Until now, harvest 2008 has been proceeding under nearly ideal conditions. Daytime highs have been in the 80s, lows in the 40s, and the grapes are ripening beautifully. Enter this weekend. A cold, dry arctic low dropped down into California about six weeks earlier than normal, and we have seen three consecutive nights below freezing in most of the Paso Robles AVA. The weather report from the Paso Robles Wine Country Alliance shows the results for last night:
As you can see, temperatures dropped well into the 20s both of the past two nights through most of the AVA. Perhaps because of our proximity to the Pacific, or due to our elevation, or due to the mountains to our north that might have helped block the cold north wind, at Tablas Creek we didn't get hit quite as hard, but we still saw some damage. The weather station is in the center of the vineyard, in a spot of more or less average temperatures, and there are always spots two or three degrees cooler.
Frosts during harvest are rarer than those during flowering, and damaging in different ways. In the spring, you are more likely to have a frost which will impact most or all of your year's results, causing uneven ripening and low, erratic yields. But, you can also mitigate spring frosts with overhead sprinkling, and most large commercial vineyards do this as a matter of course on cold spring nights. In the fall, you don't want to sprinkle overhead because of the risk of mildew and of causing the nearly-ripe grapes to swell and split (think of the problems of harvest rain). Harvest frosts don't typically render the grapes on the vine unusable. Still, if the leaves of a vine are frozen, the vine stops photosynthesizing for the year and the grapes will only accumulate additional sugar through dehydration.
At Tablas Creek, we have completed a little more than half of harvest, and already-harvested sections are unaffected by the weekend's frosts. Still, many of the areas that are still unharvested are those that were affected by this spring's frosts, and are in lower, more frost-prone areas. We have one more night where it's forecast to approach freezing, after which we'll be able to assess the damage. I'm sure it will be painful, though it doesn't so far look devastating.
Although I hope not, I would imagine that the damage in much of the rest of the AVA will be severe, adding to the pressure on many growers from a vintage already impacted by spring frosts, low rainfall, and wind during flowering that caused extensive shatter in Cabernet Sauvignon and Zinfandel vineyards.