The bad weather is expected to continue into the weekend. Areas that were burned in recent wildfires are particularly vulnerable.
"Another fast moving cold front will bring the risk of widespread moderate to heavy rainfall across southwest California today," the National Weather Service said Wednesday.
"There is also the potential for a second band of heavy rainfall tonight across the burn areas of Ventura and Los Angeles counties."
Already, preliminary numbers show that this year's El Niño weather event -- characterized by warming waters in the eastern Pacific Ocean -- is tied with the strongest ever recorded in 1997-1998.
A strong El Niño heats up the atmosphere and changes circulation patterns around the globe, especially the jet stream over the Pacific, which becomes stronger and dumps more frequent and intense storms over the western U.S., especially California.
It might not be all bad news for the Golden State.
California is in a major drought and could use the rain. The mountains could also benefit from a bigger snowpack.
But the fear is that storm after storm will batter the region, flooding it with excessive moisture.
CNN affiliate KABC reported that the storm had caused severe flooding on the 5 Freeway in Sun Valley. Multiple lanes were closed and images from the scene showed some cars stuck in standing water.
The affiliate also reported that the ground collapsed beneath a home in Pasadena because of the heavy rains.
El Niños occur every two to seven years in varying intensity, and the waters of the eastern Pacific can be up to 4 degrees Celsius (7 degrees Fahrenheit) warmer than usual.
The 1997-1998 El Niño caused havoc with weather worldwide, creating conditions that killed an estimated 23,000 people and caused as much as $45 billion in damage.