August 22-31, 2007
Midwest Weekly Highlights - August 22-31, 2007
August Winds Down
Stormy, wet weather continued across the southern Great Lakes region the first few days of the period, but the weather finally settled down as the month came to a close.
The big change the last ten days of the month was a break from the relentless heat across parts of the southern Midwest. Temperature departures this week ranged from 2°F below normal in western Minnesota to 7°F above normal in Kentucky (Figure 1). A cold front pushed well south and east of the region by the morning of August 26 (Figure 2), bringing drier and more seasonable temperatures to the Midwest.
Precipitation was again heavy in a band from Iowa and northern Missouri through the central Midwest to northwestern Ohio (Figure 3), a pattern that persisted the entire month. Most of this rain fell in the first four days of the period as waves of thunderstorms pounded the region. There was no real improvement in drought conditions over the northern and southern portions of the Midwest. Thunderstorms did bring some significant rainfall to northern Kentucky, but not enough to make a difference in the existing drought conditions. The August 28 U.S. Drought Monitor (Figure 4) indicated little change in drought status across the region. The biggest changes was in northern Missouri through western Iowa, where the abundant rainfall significantly improved conditions.
Aftermath
Flooding continued across the saturated central Midwest in the wake of the storms of August 19-21. Much of northern Iowa, southern Minnesota, and southern Wisconsin remained in a flash flood watch on August 22 as thunderstorms continued across the region (Figure 5). On August 22 in Iowa, authorities were evacuating residents along the Des Moines River in Fort Dodge, IA (Webster County) after the river rose 4 to 5 feet. In Ohio, flooding on the Blanchard River at Findlay, OH (Hancock County) reached 18.46 feet on August 22, 7.46 feet above flood stage and only 0.04 feet short of the all-time record flood level. By August 27 the river stage had dropped to under 2.0 feet. On August 22 officials in Wisconsin were monitoring seven earthen dams in Vernon County, one of which had been heavily damaged by flooding. Evacuations were in effect for residents near four of the dams until August 25. Counties affected by the severe flooding in Minnesota, Wisconsin, and Ohio were declared federal disaster areas this week.
Flood warnings were also in effect for the Mississippi River from the Iowa border to St. Louis. Minor to moderate flooding resulted from the recent rain in the upper Mississippi River basin (Figure 6). The river crested at 18.55 feet at Hannibal, MO, 2.55 feet above flood stage (Figure 7). In Illinois, major flooding was occurring on the Rock River at Joslin and Moline. Moderate flooding was occurring on the Illinois River from Henry to the Peoria Lock & Dam (Figure 8).
Central Midwest Takes Another Big Hit
The weather map on the morning of August 23 had a familiar look. A frontal system snaked its way from Iowa through central Wisconsin, northern lower Michigan, and then south through eastern Ohio (Figure 9). A surface low pressure wave on the front was located over southern Nebraska and was poised to ride along the frontal boundary during the day. The NWS Storm Prediction Center placed northern Illinois and Indiana, southern Wisconsin and southern Michigan in a Moderate Risk for severe weather during the day (Figure 10). Thunderstorms began organizing over central Iowa in the morning (Figure 11). By early afternoon the storms had reached the Mississippi River (Figure 12), and from there they quickly intensified. There was extensive damage in Galesburg, IL (Knox County) from straight-line thunderstorm winds, and one man was killed when he was struck by a falling tree limb. The storms raced eastward, reaching the western suburbs of Chicago near the start of rush hour (Figure 13). Severe thunderstorm warnings were issued for most of the Chicago metropolitan area counties. Within an hour the fast-moving storms had blasted through Chicago and were over Lake Michigan into northwestern Indiana. The storms continued on eastward across the lake into southern lower Michigan. In their wake were more than 150 reports of wind damage in a band from southwestern Iowa to Detroit, MI. The storms disrupted travel, stranding 7,000 Metra commuters because of downed power lines and debris on the rails. O'Hare and Midway airports closed for two hours. The tower at O'Hare was evacuated as the storms approached, the first time since 9/11 that the tower has been shut down for an emergency. Flooding from the heavy rain also closed expressways in the area.
There were three tornadoes reported, two in Illinois and one in Michigan, producing mostly minor damage. The damage from the 70 to 80 mph straight-line thunderstorm winds was much more extensive. In the Chicago area, trees and power lines were down over a wide area, and numerous homes were damaged by uprooted and toppled trees. At the peak, 310,000 customers of Commonwealth Edison were without power. More than 650,000 outages were reported between August 23 and August 26, and ComEd was receiving 3,000 calls per hour. On August 27 there were still an estimated 7,700 customers without power as tree damage and serious flooding made it difficult for repair crews to reach some areas. In a 30 minute period Thursday afternoon, the Chicago Office of Emergency Management received more than 1,900 calls. In the City of Chicago alone, there were 1,818 downed trees, 114 traffic signals out, 79 damaged light poles,136 wires down, and 26 flooded basements.
