August 2001
Temperature and Precipitation Anomalies.
August 2001 was a quite varied month across the Midwest, with a number of smaller areas receiving copious rain in the midst of general regional dryness. Because precipitation exceeded 5 inches in northeast Minnesota, western Wisconsin, parts of northern Illinois and Indiana, and portions of southern Illinois and western Kentucky (Figure 1), the Midwest as a whole had its 46th wettest August. One of the most noteworthy wet locations was O'Hare Airport, which is the official measurement site for Chicago, Illinois. A total of 12.25 inches of rain fell at that location, the second largest August total in Chicago's very long period of record. However, the majority of the region actually experienced below average rainfall (Figure 2), reaching as low as 25% of normal in southwestern Iowa (Figure 3). Illinois and Kentucky had their 22nd wettest August. The variegated pattern occurred as a number of frontal boundaries interacted with upper level disturbances, most of the time not producing widespread rain in any one event. Temperatures were above normal in every state and region (Figure 4), although the range extended from near normal in Iowa to 4-5°F above normal near the Great Lakes. The Midwest had its 24th warmest August on record, and Michigan had its 17th warmest.
The first two weeks of August started with a slow moving cold front edging southeastward from the Great Plains. Northeastern Minnesota experienced heavy rains on the 1st, with the heaviest precipitation sweeping southward through Wisconsin and northern Illinois on the 2nd. Up to six inches of rain fell in southern Wisconsin, while Chicago experienced an incredible 4 inches in a three hour period (Figure 5). The front sagged across the region on the 3rd and 4th, with heavy rains falling on a narrow band from northwest Iowa to eastern Kentucky. However, many areas missed the precipitation, and after the 4th, conditions turned dry and hot in much of the Midwest. A serious heat wave killed people in a number of states, as well as stressing the row crops and pastures. Another cold front moved through the area August 8-10, bringing isolated heavy rain and high winds capable of damaging crops to much of the Upper Midwest. The rain was spotty over the region, with some places receiving copious amounts and others nothing. At the same time, the remnants of Tropical Storm Barry also brought beneficial rain to the southern Midwest drought areas (Figure 6). Almost all of the Midwest, except for northern Minnesota, Wisconsin, and the southern edges received less than the normal amount of rain for the first 2 weeks of August (Figure 7). Temperatures averaged 3-7°F above normal for the period, with the extreme heat wave days tempered by the two cold front passages (Figure 8).
The third week of August proved to be cooler and more rainy than the first two weeks combined. These conditions slowed a steep decline in crop health that was occurring previously, but not before further declines were registered during the week. The Midwest experienced above normal rainfall for the August 15-21 period (Figure 9), except for all of Missouri and Kentucky and eastern Ohio. Some places received more than 4 inches of rain in heavy convective bands (Figure 10). Temperatures were 4-8°F below normal in Iowa and Illinois, and only the far north and east edges of the Midwest were above normal (Figure 11). During the week, two distinct cold fronts passed through the area, on August 15-16 and 17-19. Both were accompanied by low pressure centers passing through the upper Midwest, where substantial rains fell. The heaviest rain accompanied the latter event, and aided drought affected areas in parts of Minnesota, Illinois, Wisconsin, and Michigan (Figure 12).
The August 22-28 period was quite wet in some of the previously dry regions of the Midwest. Central Illinois, southern Wisconsin, and much of Lower Michigan received several inches of rain (Figure 13) and were well above normal for the week (Figure 14). At the same time, temperature conditions were somewhat above normal, by several degrees in most areas to more than 5°F in areas that missed the rain (Figure 15). Early in the week, a warm front brought heavy rain to the southeastern Great Lakes area, and then faded south to form a stationary front across from Missouri to Ohio. Very heavy rain fell from the 22nd to the 24th at various locations along and north of the boundary, with the area where Iowa, Illinois, and Missouri meet receiving up to 8-10 inches of rain (Figure 16). The storms along this stationary boundary caused considerable crop damage, not only in places receiving excessive rain, but in locations impacted by severe weather. In addition, widespread fog developed north of the boundary for several days. The warm front returned north on the 25th and 26th, bringing more rain to the central and northeastern Midwest. Finally, at the end of the period, a cold front swept down from the northwest, cooling conditions somewhat and drying out the region.
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). 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). The month of August both began and ended with substantial precipitation events impacting Chicago, Illinois.