Grape growers are watching the weather reports with dread.

The late-season downpour hitting Northern California is threatening to devastate this year's grape crop, making it potentially the third weather event in the last four years to wreak havoc on vineyards.

"It's causing us a lot of frustration," said Steve Hill, general manager of Durell Vineyard in Sonoma.

Most vineyards across Sonoma County are now in some stage of bloom, a process where plants engage in the delicate dance of self-pollination in order to fertilize the eggs that will become grapes.

Rainfall in June, which forecasters predict could set a new record, is expected to lower the amount of grapes that vineyards produce, potentially costing Sonoma County growers tens of millions of dollars in lost revenue at a time the industry continues to suffer economically.

"In reality, we are taking a loss every time these storms come in," said Steve Thomas, director of vineyard operations at Kunde Winery. "This crop is definitely getting smaller."

Grapevine fertilization is timed to normally occur after the rainy season. But the late rain this year is hampering the process, preventing some grapes from forming.

For instance, small caps located on the tip of a grapevine's many flowers usually pop off sometime in late May or early June. But this year's heavy rains can make the cap stick to the flower, hindering fertilization.

The wet weather also can bog down pollen spores, preventing them from alighting on the flower's eggs.

"This doesn't mean that grape growers are going to have a total bust, but we might not have as much as we'd hoped," Hill said.

The impact of the potentially record-setting rain will not be known for a few weeks, when growers will be able to identify and count how many flower clusters successfully fertilized and bear the earliest signs of grapes — called fruit sets.

Farmers expect this season's harvest will be smaller than last year's, which was about 5 percent below average because of a cool growing season and an August heat wave that cost growers about $88.4 million.

Whether or not it could be even smaller than the 2008 harvest, which was 15 percent below average due to widespread damage from a late season frost, will depend in large part on how long the rainy weather continues.

Santa Rosa typically receives .19 inches of rain for all of June. After this weekend's storm, the total could exceed 2 inches, according to weather forecasters.

"We're sitting in the barn, just watching. There is nothing we can do about it," said Duff Bevill, whose company Bevill Vineyard Management farms about 1,000 acres.

Different grape varietals and growing regions will likely be impacted differently by the rain, said Nick Frey, president of the Sonoma County Winegrape Commission.

Chardonnay and pinot noir, which tend to bloom earlier than other varietals such as cabernet, are more likely to be impacted since they are in full bloom now. The warmer growing regions, such as Dry Creek Valley and Alexander Valley where bloom usually occurs first, also are more likely to be damaged.

Last year, Dry Creek was hit hard by the August heat wave. Some farmers lost their entire crops.

"It's frustrating. No doubt," Frey said.

Other grape-growing counties such as Napa, where the bloom is farther along, could be hit even harder as the rain interrupts the vine's reproductive process.