The National Weather Service forecasts showers in the afternoon and in the evening, with San Bernardino, Fontana, Ontario and Corona possibly seeing street flooding.
As much as 1-2 inches of rain will hit the region, as burn areas are on alert for the chance of mud and debris flow.
Earlier in the day, light to moderate moved through portions of Riverside and San Bernardino counties.
The rain is one in a series of storms expected to continue into Thursday.