Workers in Northern California's renowned wine country picked through charred debris and plotted what to do with pricey grapes after wildfires swept through lush vineyards, destroying at least two wineries and damaging many others.

The wind-driven wildfires came as Napa and Sonoma counties were finishing highly anticipated harvests of wine grapes. Monday normally would have found workers picking and processing the ripe grapes to make chardonnay and other wines.

Instead, melted and blackened wine bottles decorated the ruined Signorello Estate winery in Napa Valley. People at Paradise Ridge Winery in Sonoma County posted photos of debris and haze, saying they were "heartbroken to share the news" that the winery had burned.

A maintenance worker watched and hoped for the best Monday as flames crept down a hillside by the Gundlach Bundschu Winery.

"It's right behind the main office. It's working its way down the hillside. What can I say? It's slowly working its way in," Tom Willis said.

The Napa Valley Vintners, a trade association, said Monday that most wineries were closed because of power outages, evacuation orders and employees who couldn't get to work. The organization said it did not have firm numbers on wineries burned or how the smoke might affect this year's harvest or the industry in general. But it said most grapes had already been picked.

The charred remains of the Signorello Estate winery. (Photo / AP)

About 12 per cent of grapes grown in California are in Sonoma, Napa and surrounding counties, said Anita Oberholster, a cooperative extension specialist in enology at the University of California, Davis. But they are the highest value grapes, leading to the highest value wines, she said.

It's hard to predict correctly, but she said chances are good this year's crop won't carry much smoke damage.

"Even if wines now were heavily affected by smoke, it doesn't carry over to the next season, only in the fruit itself," she said.

Gloria Ferrer, Ravenswood and Kenwood were among well-known wineries closed for the day because of the fires, according to social media posts. Chateau Montelena Winery, which helped put California on the global wine map when it won a French wine-tasting competition in 1976, escaped damage.

Wineries that escaped damage grappled with the lack of power, which they need to process the grapes.

"Some of our growers did pick for us last night. So we had to unload the fruit into our cold barrel room and wait until tomorrow to process it," said Alisa Jacobson, vice president of winemaking at Joel Gott Wines.

"I think we'll be OK, but it's not an ideal situation. But more importantly, all our employees seem to be doing OK," she said.

She said she was stunned by the speed of the fires, falling asleep around 10 p.m. Sunday only to wake during the night to the smell of smoke. By 3 a.m. people were being evacuated.

Lise Asimont, director of grower relations for the Family Coppola wineries, was among the people being urged to leave her Santa Rosa home. She said explosions that made her think of war woke her around 2 a.m. She opened the front door to a sky snowing ash.

Authorities told her family to prepare to flee, but Asimont was also worried about her grapes, four truckloads of cabernet sauvignon machine-picked in Lodi on Sunday with no way of getting to Coppola facilities Monday because of a closed highway.

She called a wine maker with LangeTwins winery and vineyards, who had a tank available to crush the grapes and was happy to be able to help. In turn, she passed on the favor to another winery.

"There's a lot of people helping each other, which is amazing," she said.