Editor's Note (11/13/12): This article was edited after original publication in the print edition to include several corrections and clarifications.
The past three winters in parts of North America and Europe were unusual. First, during the winters of 2009–2011, the eastern seaboard of the U.S. and western and northern Europe endured a series of exceptionally cold and snowy storms—including the February 2010 “snowmageddon” storm in Washington, D.C., that shut down the federal government for nearly a week. Later that year, in October, the NOAA Climate Prediction Center (CPC) forecasted a mild 2010–2011 winter for the eastern U.S., based on a La Niña pattern of cooler than usual ocean temperatures in the eastern Pacific. But even with La Niña's moderating effects, very low temperatures and record snowfalls hit New York City and Philadelphia in January 2011, catching the CPC and other forecasters by surprise.