A few of the large California reservoirs, like the San Luis Reservoir and Lake Berryessa, get their water from storm runoff in mountains that don't typically accumulate much snow, but a large amount of the water storage in California comes from Sierra or Trinity snowpack.
Unfortunately, a lot of California storm drainage is designed to move the water to the ocean as quickly as possible, to prevent flooding. I think most of the rainfall in the Bay Area that doesn't soak into the ground will find its way to the delta or to the Bay rather than a reservoir.
A few of the large California reservoirs, like the San Luis Reservoir and Lake Berryessa, get their water from storm runoff in mountains that don't typically accumulate much snow, but a large amount of the water storage in California comes from Sierra or Trinity snowpack.
Unfortunately, a lot of California storm drainage is designed to move the water to the ocean as quickly as possible, to prevent flooding. I think most of the rainfall in the Bay Area that doesn't soak into the ground will find its way to the delta or to the Bay rather than a reservoir.
Think of it this way: cumulative snowpack is a massive year-round reservoir in California. See e.g. http://www.sacbee.com/news/local/environment/article3505269....