Record Warmth Continues

Above normal temperatures continued through the third week of December. Widespread average temperatures ranged from 6-11°F above normal throughout much of the Midwest (Figure 1). Lee County, located in east central Kentucky, recorded an average temperature of 47.5°F, which was 10.8°F above normal for the week. Bureau County, located in northwestern Illinois, averaged 37.4°F, which was also 10.8°F above normal. Near normal average temperatures were observed in northwestern Minnesota. The largest deviations occurred in maximum temperatures where several locations within the Midwest recorded temperatures in excess of 10°F above normal (Figure 2). There were 546 high maximum temperature and 234 high minimum temperature records broken throughout the week (Figure 3). 152 of the high maximum temperature and 109 of the minimum temperature records were broken on December 15th alone (Figure 4).

Isolated Occurrences of Above Normal Precipitation

Precipitation departures varied from near normal to as much as 5” above normal for the week (Figure 5). In Minnesota, precipitation was up to 2” above normal. Southwestern Missouri recorded 2-5" above normal precipitation. Douglas County, located in south central Missouri, measured 5.32” of precipitation, which was 4.55” above normal. There were 93 daily precipitation records broken with the most concentrated in Minnesota, Missouri, southern Illinois, and Indiana (Figure 6).

Snowfall in Northern Midwest, leading to 5 Daily Snowfall Records

Much of Minnesota and Michigan’s Upper Peninsula recorded 2-7.5” of snowfall (Figure 7). Central Minnesota received 100-300% of average snowfall for the week (Figure 8). Cook County, Minnesota, received 9.5” of snow, which was 6.3” above normal. Isolated locations in Wisconsin and Michigan received over 100% of normal snowfall. Above normal temperatures contributed to a lack of snowfall through the rest of the Midwest. There were 5 daily snowfall records broken or tied this week (Figure 9).

Severe Storms on December 15

Another destructive weather day occurred on December 15 where several tornadoes and a derecho were reported through Iowa, Missouri, Minnesota, Wisconsin and northwestern Illinois (Figure 10). This was the first time in Storm Prediction Center history (dating back to 1990) that a moderate risk of severe weather was issued in the Iowa/Minnesota/Wisconsin region during the month of December. This was also the first known derecho in December anywhere in the United States.

The atmosphere was primed for severe weather. Average dew point temperatures were in excess of 50°F and many locations recorded maximum dew point temperatures in excess of 60°F. Creston Municipal Airport, located in southern Iowa, reported a maximum dew point temperature of 68°F, which is unprecedented for December (Figure 11). There was widespread damage from severe wind gusts in excess of 70 mph and multiple reports in excess of 80 mph. As of December 23rd, the confirmed tornado count in Iowa reached 63, which beat the previous record that occurred on August 3, 2014. Seventeen tornadoes were rated EF2. Prior to the December 15th outbreak, there had only been five tornadoes reported since 1950 in Iowa during December. As of December 29th, the NWS confirmed 20 tornadoes in Minnesota, marking the first December tornadoes on record for the state. As of December 27th, eight tornadoes had been confirmed by the NWS in Wisconsin. One fatality was reported in Minnesota after a falling tree struck a man in Rochester. One fatality was reported in eastern Iowa after high winds caused a semi-truck to roll. Outside of storm winds, the central U.S. received extreme winds in excess of 45 mph (Figure 12).

Drought Continues, but some Improvement

There were not many changes to the U.S. Drought Monitor this week (Figure 13). Abnormally dry conditions continued in Missouri, with some improvement in the southern part of the state. Additional improvement was noted in northern Minnesota, which can be attributed to the observed snowfall this week. D1-D4 (Moderate, Severe, Extreme, and Exceptional Drought) was reported for 17.15% of the Midwest, which was slightly better compared to the previous week (17.97%).