It not only rains but occasionally floods. During some rains, mudslides may and do occur, in part because of forest fires in those areas, but also in other areas because of unstable land.
The rain in southern California has - in the 40 plus years I lived there - been in relatively low amounts if you accumulate the numbers over a year's time, but in my experience, some of those rains lasted for days. I remember one four day set of storms.
As many know, California as a whole has been going through a serious drought in recent years.