A cold, heavy rain hit northern California vineyards on Oct. 13, a lot earlier than normal, and that all but sealed the 2009 harvest for most wineries.

Rain almost always hits late in the harvest season, but this year, a heavier-than-usual, earlier-than-usual storm pelted Napa, Sonoma, Mendocino and Lake counties.

The four growing regions, some of the best in California, normally don’t see more than a sprinkling of precipitation until at least a month later, but what fell on the areas has changed, to a degree, what kinds of wines 2009 will produce.

Since most all white grapes had already been harvested, just a few varieties remained unpicked when the rain hit, and with adequate warning, most farmers rushed to get grapes before the deluge.

A few grapes were harvested at sugars lower than what some farmers were hoping to get, but that merely means that some wines will have a bit lower alcohol — not altogether a bad thing.

For the most part, only two types of grapes were left to deal with: later-picked red grapes and grapes that would end up making dessert wines. Those growers who were hoping to let some red grapes “hang” on the vine for an extra week or two, to get more sugar and concentration, were pretty much out of luck.

It is true that cabernet sauvignon often can recover from a brief storm. The variety produces clusters of berries that are loosely formed, so there is air between many berries. As such, moisture that gets into the clusters can evaporate as long as the next few days are dry and low in humidity.

But humid and really overcast conditions followed the Oct. 13 storm, and by Oct. 16 it was questionable whether much post-storm cabernet will be picked.

What is likely, however, is that with humid, moist conditions, many vineyards that left grapes on the vine like riesling, gewurztraminer, and even chardonnay may well be able to wait a few more days or weeks for “rotten” grapes with which to make great sweet wine.

The word “rot” here refers to the beneficial mold called Botrytis, which turns grapes into an unattractive mass that looks totally unappetizing.

However, when courageous wine makers harvest this mess and treat it properly, some sensational dessert wines can be made. Such conditions make some of the world's greatest rieslings in Germany, which are so rare and exotic that they are some of the highest priced wines in the world.

The downside is that sales of dessert wines in the United States have never been very strong, and with the weakened economy, pricey dessert wines (as wonderful as they can be) are slow sellers.

As a result, some wineries may have the perfect raw materials with which to make great dessert wine, but will not for fear it won’t sell.

The onset of global climate change has really been a test for the California wine industry, and 2009 will go down as yet another challenge.

Wine of the Week: 2008 J. Pedroncelli Sauvignon Blanc, Dry Creek Valley, East Side Vineyard ($11): A mild melon and hay aroma and a slight touch of spritz in the mouth give this wine a zestiness that works nicely with lighter seafood dishes.

Dan Berger lives in Sonoma County, where he publishes Vintage Experiences, a weekly wine newsletter. Write to him at danberger@rocketmail.com.