July 25-31, 2008
Midwest Weekly Highlights - July 25-31, 2008
More Rain Southwest, but Dry Northeast
Several clusters of thunderstorms this week dumped unusually heavy amounts of rain on southern Iowa, Missouri, and southern Illinois, while the northeastern half of the region stayed very dry. Rainfall from southern Iowa to southern Illinois was three to five times normal for the week, with a portion of north-central Missouri receiving more then 700 percent of normal rainfall (Figure 1). Much of Kentucky received normal to above normal rainfall, as did western Minnesota. In contrast, rainfall for this week was less than 25 percent of normal from southeastern Minnesota to southeastern Ohio as a persistent northwest flow aloft steered thunderstorms from northwest to southeast well south of this region (Figure 2). Temperatures were generally near to slightly above normal across most of the region, ranging from 3°F above normal in northwestern Iowa and southern Missouri to 1°F below normal in central Missouri, eastern Kentucky, and northern Minnesota (Figure 3). Only the far southern portions of the region experienced high temperatures 90°F or above this week.
Storms Bring More Flooding to Iowa, Missouri
Several mesoscale convective systems (MCS) brought extremely heavy rains to previously saturated portions of Iowa and Missouri this week. The first of these was in progress as this week started, as thunderstorms developed repeatedly along a warm front associated with a low in the central Plains (Figure 4). Rainfall exceeded 12 inches in portions of northern Missouri, and amounts from 3 to 6 inches were reported from southern Iowa to just north of St. Louis (Figure 5), resulting in flash flood watches and warnings for much of the region (Figure 6). The largest 24-hour rainfall amount reported was 14.95 inches on mile west of Brunswick, MO (Chariton County). An observer with the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) near Kirksville, MO (Adair County) reported more than 12 inches of rain. More rain actually fell at that location, but the rain gauge overflowed and some of the rain was not measured. The flash flooding from these storms was extensive. A dam on a 2-acre pond at a country club near Kirksville was breached and water was flooding a major highway. Two men were rescued from a tree after their vehicle was swept off of a road by floodwaters in Ralls County, and authorities reported numerous vehicle rescues.
A second MCS affected much of this same area just days later, Thunderstorms developed from western Iowa into northern Missouri on July 27, dumping another 3 to 5 inches of rain on waterlogged Iowa and Missouri (Figure 7) causing more flooding and closing many roads. In Des Moines, IA (Polk County) police were going door to door in an area along Four Mile Creek warning residents of rising water. For some of this area it was the third evacuation in the past two months. Floodwaters from the creek pushed as much as 3.5 feet of water into homes in the affected area. In addition to the flooding, high winds with sever thunderstorms knocked out power to an estimated 14,000 customers of MidAmerica Energy in the Des Moines and Fort Dodge areas.
Hello Dolly
The next round of heavy rain came on July 29-30 as the remnants of Hurricane Dolly entered the Midwest (Figure 8). Heavy rain fell from north of Kansas City, MO across north-central Missouri, preventing any recovery from the flooding caused by the previous two systems (Figure 9). In Platte City, MO (Platte County), 7.70 inches of rain was recorded into the 24 hour period ending at 7:00 a.m. on July 30, and there were numerous reports of 2 to 3 inches of rain in northwestern Missouri. The heavy rain closed many roads and kept rivers and streams in flood. As the low pressure system that was Dolly continued east on July 30-31 the heavy rain moved into southeastern Missouri, southern Illinois, and western Kentucky prompting flash flood watches for much of the area (Figure 10).
In the wake of the week of heavy rain in Missouri, Mark Twain Lake, a flood control reservoir and major recreational destination, reached a record level of 640.36 feet on July 30, swelling it to twice its normal size. The previous record was 636.77 feet in 1993. On July 30 the Army Corps of Engineers closed the lake to all boating traffic, and increased the water released through the dam into the Salt River to 50,000 cubic feet per second (cfs). Releases above 12,000 cfs are unprecedented. Authorities also closed the Salt River to recreational boating traffic from the Clarence Cannon Dam to the Mississippi River because of flooding. This was having a serious impact on area businesses during the height of the tourist season. Mark Twain Lake is located in Ralls and Monroe Counties, 28 miles southwest of Hannibal, MO and 120 miles northwest of St. Louis.
Severe Thunderstorms Across the Region
Severe thunderstorms were reported in every state in the Midwest region this week. Tornadoes were reported in Iowa, Michigan, Minnesota, and Wisconsin, although most were brief, weak, and caused only minor damage. Hail two inches in diameter or larger was reported in Ohio on July 26, and in Iowa and Missouri on July 27. During the evening of July 27 hail to 4.00 inches was reported in Putnam County, MO. In Pollock, MO (Sullivan County) hail 4.50 inches in size falling for almost 5 minutes shattered car and home windows.
SDH