Midwest Weekly Highlights - August 25-31, 2009
Another Chilling Week
The last week of August was unseasonably cool across the Midwest. This was the coolest week of the month and ended with some of the coolest weather since mid May. Temperature departures ranged from 2°F below normal in eastern Ohio to 8°F below normal in a broad swath across the central Midwest (Figure 1). Temperatures in a narrower band from southwestern Iowa to southern Lake Michigan that experienced heavy rain this week were 9°F to 10°F below normal. Within this band, average daily maximum temperatures were generally 70°F to 75°F during the week, 12°F to 14°F below normal (Figure 2). Low minimum and low maximum temperature records were set somewhere in the nine-state region on six out of seven days this week, with the majority of records set August 30-31.
A slow-moving cold front helped focus heavy rain and flooding in Missouri, Iowa, Illinois this week. Precipitation from northern Missouri through southeastern Iowa, the northern half of Illinois and eastward into southern lower Michigan was two to four times normal this week (Figure 3). Significant rain also fell across parts of northwestern Wisconsin into the western U. P. of Michigan (Figure 4). The U.S. Drought Monitor for August 25 (Figure 5) depicted a one category improvement in drought status in Wisconsin and Minnesota as a result of the rain the previous week. The rain this week will likely keep conditions from deteriorating any further, but it is still dry across this area. Only light rainfall occurred across the remainder of the Midwest.
More Heavy Rain
The cold front that entered the Northern Plains on August 24 sagged slowly southward through the Midwest the first three days of this period. On August 25 showers and thunderstorms extended from southwestern Minnesota into northwestern Wisconsin and the western Michigan U. P., cutting across the heart of the area that has been in drought this summer (Figure 6). One to two inches of rain accumulated over the course of the day. The cold front continued southward, stalling out across central Missouri eastward into Ohio on August 27 (Figure 7). A low pressure wave on the front in northwestern Missouri pumped warm, moisture-laden air northward over the frontal boundary, fueling heavy showers and thunderstorms from northern Missouri into southeastern Iowa and northern Illinois late on August 26 through early on August 28 (Figure 8). Rainfall amounts from southeastern Iowa into northern Illinois totaled more than 3.0 inches, with a number locations in southeastern Iowa accumulating 5.0 to 7.0 inches (Figure 9). Cedar Rapids, IA (Linn County) totaled 7.06 inches of rain for the storm period. The heavy rain produced extensive flash flooding and also caused more prolonged flooding on rivers and streams in southeastern Iowa. At the end of the week flood warnings were still in effect for the Wapsipinicon River near DeWitt, IA (Clinton County) where major flooding was occurring, and for the Skunk River at August, IA (Des Moines County) where minor flooding was occurring (Figure 10).
Heavy thunderstorms continued to fire along the front during the evening of August 27 as it continued to push slowly to the south. Very heavy thunderstorms developed over east-central Illinois, with 3.0 to 5.0 inches of rain over western Champaign County, most of which came in a two-hour period. The U.S. Cooperative observer in Mahomet, IL (Champaign County) reported 4.71 inches of rain in a 90 minute period, with a total of 5.22 inches for the event. A CoCoRaHS observer just south of Mahomet reported a total of 5.15 inches of rain from the storms. The heavy rain caused flooding of many local roads.
An Autumnal End to August
Showers and thunderstorm continued from southeastern Illinois east through Ohio on August 28 as the low pressure wave and front crossed through the eastern half of the region (Figure 11). The system accelerated late on August 28 in response to an approaching upper level low (Figure 12), and by August 29 the low was located in Ontario, Canada with the front extending south along the Appalachians. Much colder air followed a secondary front through the Midwest, setting the stage for an unseasonably cold end to the month of August.
Record low temperatures were set across the Midwest on the mornings of August 30 and August 31 as Canadian high pressure settled over the region (Figure 13). The coldest weather on August 30 was in Minnesota, where Brimson, MN (St. Louis County) recorded a low temperature of 28°F and Babbitt, MN (St. Louis County) reached 29°F. High temperatures on August 30 were remained in the 60s as far south as central Illinois (Figure 14), aided in part by cloudiness associated with the upper level low (Figure 15). On August 31 low temperatures in the 30s occurred from the Arrowhead of Minnesota across far northern Wisconsin and northern lower Michigan (Figure 16), with several reports of temperatures at or below 32°F in northern Wisconsin. On the last two days of August more than 300 record low temperatures and record low maximum temperatures were set or tied across the Midwest.