The month has been the wettest December since 1889, the Los Angeles Times reported Wednesday. Normally, January and February are the wettest months in the region.
Bob Spencer, a spokesman for the Los Angeles County Department of Public Works, said the danger of landslides can persist for a long time after the rain stops. He said much of the area is completely saturated.
"It gets to the point where the water that's falling is no longer even going into the ground -- it's just skipping off the ground," he said. "A lot of residents are under the false impression that once the sun comes out, everything is fine. That's not the case. The soil beneath the surface can take months to completely dry out."
Douglas Morton of the U.S. Geological Survey said the greatest danger is in areas that have recently been burned over by wildfires. On steep hillsides with no vegetation, almost any rain can cause a slide, while on brush-covered hillsides it takes about 10 inches.