Dry ice pellets ensure that these just-harvested Sangiovese grapes will stay cool.

At this point, California winemakers have come to expect a heat spike around Labor Day weekend every year. Like clockwork, it seems, scorching temperatures descend on the state just as the wine harvest is getting underway, threatening to sear the grapes.

But this year’s temperature surge has proven even more challenging than usual. It's been hotter, and  lasted longer.

“It’s cooler in Phoenix right now than Sonoma County,” said William Allen, winemaker at Two Shepherds in Windsor, on Tuesday afternoon.

What resulted, during the last week, was a race through the vineyards, as many Bay Area winemakers rushed to get grapes off the vines before they baked. Some wineries harvested so much fruit all at once that they risked running out of space for it all in their wineries. Many had to contend with the possibility of rolling blackouts, forcing them to rush to finish all their work before the late afternoon, lest the power go out while a press is full of grapes.

“I think everybody is now frantically trying to get everything off (the vine),” said Bruce Devlin, winemaker at Ballentine Vineyards in St. Helena.

Virtually all California vineyards already harvest their fruit at night or in the very early morning, never during the heat of the day. The primary reason for this is worker safety. It also happens to be better for wine quality: Ideally, grapes should be cool to the touch when they arrive at a winery’s door.

When sudden weather events like this happen during the harvest season, winemakers must make a difficult decision. Because the precise moment of harvesting — and, consequently, the precise level of ripeness in the grapes — determines the quality of the resulting wine, this choice is possibly the most important one a winemaker makes all year.

The first option is to pick the grapes before the heat spike, when they may not be quite as ripe as a winemaker would like; maybe they still taste “green,” with flavors reminiscent of a bell pepper. The second option is to wait, letting the grapes mature and develop deeper flavors — but this is a risk, since the grapes could shrivel.

“It's a roll of the dice,” said Anthony Beckman, winemaker at Balletto Vineyards in Santa Rosa.

Beckman chose option no. 1, harvesting most of his Pinot Noir grapes last week before the heat wave. “In an ideal world, if I could have waited four more days, I would have,” he said. Beckman  might have regretted his choice if the heat had abated sooner. But it didn’t, and he's confident he made the better decision, he said.

Laura Barrett of Clif Family Winery in St. Helena chose option no. 2. Though most of her white grapes have been picked by now, she hasn’t felt like her red grapes are fully ripe yet. That’s especially true of her Cabernet Sauvignon on Howell Mountain, where grapes are prone to huge, astringent tannins — and the best way to get them into balance is to let the grapes hang on a little longer.

“There’s a little bit of raisining on the sun-exposed sides of the clusters,” she said, “but nothing that's alarming to us yet.” She’s hoping to wait until next week to start picking reds, but there’s always the option to call a last-minute pick if the grapes begin showing signs of distress. The weather station at her vineyard in Napa’s Oak Knoll District topped out at 114 degrees over the weekend.

Extreme bursts of heat can adversely affect wine grapes in several ways. The heat can literally turn the grapes into wrinkled, dehydrated raisins. White grapes can get sunburned, turning from a greenish color to a purplish-gray. At a certain temperature threshold, vines actually shut down entirely, going into survival mode and ceasing photosynthesis.

In certain volumes, these heat-stress effects are manageable. “Small enough quantities of raisining won't affect the wine that much,” said Devlin, who picked all the grapes for his Ballentine wines before the heat wave. Winemakers will simply discard the raisinated clusters.

But the chaos of a heat wave like this one ultimately makes it more difficult for a winemaker to have total control over their production, said Allen of Two Shepherds. He counts himself as  a natural winemaker, so if grapes arrive at his winery too ripe or underripe, he won’t adjust their color, acidity or sugar levels — he just lives with whatever he's got. 

“It's going to be a tough year for natural winemaking,” he said.

Still, there's one tool available to every winemaker, natural or otherwise, that may come in handy for this year's wines: blending. Combine a slightly underripe wine with a slightly overripe wine, and maybe, Allen said, you'll end up with something that tastes perfectly balanced. 

Senior wine critic Esther Mobley joined The Chronicle in 2015 to cover California wine, beer and spirits. Previously she was an assistant editor at Wine Spectator magazine in New York, and has worked harvests at wineries in Napa Valley and Argentina.