Midwest Weekly Highlights - August 11-17, 2009

A "Typical" Week of Summer

For the first time in over a month, most of the Midwest was warmer than normal. It was much warmer in the upper Midwest, where average temperatures this week were 3°F to 6°F above normal (Figure 1). Temperatures were near average from Iowa southeast through much of Illinois and Kentucky, and near to slightly below average in central and southwestern Missouri. Temperatures this week were consistently in the 80s, with a few 90s in western Minnesota and in the far southern Midwest. Late in the week cooler weather settled in over Minnesota, keeping high temperatures in the 70s. Record high minimum and maximum temperatures were scattered across the region during the week, with the most records set on August 15-16 as warm, humid air overspread most of the region.

Showers and thunderstorm were most prevalent beginning on August 13 through the end of the period. Heavy rainfall occurred in parts of the upper Midwest that have been in severe drought, and the storms brought some relief from the dryness, but also some flooding. A moist southerly flow of air from the Gulf of Mexico fueled storms throughout the region aided by the presence of a frontal system pressing southward. The pockets of heavy rain occurred in northwestern Minnesota, northern lower Michigan, Missouri and western Illinois and northeastern Indiana (Figure 2). Rainfall in these areas reached 300 to 400 percent of normal. There were also a number of all-time daily record precipitation amounts during the week. Rainfall was less than 50 percent of normal in the Michigan U. P. south through southeastern Wisconsin, and from southern Indiana and Ohio south through the western two-thirds of Kentucky. The August 11 U.S. Drought Monitor continued to depict Severe to Extreme drought in Wisconsin and Minnesota (Figure 3), but some heavy rain fell in the southern portions of the drought area this week.

Despite numerous heavy thunderstorms there were only scattered reports of severe weather during the week. Flooding from the heavy rain was a more serious problem.
 

Flooding in Wisconsin

On the night of August 13 parts of western Wisconsin received more than 7 inches of rain in as little as two hours (Figure 4), causing a number of creeks and rivers to rapidly rise. In Arkansaw, WI (Pepin County) the Arkansaw Creek, normally only a few inches deep, was estimated to be up to 15 feet deep from flooding. At least a dozen homes and three business in the village were damaged, and two bridges were reportedly washed out. An observer with the Community Collaborative Rain, Hail, and Snow network (CoCoRaHS) in Arkansaw reported 7.34 inches of rain for the 24 hour period ending at 6:00 a.m. on August 14.
 

Flooding in Missouri, Western Illinois

The southerly flow of warm and moisture-laden air continued to feed into the Midwest during the week. A cold front stalled over the western portions of the region on August 16-17 and was the focus of heavy thunderstorms from northern Missouri to western Illinois (Figure 5). Flash flood watches and warnings extended across this entire area (Figure 6). Rainfall amounts in the heaviest band across northern Missouri into western Illinois were commonly between 3 and 6 inches (Figure 7). 7.16 inches was reported by a NWS Cooperative Observer in Chillicothe, MO (Livingston County), and 6.11 inches was reported just east of Chillicothe by a CoCoRaHS observer. The rain caused widespread flash flooding in Missouri with numerous roads closed across the affected area. Much of this same area received another inch or more of rain during the day on August 17.
 

..and Flooding in Indiana

Northern Indiana had been rather dry so far this month, with less than 50 percent of normal rainfall. That all changed late this week. Thunderstorms developed early on August 17 over northeastern Indiana and northwestern Ohio and continued moving over the same areas during the day. These training thunderstorms continued until after midnight, leaving more than two to three inches of rain in their wake (Figure 8). The heaviest rain of 7 to 9 inches fell in northeastern Kosciusko County (Figure 9). Rainfall rates exceeded 3 inches per hour at times. A CoCoRaHS observer southeast of Milford, IN (Kosciusko County) recorded 9.09 inches of rain for the 24-hour period ending at 7:00 am EDT on August 18, with a two-day total of 9.90 inches. There were more than 40 street closures in Fort Wayne (Allen County ) due to the heavy rain. In Nobel County, IN a mud slide deposited two feet of mud across a road.
 

SDH