Midwest Monthly Highlights - September 2007

Warm and Dry - Cool and Wet

The month of September had temperatures ranging from near normal to well above normal across the Midwest. The monthly departure map illustrates that the cooler areas were collocated with regions that also received heavier precipitation. Southwestern Iowa was the only large scale area to see temperatures closest to normal. Isolated areas in northern Minnesota also saw temperatures very close to normal (Figure 1). Areas in the eastern part of the Midwest were warmer than normal. Missouri and Wisconsin ranged from 1°F to 3.5°F while Illinois and Michigan generally dealt with temperatures ranging from 2°F to 4°F above normal. Indiana, Missouri, Ohio and Kentucky were the warmest as the average temperatures jumped to over 5°F above normal for this time of year.

Precipitation was quite variable during September. Rain fell where it was cooler and was sparse where it was warmer (Figure 2). Minnesota, Missouri and the Upper Peninsula of Michigan were the only areas to receive an estimated 10" of rain or more. Many locations in Minnesota were close to setting records in September as rainfall totals were over 200% of the typical monthly values (Figure 3). The arrowhead of Minnesota was suffering from Extreme Drought (D3) conditions, and the near continuous rainfall helped to nearly wipe out the excessive drought conditions. Overall the drought seems to be improving throughout the northern half of the Midwest. All of the states have seen improvement, nearly eliminating the Extreme Drought conditions (Figure 4). Conditions across the southern part of the region have deteriorated some, especially in Kentucky where over half of the state has seen less than 50% of the rain that typically falls during the month. A glimmer of hope appeared during the third week for this area as a large swath of Extreme Drought (D3) in the central portion of the state had improved one category to Severe Drought (D2) classification with the rains that week. Unfortunately, drier conditions similar to the early part of the month occurred during following week, and Extreme conditions overtook more than 88% of the state (A 17% increase in D3 Drought area from the previous week). Drought conditions are also creeping slowly northward into central Illinois, Indiana and Ohio after a slight retreat during the third week.
 

One After Another

September felt like a broken record with the same atmospheric setup occurring over and over. Low pressure would develop over the Rockies and warm air would surge through the Plains warming the area considerably. That warmth would be followed by a strong cold front and occasionally a weak backdoor cold front behind that which significantly cooled temperatures in the area. Temperatures on September 15 dropped to record levels after a strong front passed through the area and high pressure settled in (Figure 5). Temperatures dropped to freezing levels in Minnesota, Michigan, Iowa, Wisconsin and Illinois. The first fall freeze typically ranges from mid September in Northern Minnesota and Michigan to October further south.

While temperatures seemed relatively warm for the majority of the month, no records were set. Many locations will see the month of September go down in the record books as one of the top 20 (or top 10 in a few locations) warmest Septembers.

SSS