In the Spring a Subtropical High begins to form off the coast of CA. As the days pass this High strengthens and moves to the north. This High directs the Jet Stream to the north effectively blocking most storms from CA.
Another effect of the High is its strong circulating winds. These winds blow parallel to the Bay Area Coast line. They are strong enough to force the surface waters away from the coast. The surface waters are replaced by cold water uplifted from the deep ocean. The result is the water off the coast is very cold. Surfers wear wetsuits because they need to and not as a fashion statement.
The very cold water contrasts with what is happening on the land. The nearly cloudless days and high sun angle heat the inland areas. One author described this as desiccating.