In addition to flooding in Central and Northern California:

They also expect 2 to 5 inches of rain from around Santa Barbara to San Diego and into Arizona likely to within the next 24 hours. So, being out tonight might be a problem on the roads of North, South and Central California.

The main problem in California comes from a lot of rainfall in mountains and hills that then concentrate into Valleys and Rivers at times like this. 

So, presently I would avoid traveling between Paso Robles and Salinas on Highway 101 (at least for tonight).

So, it's quite likely that the Salinas River could flood anywhere from Paso Robles north to the ocean as well, especially tonight or tomorrow when the rains might subside. Too much rain has fallen in too little time. This is the problem.