Monthly values of the North Atlantic Oscillation (NAO), which compares the relative strength of semi-permanent high and low pressure patterns in the North Atlantic Ocean. When the NAO is positive, pressure patterns favor mild winters in the U.S. East. When the NAO is negative, pressure patterns favor stronger cold-air outbreaks and increased storminess in the eastern U.S.
Late winter temperatures compared to the 1981-2010 average when the North Atlantic Oscillation (NAO) was strongly negative (top, Jan-March 2010) and when it was strongly positive (bottom, January-March 1990). Winters are often cooler than average across the mid-latitudes when the NAO is negative, and warmer than average when it is positive.