Midwest Overview - September 2012

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Precipitation Gradient

Precipitation totals across the Midwest ranged from less than 1" in most of Minnesota to more than 14" in southern Illinois (Figure 1). Viewed as a percentage of normal for September, the monthly values ranged from less than 10% of normal in Minnesota to more than 300% of normal in southern Illinois (Figure 2). The northern half of the region had below average totals except for part of Upper Michigan while the southern half of the region saw above normal totals except in a small pocket of southwest Missouri. The pattern of rains in the southern Midwest started with the remnants of hurricane Isaac moving through the area in the first few days of the month (Figure 3) but the pattern continued for the rest of the month as well. Daily precipitation records were far more common in the southern states (Figure 4) but were spread throughout the month when viewed by date (Figure 5). Statewide precipitation totals for various numbers of months range from among the driest in many cases to among the wettest in a few cases (Table 1).

Below Normal Temperatures

The string of months with above normal temperatures came to an end in September with a regional temperature 0.6° below normal (Figure 6). Temperatures for the Midwest as a whole had been above average for 11 straight months, October 2011 through August 2012. It was the first month since January 2011 that none of the nine Midwest states were above normal (Figure 7). Daily temperature records were mostly record highs early in the month but switched to mostly record lows by the end of the month (Figure 8). Temperatures ranged from about 3°F below normal near Lake Superior to about 1°F above normal in southwest Missouri (Figure 9). Despite the cooler September, 2012 remains the warmest or second warmest on record in all states (Table 2).

Early Freeze in Iowa, Minnesota, and Wisconsin

The freeze event around the 23rd and 24th (Figure 10) brought freezing temperatures to Iowa, Minnesota, and Wisconsin earlier than normal. In many cases, the freeze occurred two weeks ahead of schedule. In Iowa, many locations recorded their earliest freeze since 1983. Early crop maturity and harvest progress limited the impacts of the early freeze.
 

Big Drought Changes

September rains led to improvements in the southern Midwest according to the US Drought Monitor. In the northern parts of the region where precipitation was limited, drought expanded and intensified during the month (Figure 11). The area of the Midwest in drought increased from about 82% to 91% in September but there were reductions in the area in severe drought (50% to 42%), extreme drought (33% to 15%), and exceptional drought (7% to less than 1%). The biggest improvements were seen in Missouri (Figure 12) where extreme drought decreased from 97% of the state to just 17% but the entire state remained in drought. Minnesota saw the most expansion and intensification (Figure 13) with drought areas increasing from 38% to 96% and extreme drought increasing from 0% to 20%.
 

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The Indiana State Climate Office also contributed to this report.
The Iowa Climatology Bureau also contributed to this report.
The Kentucky Climate Center also contributed to this report.
The Minnesota State Climatology Office also contributed to this report.
The Missouri Climate Center also contributed to this report.