The third and most powerful in a series of storms pounded Southern California on Sunday, dropping nearly 4 inches of rain south of Los Angeles, flooding freeways and raising concerns about damaging mudslides. Commuters could expect a messy drive to work Monday in several areas, with rainfall expected to ease slightly but not taper off until Tuesday. Flash flood watches and warnings were in effect for swaths of greater Los Angeles and across Southern California where multiple roads were closed Sunday or blocked by fallen trees.