National Weather Service meteorologists said showers and thunderstorms could break out along the coast, in the valleys and the mountains as early as this afternoon, and overnight in the deserts.
"The long awaited return of the upper low system is beginning to approach our region from the southwest early this morning,'' according to the weather service.
Temperatures throughout the county are expected to go from slightly above normal today to about average this weekend as a second storm system moves into the region. Forecasters said there would be an even greater chance for rain starting Thursday.
Today, partly cloudy conditions are expected, along with high temperatures of 79 to 84 degrees along the coast, 83 to 88 degrees inland, 85 to 90 degrees in the valleys, 80 to 86 degrees in the mountains and around 96 degrees in the deserts.
On Thursday, highs are expected to range from 75 to 80 degrees near the beaches, 76 to 81 in the valleys, 73 to 79 in the mountains and around 92 degrees in the deserts.
Forecasters said temperatures should gradually sink back into the near- normal range by the end of the week as a second low pressure system moves into the area.