Two words are expected to define this year's grape harvest: Late and light.

The harvest is expected to start several weeks later than normal, perhaps kicking off in late August due to cool spring weather that stalled the growing season.

Then, once farmers do start harvesting, most expect a light crop due to a burst of unseasonable rain in June that interfered with vine fertilization and resulted in inconsistent grape growth.

"It's going to be as small as we've seen in many, many years," said Bob Iantosca, director of winemaking at Gloria Ferrer in Sonoma. "We had a cool spring and a lot of shatter."

Iantosca is referring to a rare phenomenon that growers call "shatter," which occurs when rain interrupts the climax of a vine's reproductive cycle.

Normally, a grapevine self-fertilizes by dripping pollen from its stamen down onto its eggs — a process that results in the formation of grape bunches.

But this June, when many vines had started to drop millions of pollen seeds down onto their eggs, heavy rains roared down from Alaska. The unseasonable rain washed away pollen and in some cases prevented a little cap on the stamen from popping off — in effect creating a natural prophylactic that stymied all those reproductive cells from reaching the vine's eggs.

"Now this season is a big question mark," said Chris Bowen, vineyard manager at Hunter Farms, usually one of the first to harvest grapes in Sonoma County. "I'm going to be at least 25 percent down in pinot noir."

Iantosca said this year's crop "could be historically low, aside from the severe frost years" such as 2008, when freezing weather in late spring destroyed a large swath of the year's crop.

Many growers are trying to determine just how small their crops will be.

"It varies a little bit by grape variety, but just about everything is affected," said Nick Frey, president of the Sonoma County Winegrape Commission. "I think some growers will be under financial pressures because of the low yields."

Predicting the exact impact of any natural event on a crop's size is notoriously difficult, growers say. Last year, due to the abnormally cool spring and summer, many estimated Sonoma County's grape crop would be 10 percent to 15 percent smaller than the 200,000 tons harvested in an average year. But by the time the last grape was hauled in, the county's crop size was down only 5 percent from normal.

Despite this year's natural calamities, farmers' spirits have been buoyed by a recovering grape market. Prices for grapes are on the rebound as more buyers return to the market.

"Buyers know this is a small crop, and with wine sales up, there is increased demand for grapes," Frey said. "We're even seeing some multi-year contracts being offered."

Yet despite the rebounding market, growers will likely face another natural obstacle. Next year's crop will likely be smaller, due to this year's cool spring weather.

A vine determines how many grapes to produce based on the previous year's spring weather. Warm weather means more grapes. Cool weather usually results in a smaller crop. The last two springs have been cool.

That means this year's crop was pre-destined to be smaller, and the rains only made matters worse.