The spider in your photo is a type of orb-weaver spider in the family Araneidae. Your photo is a ventral view of the spider, and several species have these same or very similar ventral markings. I would need to see an image of the top side of the spider to be absolutely sure of its identification. However, I believe this is most likely the arboreal orbweaver or spotted orbweaver, Neoscona crucifera, which is a mainly eastern species that has recently extended its range into southern California. It is supposedly common in yards and suburban areas, frequently building its large webs on fences, patios, buildings or near lights. It hides in a secluded place during the day and usually only comes out onto its large orbweb at night. It can be quite variable in color and dorsal patterns. Like most of these types of spiders, they are generally considered to be harmless. I have included some links below to webpages with more information and photos of this spider. Perhaps you can use these to compare to your spider if it is still there. If it doesn't look the same, then it might be another species and I would need to see a good topside photo to be able to identify it further.

http://bugguide.net/node/view/28154

http://staffhttp://www.fullcoll.edu/lvincent/spiderguide//xtra/D1X.html

http://www.spiders.us/species/neoscona-crucifera/