El Nino in Southern California

El Nino happens when an increase in the ocean surface temperatures in both the central pacific ocean and eastern ocean. The warm events happen randomly but usually ranging from 6 months to 7 from the last one. Even though it is only present in the tropical Pacific it can affect different parts of the world as well with unusual precipitation patterns.

This is what "normal" condition's for weather and temperature without El Nino

This is the difference with El Nino.

El Nino is the extreme warm phase. 

Because of these changes Southern California receives much more rainfall than usual during these times. In general, the effect of El Niño on California is increased rainfall with accompanying floods, landslides, and coastal erosion. The effects are variable across the state and are more predictable in Southern California. This can be detrimental and dangerous for the state causing things from houses being damaged to people dying. 

This is typically the expectation of rain for a stronger El Nino

And the picture below would be expectations for a weaker El Nino

Basically, the warmer the currents, the more moisture in the air adding to much more rainfall than usual. This is something Southern California is used to dealing with but still is preventable. Little things can be done for rainfall proactive planning but other than that when the rain comes the rain comes!

http://ggweather.com/enso/calenso.htm

http://www.ucmp.berkeley.edu/education/dynamic/session4/sess4_hydroatmo3.htm