The Midwestern region of the United States experienced its second coldest December in the 106 year record of observations. The December 2000 average temperature was 14.3°F, just missing the 1983 record of 13.9°F. A number of first-order stations broke their all-time cold records for December, including South Bend, IN; Chicago-Midway, Rockford, and Moline, IL; and Louisville and Paducah, KY. The center of unusually cold conditions was in Iowa, Missouri, and Illinois, where temperatures averaged 12-15°F below normal over most of the three states (Figure 1). Even in relatively warmer locations such as northern Michigan, temperatures were still more than 5°F below normal. The temperature departure pattern in December 2000 was similar to that in December 1983, in the sense that the largest negative anomalies were in the western half of the region (Figure 2). The causes were similar, too. Both cold months occurred during a somewhat neutral to slightly La Niña oriented season, with central equatorial sea surface temperatures slightly cooler than normal. The resulting upper air patterns indicate the preference for a strong trough to develop over the central and eastern United States in both seasons. During December 2000, a strong ridge dominated the western coast of North America, helping to accentuate the north-south delivery of very cold air in the Midwest trough (Figure 3). December 1983 was similar; while the amplitude of the western upper air ridge was less than in 2000, the eastern trough was deeper (Figure 4).
While the amount of liquid water in the precipitation that fell during December 2000 is only slightly above normal overall in the Midwest, some more active regions can be seen in Iowa and the northern tier states (Figure 5). It is unusual for a cold month during winter to also be a month with normal or above normal precipitation. The remarkable aspect of the precipitation in December, though, was that almost all of it was delivered in the form of snow, leading to a widespread deep snow pack that helped to maintain cold temperatures. The snow totals reported in real time in December 2000 vary widely depending on the reliability of reporting, but can be seen to include many very large values, especially in the north-central latitudes of the Midwest (Figure 6). A subset of stations with good climate records and available real-time data show that the snow fall totals were typically more than 10 inches above normal in most of the Midwest (Figure 7). Snow fall exceeded 300% of normal in the central and southern Midwest (Figure 8). While there were several major snowstorms that traversed the region during December, the sheer number of smaller "clipper" systems originating in the northern Rocky Mountains contributed greatly to the overall snow fall totals. At least 15 first-order stations broke all-time records for December snowfall, and five of these set their all-time record snowfall for any month of the year (Table 1). Most of these stations are located in a belt from central Iowa and southern Minnesota eastward through the Great Lakes region (Figure 9). As might be expected, December 1983 was similarly snowy, with the axis of heaviest snow perhaps shifted somewhat to the northern part of the Midwest (Figure 10).
Overall, conditions during December 2000 were quite extraordinary in the Midwest. Illinois experienced its single coldest December in 106 years, while Iowa experienced its largest December snow fall state-wide (Harry Hillaker, Iowa State Climatologist). The December 2000 temperature rankings for each of the nine states in the Midwestern region are available in Table 2, and the temperature and snow fall rankings for a list of major Midwestern cities is given in Table 3. The Climate Prediction Center in Washington, D.C., has indicated an appreciable likelihood for cold weather to continue through the upcoming winter months, especially in the northern Great Lakes region. However, it should be noted that following the record cold December 1983, January-February 1984 was the 25th warmest on record for the Midwest.