The National Weather Service flash flood watch for the valleys, mountains and deserts is set to expire at 5 p.m. Forecasters said rainfall in excess of an inch per hour would be possible and could lead to dangerous flash flooding.
"A trough of low pressure and associated deep layer moisture will bring the potential for thunderstorms with heavy rain and flash flooding through this afternoon,'' according to the weather service. "The greatest potential will be over the mountains and deserts this afternoon, with smaller chances in the San Diego County valleys and the Inland Empire.''
Although the coast was not included under the watch, forecasters predicted a chance of showers and thunderstorms Tuesday morning and a slight chance for rain this afternoon.
Locales in and below recently burned areas will be particularly susceptible to heavy runoff and debris flows should strong thunderstorms develop nearby. Forecasters advised residents with outdoor plans or who live near a flood-prone area to "be alert to rapidly changing weather conditions.''
The risk of flooding is expected to subside this evening into Wednesday morning. Forecasters said drier conditions would prevail later this week as high pressure rebuilds over the southwest.