From NOAA yesterday:

Confidence is increasing for a more notable rain system to impact
the region starting on Friday into the early part of next week.
Ensemble models guidance shows an upper level trough in the Pacific
approaching the West Coast and extending into the western United
States. This in turn will bring moisture from the tropics west of
Hawaii through California. At this time, the ensemble model guidance
is suggesting a low end atmospheric river. However, the guidance
also shows that the moisture tap is quite dynamic, instead of being
fixed to one region, the end of the plume touches the central
California coast, and quickly moves south towards Baja California
before another plume comes up behind it. At this time, the forecast
calls for a couple periods of moderate to heavy rain from Friday
into the weekend, with lingering rain through Tuesday of next week.
Rainfall totals from Friday into Monday look to be highly
significant, with early estimates suggesting 1.5-3" in the Bay Area
and Central Coast valleys, 3-5" in the North Bay valleys, and 4-6"
in the coastal ranges with localized amounts up to 8". Details
should be refined through the week. In the meantime, it is a good
idea to plan for wet conditions through the weekend, and stay tuned
to forecast updates.