San Diego County will experience a second day of heavy rain Wednesday from a cold storm out of the north that tapped into tropical moisture from the south as it surged into Southern California, according to the National Weather Service.

The entire region will get soaked, and all areas are under a flash flood watch. But the rain will be heaviest in northern and central San Diego County, especially along Interstate 15, from roughly Escondido north to the county line, forecasters say.