August 29-31, 2001:

The last three days of the month were quite eventful in the Midwest. A strong cold front entered the northwest part of the region on the 29th and swept through the region on the 30th and 31st, bringing widespread rain and isolated zones of flash flooding and severe weather. The southern Midwest experienced loosely organized but severe convective storms on the afternoon of the 29th along a stationary front (Figure 17, UIUC Dept. of Atmospheric Sciences). The cold front passage was responsible for the rain elsewhere in the Midwest during the remainder of the period (Figure 18). Temperatures were above normal ahead of the frontal passage, as a strong flow of warm, moist air preceded the cold front (Figure 19). It is this moist air supply that was converted by the approaching front into both scattered severe storms and organized squall lines with some gusty winds and spectacular lightning and precipitation rates (Figure 20, UIUC DAS). The month of August both began and ended with substantial precipitation events impacting Chicago, Illinois.