The National Weather Service issued a flash flood watch for San Diego County, San Bernadino County Mountains, Riverside County Mountains, Apple and Lucerne Valleys and the Coachella Valley.
The watch is in effect until 8 p.m. on the evening of Aug. 21.
The southland has seen several days of flash flood watches and thunderstorm warnings over the past couple weeks due to bands of moisture moving through southern California.
The increase in moisture and instability during the past 12 hours points to moderate to high flash flood threat over the mountains and deserts from late morning through this evening, the NWS advised.
There is a slight chance that thunderstorms may continue overnight, mainly over San Bernadino County mountains and deserts. Should this threat increase, the flash flood watch will be extended beyond 8 p.m.
Excessive runoff from heavy downpours may cause rapid filling and overflowing of normally dry washes, creeks, small streams and low-lying areas.
In addition there is a threat of rock and mud slides below steep terrain in the mountains and recently burned areas.