If you can make it out to the Santa Cruz mountain range, especially a day or so after a nice rain, you will have luck at this time.
We had rain in the Bay Area yesterday and today in the morning, around 10am, I located a gorgeous Rubber Boa in San Mateo County under a large, heavy wooden board. Many other snakes were found throughout the mountain range today as well.

As stated by Richard Hoyer, the spreading of artificial cover, such as tin, wooden boards, tarp, etc will greatly improve your chances of locating a boa. Not only does it add more habitat cover for the boas, but it will also give you more items to flip, adding to a higher percentage of seeing something.