My colleague at AccuWeather, Ken Clark, has a good article outlining preparations Californians should be thinking about in advance of the wave of storms that will begin this weekend in Northern California, will spread south as next week progresses, and continue through at least the following weekend. Very heavy rains are likely with significant flooding a possibility.
I wish to add one caution of my own: It would not surprise me if one or more tornadoes occurred in Southern California with this wave of storms. The Los Angeles Times has a photo of a Southern California tornado here. Movie of two Southern California tornadoes here, the best footage starts around six minutes in. Finally, here is a brief summary of a major tornado in downtown Los Angeles during the 1983 El Nino,
On 3/1/1983, a category F2 (max. wind speeds 113-157 mph) tornado 3.1 miles away from the Los Angeles city center injured 30 people and caused between $5,000,000 and $10,000,000 in damages. The tornado did significant damage to the Los Angeles Convention Center which required $5,000,000 in repairs (1983 dollars).