As we work through the latter stages of summer and towards autumn, thoughts of cold and snow will gradually increase as the weather eventually (although not for a while) turns colder. One of the principal drivers of weather in winter is the tropical Pacific -- specifically the Nino/Nina pattern and which of those ultimately sets up shop across the Equatorial Pacific.
A few months ago, thoughts of El Nino by late summer were en vogue across the Pacific as modeling and forecasting suggested a weak Nino may be coaxed out of the pattern. However, as we drift into late August the tropical Pacific, specifically south of the equator and east of 120 degrees west (the dotted line that crosses north into California) we are not seeing the Nino develop. In fact, a cooling trend has taken hold over the past several weeks.
Modeling is realizing this and is slowing any warming trend in the tropical Pacific, delaying any possible onset of Nino until Spring 2014 at the earliest. The probability charts below show a slight tendency to La Nina (the bar graph) developing in the tropical Pacific over the next few months; however, this Nina may be very weak and perhaps not a classified La Nina in the technical sense.
Ninos and Ninas are but one piece of the puzzle in determining the final winter picture for North America. Arctic and North Atlantic blocking regimes, as well as the North Pacific and how powerful the Pacific jet is, will also drive the bus as well. La Nina patterns bring varied winters to us -- some years like 2010-11 produce an epic stretch of winter...the following winter was also La Nina and one of the least snowy on record. The Nino or Nina also does so much -- but it is a factor in the weather and one that we'll continue to look at over the coming weeks.