Dark clouds loomed over most of this grape growing season — at least figuratively speaking.

Now those big shadowy balls of moisture are literally racing across the Pacific Ocean. They're expected to dump an inch or more of rain across Sonoma County by Monday, possibly cutting short a year already plagued with problems.

People are working nearly around the clock to harvest the fruit and move it through the initial stages of the winemaking process.

Weather forecasts on Thursday called for scattered showers overnight followed by a period of gloomy calm before back-to-back storms are expected to drench the North Coast on Saturday and Sunday.

Growers say the weekend storms come as a mixed blessing, noting that until now October has offered near perfect conditions. But they say this year's late-ripening crop could have used a little more time.

"Until now, the weather has been great in the month of October," said Steve Hill, general manager of Durell Vineyard in Sonoma. "We were lucky to have gotten this many grapes off in the last two weeks."

Farmers are expected to be about 70 percent done with harvest by Friday night, said Nick Frey, president of the Sonoma County Winegrape Commission.

"It's pretty urgent to get the thin skinned grapes (such as chardonnay) harvested by Friday night," Frey said. "What will be left out there are cab and other thick-skin varietals that can hopefully take the rain reasonably well."

A few inches of rainfall are not unusual for October, but this year's harvest is a couple weeks late in ripening due to a cool summer.

This time of year, wet chardonnay grapes begin to mold after about 48 hours, Hill said. Most red grapes handle wet conditions much better than that, but if next week remains cool and moist, it could result in widespread outbreaks of mold.

The harvest is already expected to be about 20 percent smaller than normal in part because Mother Nature let fly a burst of heat in August, scorching grapes that had grown accustomed to a record cool July. Grapes baked on the vine during two days of 100-plus degree heat and within days began withering into little raisins.

Some regions and grape varietals were impacted harder than others, such as zinfandel grapes in the Russian River Valley and Dry Creek Valley.

Brandon Lapides, winemaker at Armida Winery in Dry Creek, said the sun fried about 75 percent of the zinfandel grapes in their 5-acre vineyard.

As a result, he will probably make only 6,000 cases of wine this year, down about 30 percent.

The damage has complicated harvest. Lapides and his crew are forced to carefully sort grapes to ensure raisins and mold don't get crushed along with healthy, plump grapes.

"We're doing double sorting on every grape we get in here just to make sure the quality is where we want it," he said. "It is a little bit like pulling teeth."

He said the grapes being crushed are of an excellent quality.

Still, he voiced the concern of many in the industry when he said, "I'm sure the 2010 vintage will probably get panned in the press."

Lapides acknowledged that questionable fruit is being picked by some growers. In fact, he decided not to buy fruit on the spot market because of those concerns.

But that's not the only story, he said. This year's growing conditions resulted in some excellent fruit, albeit at far smaller volumes.

That sentiment is shared by many in the industry. Hill said this year's fruit was either excellent or not very good.

"We haven't just picked bad grapes," he said. "We have picked some terrific grapes that will make some very good wine."

For instance, fruit from the Petaluma Gap region is the best grown in several years, said Glenn Alexander, founder of Bacchus Vineyard Management in Fulton.

"There were some grapes that really were perfect," he said.

Bob Iantosca, winemaker at Gloria Ferrer in Sonoma, said he hasn't tasted anything unusual in the recently crushed wine.

"We're not noticing any raisiny, cooked characteristics," he said.

As the season comes to a hectic close 71-year-old Nick Leras, who is finishing his 53rd Russian River harvest, put things into perspective.

"I'd rather be growing grapes than playing golf or fishing," he said. "I just love it. This is what life is all about."