Don't forget your sunglasses and sunscreen if you're visiting Sonoma in June, July, or August, . The heat here is dry, and midsummer temperatures can range from 80 to 100 degrees F (26-37 C).
On the other hand, Sonoma is only an hour from both San Francisco Bay and the Pacific Ocean, so the weather can also turn foggy and cool, even in the summer. Pack a sweater just in case.
Wear comfortable shoes when visiting wineries. One of the best parts of visiting wineries is walking in the vineyards. Vineyards are dusty, uneven and often difficult to manuever. Open sandals, nice shoes, and high heels are definitely NOT recommended.
In the winter, wineries are really, really cold. Dress accordingly.
Summers in wine country have typical day temperatures in the high eighties or low nineties, but night time temperatures usually go down into the fifties. Light jackets are recommended.
Miscellaneous: Never go to wine country without a corkscrew. Bring a couple of real wine glasses. They will make your picnics much more elegant and they make the wine taste better.