I had thirty years of records for Oceanside, California where I lived and averaged 12.57." Neutral seasons in that period averaged the driest. From 1980-1999 the wettest La Nina I had was 8.18". Ironically, I was reading about how dry La Ninas were in southern California in 2000. The next three La Ninas produced above normal rainfall culminating in the 2010-2011 season that produced 22 inches! Combined they produced over 50 inches in a three year period. No matter how much knowledge one has, it will be difficult to predict. There will likely be some new fly in the ointment of predicting that occurs that no one saw before.