SAN DIEGO (CNS) - Summer thunderstorms threatened San Diego County Friday, bringing the likelihood of scattered showers and cooler temperatures to the region.

"An upper level low will remain over Southern California today, with a very moist and unstable air mass in place," according to the National Weather Service, which issued a flash flood watch for mountain and desert areas through Friday evening.

Friday's thunderstorms will be capable of producing heavy rain, leading to the possibility of excessive runoff and rapid filling and overflowing of normally dry creeks and streams, according to an NWS advisory.