Droughts are common in California, a large, generally dry, and hydrologically complex place.  So it is hard to rely on a single index of the end or beginning of a drought.  A single storm is rarely enough to end a drought in California, especially a long drought like the one that seems to be mostly ending now.  Regular hydrologic statistics can be used as indicators of drought, but these do not do justice to how droughts actually end (or begin).