Southern California experienced a drastic drop in temperatures on Monday, with wet weather expected through the week, while a powerful winter storm hit Northern California started late Monday and brought several inches of rain and heavy winds.The National Weather Service issued a series of updated warnings targeting communities in the area, including a flood watch for cities like San Francisco, Watsonville, Pacifica, Santa Cruz, Scotts Valley and Boulder Creek.By Tuesday morning, experts forecasted that significant runoff from surrounding mountain ranges could bring flooding to rivers, creeks, streams and low-lying areas, the weather service said in an advisory, adding that flooding could occur in urban areas as well as areas with poor drainage systems."Localized flooding will likely occur, particularly in urban areas where ponding on roadways or poor drainage is a common issue," weather service forecasters said.Meanwhile, Northwest Oregon and Southwest Washington face a storm system that is expected to bring heavy rain and strong winds starting Monday evening that could lead to minor flooding along rivers and creeks, according to NWS Portland."