West coast weather—even as close to Southern France as this—can have bad storms even during the summer because of the nearby Atlantic, but this is the case with a lot of the south of France. Happily, storms or bad days don't hang around for long.
The thing to keep in mind is when you come. French people usually vacation within their own national borders, so that means mid-July to the end of August is when you'll have company, and lots of it, especially in the more famous destinations.
Spring and fall are the best times to visit—there aren't as many tourists around, and the weather is still pleasant.
The vendanges (grape harvests) usually begin about mid-September in the Bordeaux region (though you can't visit the wineries at this time), and two weeks later in the Cognac region, to the north.