 2,500 firefighters are battling the Glass fire, which broke out last Sunday near the resort community of Calistoga, some 120 km (75 miles) north of San Francisco, already destroying 248 homes and other structures.
Among them were the stately Chateau Boswell Winery north of Napa, and a farmhouse at the landmark Castello di Amorosa winery, where the castle-like main building survived.
Thick smoke poured over deserted Napa Valley communities that would typically be bustling with visitors for the grape harvest.
Winery employees stand guard
"Everyone is hunkered down, it's very, very quiet," said Kari Corte, who works at the Ghost Block Estates Winery outside Oakville. She was in her home in Zinfandel, the fire burning less than three miles to the north.
"I've got my bags packed and I'm ready to go," Corte said, recalling that in 2017 she had been forced to flee the town of Atlas Peak during another bad fire year for the wine country.
At some wineries employees stand guard over buildings and crops with bulldozers and water tankers, hoping to serve as a desperate last line of defense if flames made a run out of the foothills.
The Newton Vineyard winery went up in flames on Wednesday, as rivulets of red wine mixed with ash flowed down its main access road.
Vintners in the area fear the smoke has spoiled much of this year's vintage.
Rain the forecast
Roughly 52,000 residents are under evacuation orders at least through the weekend, with the fire at only 5% containment, according to the California Department of Forestry and Fire Protection, or Cal Fire.
The National Weather Service has issued red flag warnings for high winds over the next two days.
Rain storms are forecast for the region at the end of next week, and firefighters hope they cabn keep the blaze from entering populated areas again until showers come.
"It's going to be a big firefight for us in the next 36 hours," Cal Fire operations section chief Mark Brunton said on Thursday.
Resources were stretched due to the high number of fires burning across the state, he added.
The Zogg Fire
Some 320 km (200 miles) to the north, crews have made significant progress against the Zogg fire in the foothills of the Cascade mountain range.
Containment reached 46% as of Friday afternoon, up from 26% a day earlier.
Four people have been killed in the Zogg fire, which has also destroyed nearly 150 buildings near the town of Redding.
No casualties have been reported in the Glass fire.
Overall, at least 30 people have died since mid-August in a devastating string of wildfires across the US West that have been stoked by bouts of extreme heat, winds and dry-lightning storms that scientists attribute to climate change.
California Governor Gavin Newsom has also blamed decades of poor forest management.
California fires have scorched nearly 15,800 square km (6,100 square miles acres) since January, and far exceeding the acreage burned during any previous year on record.