A blast of hot weather has kicked an idling harvest into high gear this week, sending vineyard workers scrambling for fruit that would normally have been picked weeks ago.

"We finally got summer," said Saralee Kunde, a Russian River Valley grape grower whose vineyards supply 60 winemakers. "This heat has got things rolling."

Harvest had been off to its slowest start in more than a decade after weeks of cool weather delayed ripening of a crop already thinned by mid-summer rains.

As of last week only about 2 percent of the county's $400 million grape crop was in wineries' tanks. Normally, harvest would be a quarter to halfway over by mid-September.

But temperatures have climbed this week as summer prepares to transition to autumn, which begins Friday. Fogless mornings and sunny days this week have helped push sugar levels in many grapes up over the threshold needed to ferment their juice into wine. Temperatures reached 98 degrees Monday and again Tuesday at the Charles M. Schulz-Sonoma County Airport, the hottest days of the year.

Suddenly some growers have gone from a near standstill to a sprint that is likely to last through the weekend, when daily highs are forecast to dip back into the 80s.

By the weekend, growers should have harvested 10 percent of the county's grape crop, said Nick Frey, president of the Sonoma County Winegrape Commission.

To beat the heat, many growers are waking up in the middle of the night and picking grapes before the sun rises.

"We are hammered because of the heat," said Glenn Alexander, owner of Bacchus Vineyard Management, whose workers harvest 600 acres of vineyards throughout Sonoma County and near Calistoga. "Starting at 1 a.m. Wednesday morning, we are overbooked through Sunday."

Last year, a more intense heat wave in September caused panic in some growers as temperatures in the 100s threatened to bake grapes already damaged by a sudden spike in temperatures a month earlier.

But growers and vintners said this week's more mellow climb in temperatures is welcome news.

"It was just a nice little catalyst to kick things right over the edge," said Randy Ullom, winemaster for Kendall-Jackson Wine Estates. "The flavors were there. We were just one step away from being ready. Now we are good to go."

Still, good weather in September can only do so much to make up for June storms, cool August days and the appearance of moisture-related diseases. Frey said it's too early to say how big the 2011 crop will be, but it will be smaller than last year.

"We'll definitely be below last year, which wasn't a large crop by any means," Frey said.

Kunde said she expects her total harvest to be down as much as 15 percent, with some varietals faring significantly worse. Kunde and her husband, Rich, grow 18 varietals of grapes on more than 300 acres of vineyards in the Russian River Valley.

Her pinot noir vineyards, which produce one of her largest crops, look set for a good harvest. But her gewurztraminer is decimated, the victim of summer rains that interfered with pollination.

"It looks like we already picked it," she said Tuesday morning, surrounded by acres of pinot gris outside Windsor. "And we haven't even touched it."