There are important changes going on in the tropical Pacific Ocean in terms of sea surface temperatures that will likely have ramifications on the overall US and tropical Atlantic weather for the next several months. For almost two years now, sea surface temperatures in the tropical Pacific have been below normal (La Nina). It now appears, however, that an El Nino is developing with above normal sea surface temperatures off of the west coast of South America and that transformation could very well last right into the 2012-2013 winter months. In terms of possible effects on the weather in the US and tropical Atlantic this summer, the developing El Nino could do the following based on analog years:
1) diminish the chances for widespread and long-lasting heat 2) diminish the threat for drought in certain places like the southeastern part of the country and 3) diminish tropical Atlantic storm development as there is a tendency for greater vertical wind shear in the Atlantic which inhibits storm formation.
Looking way ahead, the newly emerging El Nino could have an impact on the upcoming winter weather here in the US, but much of that will depend on the strength that it ultimately reaches. In general, strong El Nino events feature less snow in the eastern US whereas weak-to-moderate events have been known to produce heavier wintertime snowfall amounts. My guess at this point is that this El Nino will remain in the weak-to-moderate range as winter approaches, but we’ll continue to monitor that over the next several months. El Ninos tend to be weaker and less frequent during periods of "negative" Pacific Decadal Oscillations (PDOs) as were are currently experiencing in the northern Pacific. One thing is pretty certain at this point in time, the overall pattern during the upcoming winter will very likely be quite different in the tropical Pacific compared to last winter when there was a moderately strong La Nina.