A National Weather Service heat advisory for the coast and the valleys will be in effect from 9 a.m. to 6 p.m. Tuesday.
High temperatures this afternoon are expected to range from 88 to 93 degrees along the coast, 93 to 98 inland, around 96 in the western valleys, 86 to 91 near the foothills, 76 to 84 in the mountains and 89 to 94 in the deserts.
Forecasters said there would be a slight chance of morning sprinkles followed by possible showers and thunderstorms this afternoon and evening in most areas other than the immediate coast and slightly inland.
"Upper level lower pressure over northern Baja will drift northward over Southern California, bringing clouds, higher humidity, cooling and a chance for scattered showers and isolated thunderstorms, especially over the mountains," according to the weather service.
Forecasters warned that overexposure to the heat could lead to health problems, especially in children and the elderly. Pets are also at risk for heat-related illnesses.
Residents in the hot spots were advised to stay well-hydrated, avoid working in the sun, use air conditioning and check on neighbors and relatives.
Slow cooling is expected through Thursday in San Diego County with continued chances of showers and thunderstorms. A low pressure system moving inland Friday through Monday is expected to send temperatures down to slightly below seasonal averages, according to the NWS.