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Feb 18, 2012
The warm Wisconsin winter of 2012 continues to be above normal in terms of temperature. Wisconsin has been under the influence a La Nina, which usually favors colder temperatures. However, there are more weather variables that cause warm and cold spells over a geographic region.
The 2010 - 2011 winter brought major snowstorms and cold temperatures for much of the United States thanks to a La Nina pattern that was in place. During the spring of 2011, La Nina weakened but it strengthened during the fall months. Knowing this, many meteorologists expected this winter to be cold and potentially face repeated bouts of heavy snow for most of the Midwest. AccuWeather even made a bold, confident prediction that Chicago would have a very harsh winter, but a warm winter has been persistent since the start.
What has caused the warm weather? The key player so far this winter has been the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). Both indices have been positive for the most of the 2011 - 2012 winter. When these values are positive, the brutal cold air is parked close to the north pole. When the indicies become negative, this signals a shift in the jet stream that enchances the cold air to invade most of North America, including Wisconsin.
 
The AO was negative for the first half of February (shown above, left) that brought colder temperatures to Wisconsin. Both the AO and NAO are now back in the neutral to positive territory that will help keep temperatures above normal for most of Wisconsin for the next week. If these values remain positive, the warm Wisconsin winter will persist and keep temperatures above above normal.
Make sure to browse the news section for additional U.S. and Wisconsin weather news information.
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