Growers in Napa, already facing an estimated $5m loss in the Chardonnay crop to frost damage in April, have taken another hit from Mother Nature. Three days of extreme heat, with temperatures spiking at 102 degrees Fahrenheit hit the vineyards in late May, scorching the blooms on late flowering Merlot and Cabernet Sauvignon.

While much of the crop was past the bloom stage, a significant portion was still in bloom, according to Napa County Farm Advisor Ed Weber.

Pat Garvey of Flora Springs Winery said about 40% of grapes in any given cluster mature at a slower rate, so damage could be substantial. "We got the double whammy this year," he said, referring to first frost, then heat. "The clusters we had going through bloom were burned to a crisp," he added.

The extent of the heat damage is still uncertain. It depends on a number of factors, such as location of the vineyard, soils and how the vineyard was pruned. No one will know what kind of damage both the frost and the latest heat caused until harvest.

The damage to Napa's three most important varietals could have significant impact on grape prices this fall, especially since Napa is one of the few growing regions in California where prices have remained firm in the face of uncertain market conditions.