It's true that California has been unusually rainy so far for a La Nina winter. Most La Nina winters have been dry in southern California, as Mike's latest Winter Outlook post clearly shows, but there have been exceptions, and perhaps this will be one of those exceptions (but we still have most of winter yet to come!). This is another case that clearly shows why seasonal forecasts are issued as probabilities - even if one particular outcome is favored (like a dry southern California), the influence of chaotic weather variability always has a chance to tip the scale in another direction. We certainly will look more closely at precipitation in California at the end of winter. 

I also want to echo Michelle's response above, which notes that the La Nina dry signal in southern California is stronger in late winter than in early winter, so we will see if the second half is notably drier than the first half.