Satellite images showed the first of two back-to-back storm systems approaching the US west coast on Wednesday, 31 January.
Two atmospheric river storms are forecast to bring several inches of rain in some areas of California by this weekend.
Excessive rainfall is predicted from southern Oregon to central California on Wednesday, the National Weather Service (NWS) reported.
Higher elevations should experience heavy snow, and Los Angeles is expected to receive up to four inches of rain by Thursday.
Scattered flash flooding and landslides, due to oversaturated soils, was possible, officials warned.