While the winds caused immediate damage, the rain was a growing problem. More than an inch of rain fell in 30 minutes or less with the first wave of storms that brought most of the severe weather. Two more lines of thunderstorms came through northern Illinois during the evening and prior to midnight dumping more heavy rain on already saturated ground. The Chicago Deep Tunnel System was overwhelmed with storm runoff, forcing officials to release storm water and untreated sewage into Lake Michigan. Many of the rivers in northern Illinois quickly rose from the storm runoff. Residents were sandbagging along the Des Plaines and Fox Rivers. In (DeKalb County, the Kishwaukee River approached record flood levels (Figure 14). Classes were cancelled at Northern Illinois University on August 24 because limited access to campus caused by the flooding (Figure 15). Numerous smaller creeks and retention ponds throughout northeastern Illinois were out of their banks. There was also damage to agriculture, and Crop Insurance & Financial Services reported they had received 900 claims in Illinois due to water and wind damage from the August 23 storms. (Cook, DuPage, Lake, Kane, McHenry, and DeKalb counties were declared state disaster areas by Illinois Governor Rod Blagojevich.
Storms Spawn Tornadoes Michigan, Cause Flooding in Northwestern Indiana
The line of storms continued eastward on August 23 into southern lower Michigan, where straight-line winds knocked out power to an estimated 310,000 customers. A tornado briefly touched down about 50 miles northeast of Grand Rapids in Moncalm County, causing minor damage. One fatality in Michigan was attributed to the storms when a 77 year-old motorcyclist hit a tree that had fallen across a road. In Indiana, flooding from the torrential rain closed a three-mile stretch of the eastbound and two westbound lanes of Interstate 80/94 in Hammond. The storms knocked out power to more than 81,000 Northern Indiana Public Service Co. customers.
The action continued along the slow-moving cold front on August 24, with severe storms reported from southern Michigan through central Illinois and into southern Missouri. Four tornadoes touched down in southern Michigan. One tornado, rated EF2, produced significant damage in Fenton, MI (Genessee County) when the tornado path was about one quarter mile wide. Another tornado, rated EF3, touched down in Eaton County. A second tornado struck just west of Lansing, MI (Ingram County) and moved across the southern part of Lansing. Two weak tornadoes also touched down during the afternoon in south central Missouri producing minor damage.
Ohio Next in Line
By the morning of August 25 the cold front triggering the storms had made its way only to northern Indiana and central Illinois. That set the stage for more thunderstorms ahead of the front in Indiana and Ohio during the day. While the number of wind damage reports was not very high, the storms disrupted power for more than 100,000 customers in central and southeastern Ohio.
Heat and More Thunderstorms, then Finally Some Relief
Hot weather began to build back northward August 27 to August 29. Temperatures reached 100°F again in southern Illinois, and on August 28 90°F highs could be found as far north as Green Bay, WI. However, a cold front had already made its way into Minnesota and northern Wisconsin. An interesting phenomenon occurred on August 27 in south-central and southeast Wisconsin in the wake of a decaying thunderstorm complex moving ahead of the front. A large pressure gradient developed to the rear of the thunderstorm complex, producing strong, non-thunderstorm winds. The winds, gusting 45 to 50 mph, resulted in some tree and power line damage.
By the morning of August 30 the cold front had pushed south of the Ohio River (Figure 16), and refreshingly cooler and drier air spread through the region. High temperatures on August 30 ranged from the low 70s in northern Wisconsin to around 90 in southern Kentucky, and by late afternoon dew points were in the low to mid 50s in the northern Midwest and low to mid 60s along the Ohio River. The cold front did set off thunderstorms in its passage south, mostly over southeastern Illinois, southern Indiana, and western Kentucky, areas that badly needed the rain (Figure 17). Amounts in western Kentucky generally ranged from 0.30 to 1.50 inches. However, Marion, KY (Crittenden County) received 3.10 inches of rain and Woodbury, KY (Butler County) picked up 2.66 inches. By the morning of August 31 the cold front had moved to the Appalachians (Figure 18) and the entire region enjoyed cooler weather. High temperatures on August 31 ranged from the 70s north to only the mid 80s in southern Kentucky (Figure 19). Daily temperature and precipitation records can be viewed in Table 1.