I am sure that many forecasters and research scientists will do a more comprehensive post-mortem once winter is over, especially if these wet conditions continue, but I will just note a couple of points in two of the responses above. 

First, as Michelle noted above, the La Nina signal for a dry southern California is stronger in late winter than in fall and early winter. From Emily's earlier post, the October-December La Nina precipitation composites do not show much of a dry signal over southern California, but the January-March average shows much drier conditions. So, we will be looking to see if drier conditions take hold in the second half of winter. 

I also note that although most La Nina winters have been dry in southern California, Mike's latest Winter Outlook post clearly shows that there have been exceptions, and perhaps this will be one of those exceptions. 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.

Perhaps this is not a fully satisfying answer, but we need both a full season for evaluation and time to perform a more thorough attribution.