The strongest storm to hit Southwest California in several years is expected to bring torrential rain, flash flooding and powerful winds on Friday, forecasters say. The storm is expected to dump up to 6 inches of rain on Los Angeles County beaches and valleys and 5 to 10 inches on south-facing foothills and coastal mountain slopes, according to the National Weather Service.  A flash flood watch has been issued for Santa Barbara, Ventura and Los Angeles counties from Friday morning through Saturday morning.