Sonoma County's climate is Mediterranean — warm, dry summers and cool, wet winters. The best times to visit are late spring (May–June) and autumn (September–November). Late spring brings wildflowers on the hillsides and comfortable temperatures before the summer crowds arrive. Autumn is harvest season — the most dramatic time in wine country, with the vineyards turning and harvest activity at every winery.
Summer weekends are the busiest period, with tasting rooms crowded and restaurant reservations essential. The Sonoma International Film Festival in March is a good reason to visit in the cooler season. January through March can be rainy, but it is the least busy time and the valley is green and beautiful. Fog from the coast rolls into the Carneros region at the southern end of the valley most mornings, burning off by midday.