There has been much excitement in the news about the possibility of a "double dip" La Nina. A large part of that has been simply because it made it into the news wires. Actually the Climate Prediction Center (CPC) put out the La Nina Watch back on August 4th. So what's up?
Back as early as late June some of the computer models that forecast the sea surface temperatures (SST) for the tropical Pacific (this is what is used to define El Nino/La Nina) started to shift from forecasting a Neutral winter to a La Nina. Now more of these models are forecasting La Nina Conditions for the coming winter.
As of their last report, CPC said that the bulk of the models indicated neutral conditions for the fall but now about 50% of the models are forecasting a return to La Nina conditions duirng the winter months. The possibility is increased because there are still remnants of last winter's La Nina apparent in both the atmospheric and ocean circulations. This may be one of the reasons we have had such a hard time finding summer weather this year.
Also "double dip" La Ninas have been occurring more often in the past couple of decades (and even triple dip), especially following significant La Ninas (Last winter was a moderate to strong La Nina). Ordinarily the second La Nina SST anomaly isn't as strong, but it's strength doesn't necessarily affect its effect on us.
What does it mean if we do get another La Nina winter? As it always does, a La Nina tips the odds towards towards a winter with above normal rainfall and possibly cooler temperatures. Will it be a replay of last winter? Probably not as each winter is unique and La Nina is only one of many factors that affect our winter weather ( many of them are yet to be discovered - much less understood).
All that said, our last "double dip" was 2007-2008 and 2008-2009 and, as you may recall, December of 2008 was a big producer of lowland snow. Personally, I am going to glory in the perfect Northwest summer weather we have headed our way for next few days and wait until the fall to worry aobut the winter. Stay tuned.