Most people will gladly take hot weather on Labor Day—one last hurrah before summer comes to an end. But California vineyard owners aren't worried about hitting the beach; they're concerned about the grape harvest. And over the holiday weekend, unseasonably hot weather played havoc throughout California wine country, with some winemakers complaining that their grapes have shriveled into raisins.

Temperatures on September 1 spiked to between 105 and 109 degrees in major wine regions, including Napa and Sonoma—the hottest weather those two areas have seen on the first day of September in at least 40 years, according to Farmer's Almanac data published by the San Francisco Chronicle.

"I've been making wine for 34 years, and I don't think Napa's ever seen this excessive heat at this stage of ripeness," Pam Starr, co-owner of Crocker & Starr Wines in St. Helena, told the paper. In her part of the Napa Valley, temperatures were over 110 degrees three days in a row. "I thought we were going to make it through without a lot of repercussions, but that's not the case."

The result of such heat is a well-known phenomenon: The grapes can turn to raisins while still on the vine. Starr said irrigation helped fight against some of this shriveling, but she still predicted that some blocks of her vineyard lost half their crop or more.

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Though the heat was an issue all over, other winemakers said their losses weren't as bad. Jasmine Hirsch, of Hirsch Vineyards in Cazadero in Sonoma County, said yields would probably be down about 20 percent due to a mix of raisining and water evaporation. But she still said the heat was unprecedented. "The extensiveness of the dehydration was something I've never seen before," she was quoted as saying. "It's exceeded 104 degrees here before, but never during harvest."