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Midwest Weekly Highlights - December 15-21, 2015

Well-Above-Normal Temperatures Continue

The warm start to December continued into the second-half of the month, as temperatures were above normal across the entire region (Figure 1).  Average temperatures in Wisconsin, Iowa, Illinois, southeast Minnesota and northern Missouri were mainly 8-12°F above normal.  Warm temperatures in the 6-9°F range above normal were also seen in Indiana, Ohio, Missouri, and Michigan.  Kentucky was mainly 3-6°F above normal.  Minimum temperatures were even warmer in many areas, with parts of northern Wisconsin and Minnesota 10-13°F above normal (Figure 2).  Closer to normal maximum temperatures in northern Minnesota caused average temperatures to only be 4-7°F above normal, however (Figure 3). While the warmth during the period paled in comparison to the previous week, December average temperatures through the 21st remain 8°F or more above normal for almost the entire region (Figure 4).

Wet Great Lakes, Dry Ohio Valley

Above-normal precipitation was common across the Great Lakes while drier conditions prevailed in the Ohio and mid-Mississippi River Valleys (Figure 5).  Precipitation amounts of less than a half-inch were common across Missouri, Iowa, Illinois, Indiana, Ohio and western Kentucky (Figure 6).  Most of Minnesota also had less than a half-inch of precipitation, but this was above normal for most of the state.  Precipitation values in the 1-2 inch range were common across northern Wisconsin and the U.P. of Michigan as these areas received more than three times the normal amount of precipitation (Figure 7).  Portions of this precipitation fell as snow across the U.P. of Michigan and northern Minnesota.  Most of these areas received 4-6 inches of snow over the period, with isolated amounts up to 10 inches (Figure 8).

Mild Start to Winter According to the AWSSI

With well-above-normal temperatures and below-normal snowfall across most of the Midwest, winter conditions have been quite mild.  The Accumulated Winter Season Severity Index (AWSSI), which quantifies the severity of winter, rated all but two of the more than 30 Midwest stations available as mild, the least severe category, as of December 20 (Figure 9).  Accumulated AWSSI values through this time of year were near record minimums in some areas, with stations in Michigan and Missouri setting new record mild accumulations. Mild and record mild values of AWSSI were not just isolated to the Midwest, as almost the entire eastern half of the U.S. has been mild.  Read more about how the AWSSI accumulates, for over 100 cities nationwide, on our website.