From Daniel Swain at the California Weather Blog:
The prospect of an El Niño event in the Pacific Ocean always generates quite a bit of interest in California. This attention largely stems from the fact that two of California’s wettest years on record—1982-1983 and 1997-1998—occurred during the strongest El Niño years in living memory. The popular perception that El Niño always brings a lot of water to the Golden State, though, is not particularly accurate.
The reality is a bit more nuanced: particularly strong El Niño events exert a powerful influence upon the atmosphere over the northeastern Pacific Ocean, and really do have a tendency to enhance the storm track in a way that favors greatly enhanced precipitation across the entire state of California. But more middling weak to moderate events don’t have nearly as pronounced an effect, and in many cases don’t meaningfully affect the odds of seeing wetter or drier than average conditions in California.
The main reason for this nonlinear effect is that other periodic oceanic and atmosphere oscillations (other than El Niño) still play a major role in California’s winter weather, and unless El Niño is powerful enough to consistently outweigh all of them, the net effect can swing either way. The key message here: strong El Niño events are the ones to watch out for from a California weather perspective, and it’s reasonable to expect that such events greatly increase the odds of wet conditions throughout the state.
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