Storms are not common in San Francisco, but they do happen occasionally. We usually get at least one or two good thunderstorms with high wind every winter.

There was a big one in February that eroded away a huge chunk of beach: http://www.sfgate.com/bayarea/article/Old-ruins-emerging-on-...

It seems that the local CBS has stories about storm damage in the bay area http://sanfrancisco.cbslocal.com/tag/storm-damage/