11:45 AM | La Nina Update and its possible effects on our winter weather


The latest Pacific Ocean sea surface temperature (SST) anomaly readings indicate that weak La Nina conditions (colder-than-normal) currently exist in the equatorial region of the Pacific. In the 2011-2012 Winter Outlook video (under Seasonal Forecasts) I discussed the likelihood of an “east-based”, weak-to-moderate La Nina during the upcoming winter season and I continue to favor that scenario. The difference between an “east-based” and a “west based” La Nina can be quite important with respect to winter temperature patterns in the Mid-Atlantic region. In short, an “east-based” La Nina typically leads to a colder nation, including the Mid-Atlantic region, compared to a “west-based” condition which often results in much warmer-than-normal winter weather in the eastern states. The latest information show that the most substantial cooling over the past 4 weeks (relative to normal) has occurred over eastern and central portions of the tropical Pacific and that should continue for the foreseeable future which favors the “east-based” scenario. As far as the strength of the La Nina signal is concerned, NOAA maintains an oceanic index that represents the 3-month running mean of SST anomalies where negative values indicate La Nina conditions. This index has now moved from -0.2 to -0.4 in the latest reading suggesting some strengthening of La Nina has recently occurred. In fact, I expect La Nina to strengthen a little bit more over the next few months with the oceanic index moving to between -0.5 and -1.0, but this is generally regarded as still in the weak-to-moderate range. We’ll continue to monitor and report on the Pacific Ocean cycles as well as on the North Atlantic Oscillation (NAO) over the next several weeks here at “thesiweather.com” since both are critical to winter weather conditions in the Mid-Atlantic region.