This is a rather long explanation of what is coming, what is likely, and what is possible, although improbable. From NOAA/NWS:
Early morning satellite imagery shows two features of interest over the eastern Pacific approaching the California coast. The first is a comma-shaped cloud mass associated with an upper low currently centered several hundred miles west of San Francisco near 35N/145W. The second is an elongated area of cloudiness to the south of the upper low associated with a plume of subtropical moisture that is taking aim at south-central California. Models have been consistent in keeping these two features largely independent of one another as they approach the coast over the next 36 hours. What this means is that the subtropical moisture plume, and its heavy rain potential, will likely remain mostly to the south of our area. Models generally agree that rainfall associated with this plume will spread onshore across southern Monterey County by around midday Tuesday, but then mostly remain to the south of our area through Wednesday, generating the most intense rainfall across San Luis Obispo and Santa Barbara Counties. Meanwhile, the upper low will track towards the northern California coast and spread widespread rainfall across our area from late Tuesday into Wednesday. The upper low will entrain enough moisture to generate moderate rainfall amounts across our area, and brief locally heavy rain is possible, especially late Tuesday night and Wednesday morning. Rainfall totals from Tuesday through Wednesday are currently forecast to range from a half inch to an inch in most urban/coastal/valley locations, and from 1 to 2 inches in the hills, except up to 3 inches in southwest Monterey County. Rainfall totals of this magnitude combined with saturated soil could result in minor nuisance flooding in low lying areas, but river flooding is not anticipated. Will need to be wary, however, of any indication that the upper low is entraining more significant moisture from the moisture plume to its south. If such a development were to occur, rainfall totals could be significantly higher across our area, and more widespread and serious flooding issues could develop. But given the how consistent the models have been as of late, this more dire scenario seems unlikely.
From John Lindsey, SLO: “This morning’s European (#ECMWF) model run is indicating between 2 and 4 inches of rain starting Tuesday afternoon and continuing through Thursday afternoon.”