During December of 2000, much of the nation was gripped in unseasonably cold conditions. According to data compiled by the National Climatic Data Center (NCDC), mean temperature departures from the 1961-1990 normals were as much as -9.0°F or below for much of the eastern two-thirds of the continental United States. According to data from the U.S. Historical Climate Network (USHCN) and the Climate Division Database, December 2000 ranked as the seventh coolest December since such records began in 1895. The national average temperature as computed preliminarily by NCDC was 28.9°F, 4.4°F below the long-term mean, making December 2000 the first December in eight years to fall below the long-term mean.

One year later, much of the eastern two-thirds of the nation sat comfortably in the midst of an unseasonably warm weather pattern, in which mean temperature departures from the 1961-1990 normals were as much as +9.0°F or above. Similar data from the USHCN and the Climate Division Database ranked December 2001 as the 12th warmest December of record, with a national average temperature of 36.6°F, 3.2°F above the long-term mean.

A time period consisting of the first twelve days of each month was chosen based largely on the fact that much of the contrast occurred during the first portion of each month. Maps of the means of several different variables for this time period during both months are presented to illustrate the stark circulation contrast.