Midwest Overview - December, 2005

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A Cold, Snowy Start, A Mild End

The first three weeks of December were cold and snowy throughout most of the nine-state Midwest region, with only Kentucky missing out on much of the snow.  Temperatures the first three weeks of the month ranged from 6°F to 12°F below normal from central Minnesota and Wisconsin on south through the region (Figure 1).  Only the far northwestern corner of Minnesota experienced near normal temperatures.  A number of locations in the central Midwest set record low maximum and low minimum temperatures the first week of the month.  The upper level wind pattern shifted to a more west-to-east (zonal) flow the last 10 days of the month, shutting off the flow of cold air and replacing it with milder air of Pacific origin.  As a result, temperatures the last ten days of the month ranged from 2°F above normal in southern Kentucky to as much as 20°F above normal in northwestern Minnesota (Figure 2).  The warm period was not enough to totally erase the effects of the first three weeks of the month, and December temperatures ranged from 2°F to 4°F below normal across the southern two-thirds of the region to 7°F above normal in northwestern Minnesota (Figure 3).

Snowfall was abundant during December (Figure 4), with significant snowfalls occurring as the month began.  By December 9 a series of storms had laid a blanket of snow across the Midwest, and snow cover of at least an inch extended as far south as the Ohio River (Figure 5).  Only a small portion of southwest Missouri and western Kentucky received below normal snowfall.  However, by Christmas the snow cover had retreated to a line from central Iowa to northern Illinois and northern Indiana. Despite the abundance of snow across the region, precipitation was generally well below normal except across the northern Midwest (Figure 6).  Much of the snow that fell in the early part of the month came during very cold conditions which favored snow to water ratios on the order of 15 to 1 or higher.  At a ratio of 15 to 1, 6 inches of snow only results in .0.40 inches of water.  In the northwestern Midwest, frequent snows did produce above normal precipitation.  Duluth, MN received 34.5 inches of snow during December, the sixth highest total on record (the record is 44.3 inches set in 1950). Minnesota is the only state in the nine-state Midwest region that is not affected by the on-going drought.   The driest area during December was the southern quarter of Missouri, which received less than 25 percent of the normal monthly precipitation.  Rainfall in southwestern Missouri was below normal for much of the year, and the drought is affecting streamflows in that part of the state. The U.S. Geological Survey reports that many streams were flowing at a rate below the 10 percentile (flows have been lower only 10 percent of the time).  Similar streamflow conditions also exist in northwestern Illinois, which has been experiencing Extreme Drought (as depicted on the U.S. Drought Monitor) since late summer.