"El Niño, California Precipitation, and Landslides" <https://www.conservation.ca.gov/cgs/landslides/el-nino>
El Niño is the “warm phase” of the El Niño-Southern Oscillation and features warmer than average ocean surface temperatures off the coast of Ecuador and Peru in the equatorial eastern Pacific Ocean.... Historically, the presence of El Niño conditions tilts the odds in favor of wetter than normal precipitation across the southern third of the state. The relationship between El Niño and cool season precipitation is weaker in the central and northern parts of the state.
Flooding is possible, particularly if dams exceed capacity, perhaps exacerbated by earlier snowmelt and peak runoff. Landslides are a more widespread, and less manageable problem, particularly as loose-material slopes are waterlogged. The deluge -> high growth -> heavy fuel load -> large wildfire -> landslide cycle is one that's become fairly well-known in California.
Levee failures within the Sacramento Delta or Central Valley could result in very large-scale flooding, affecting agriculture or communities.
The San Francisco Bay Area may see "king tides" and general flooding throughout the bay and delta region, particularly during December and January, and all the more so if high tides coincide with heavy river run-off. Remember that the entire Central Valley of California, nearly 500 miles long north-to-south, drains out through the Bay. Prevailing winds may create a wind setup or seiche which can result in flooding along a some but not all shorelines, or alternating across a body of water.
The aquifers are all extremely depleted and the reservoirs nearly empty. Extra rain will be very good. If private interests are allowed to graze and log public land, fire risk can be mitigated.