A deep marine layer will keep the weather cool on Sunday, with periods of gusty mountain and desert winds through Tuesday, forecasters said.
An increase in onshore flow will bring back low clouds across the coast and valleys Sunday night, the National Weather Service said, with very light showers across the mountains and valleys.
Sunday's highs were forecast in the 60s across the county, with overnight temperatures expected in the low 50s.
Starting Wednesday, a large area of high pressure to the southwest will move into Southern California, forecasters said. Onshore flow will slowly ease, giving way to warmer, drier and less windy weather.
As the ridge amplifies later in the week, the county will see some areas 10-15 degrees above average by Friday when temperatures really start to warm up.
"This will be the first prolonged heat event of the season," the weather service said. "Be prepared for the upper 80s to low 90s across the inland valleys and near 100 degrees for the low deserts as we get into next weekend. This warm and dry pattern looks to stick around through at least early next week.''