Midwest Weekly Highlights - August 1-10, 2009

Cool and Stormy Start to August

The unusually cool weather that characterized July 2009 continued well into the first ten days of August. Temperatures during this period ranged from near normal across much of the southern half of the region to -5°F across northern Minnesota (Figure 1). Record temperatures were set every day this period. Record low maximum and minimum temperatures were set in the northern portions of the region early in the period, and record high minimum and maximum temperatures were set the last three days. Temperatures from August 1-6 ranged from near normal in Missouri to 7 °F below normal in northern Minnesota (Figure 2). The weather warmed considerably August 7-10, with temperature departures running from +4°F in western Missouri and Iowa to -3°F in northwestern Minnesota (Figure 3).

There were three distinct bands of heavy precipitation August 1-10 (Figure 4). One band extended from southern Minnesota eastward through much of lower Michigan. The second area extended from southern Wisconsin through much of Iowa into northern Missouri, and the third extended along the Ohio River. Precipitation in these areas ranged from 150 to 200 percent of normal. Rainfall for the first ten days of August was less than 50 percent of normal in southwestern and central Missouri and from northern Indiana across the northern two thirds of Ohio. The rain in the upper Midwest did benefit the southern portions of the drought areas in Wisconsin and central Minnesota, where Severe to Extreme drought was depicted by the U.S. Drought Monitor (Figure 5).
 

Storms Roll Across Central Midwest

Another cold front was pushing through the Midwest on August 4 (Figure 6). A complex of thunderstorms developed along the front in Missouri late on August 3, then moved through Illinois into Indiana during the morning of August 4 (Figure 7). There were numerous severe thunderstorm warnings issued in advance of the storms, and many reports of severe weather, mostly damaging winds, were received across central Illinois, Indiana, and in Kentucky. A section of Interstate 65 was closed south of Franklin, IN (Johnson County) when power lines came down across all four lanes. More than 78,000 customers were without power in central Indiana due to the storms. Heavy rain also caused urban and street flooding in Indianapolis.
 

Devastating Flooding in Kentucky, Indiana

The front slowed as it approached the Ohio River, concentrating heavy showers and thunderstorms in the Ohio Valley. Training thunderstorms occurred over northern Kentucky, southern Indiana, and extreme southwestern Ohio on the morning of August 4, dropping more the seven inches of rain on Louisville, Kentucky (Figure 8) and three to more than four inches on surrounding areas. The National Weather Service office in Louisville reported that five inches of rain fell in 90 minutes from 7:45 a.m. to 9:15 a.m. EDT, and rainfall rates of nearly 9 inches per hour were reported. Officially 4.53 inches of rain fell at Standiford International Airport in Louisville, which broke the old record for highest rainfall in a single day in August (set in 1879).  Three inches of that rain fell in just one hour. National Guard flood rescue teams were sent to Louisville in response to the flooding, and the American Red Cross opened two shelters in Louisville and one in Clarksville, IN (Clark County). The flooding in Louisville was extensive. A dozen buildings were flooded on the University of Louisville campus. The Louisville City Library lost tens of thousands of books worth a million dollars when floodwaters inundated the library's lower level to a depth of 3.5 feet. Two hospitals discharged or transferred almost 200 patients late on August 4 and remained closed due to water damage. At Churchill Downs two dozen horses had to be relocated to a nearby training training facility because of floodwaters. The Derby Museum, floodwaters inundated the basement and caused damage to the main floor. Preliminary damage estimates and cleanup costs were in the millions of dollars. Flooding also closed many roads throughout the region, including sections of Interstate 74 and Interstate 65. nearly 200 people were rescued from the flooding, but fortunately no injuries or deaths were reported. The thunderstorms also caused wind damage in many areas. Downed trees in McCreary County shut down the Norfolk & Southern Railroad for three hours.
 

The Heat of Summer Returns

A high pressure system over the Great Lakes kept the weather cool and comfortable for most of the region through August 6. On August 7 warm air was beginning to make its return to the region as a low pressure system wound up over the Northern Plains and and a warm front extended southwest from the low through central Missouri (Figure 9). High temperatures reached well into the 90s over much of Missouri and western Iowa on August 7 (Figure 10), with the 90s spreading as far north as southern Wisconsin and Michigan and as far east as eastern Ohio on August 8 (Figure 11). This was the first significantly hot weather for much of this region since late June. Along with the heat, dew points reached into the mid 70s over the central Mississippi River Valley (Figure 12).

Heavy thunderstorms developed along the leading edge of the warm, humid air in Minnesota and Wisconsin on August 7 and into August 9, bringing rain to the southern portions of the drought area. The heaviest band of rain August 7-8 extended from south-central Minnesota into west-central Wisconsin, with amounts from 1.50 to 3.00 inches reported in many areas, with 6.21 inches at Chaska, MN (Carver County), 6.01 inches at Lester Prairie (McCleod County), and 4.95 inches at the National Weather Service office at Chanhassen (Carver/Hennepin Counties) (Figure 13). A number of the storms during the evening hours of August 8 were severe in Minnesota. Two tornadoes touched down in eastern Minnesota and one in western Wisconsin, causing some damage.

Severe storms also erupted from southwestern Iowa across southern Wisconsin on August 9 along the leading edge of cooler and drier air (Figure 14) with many reports of wind damage. On August 10 the focus of the shower and thunderstorm development shifted south into Missouri, where 1.50 to more than 2.50 inches of rain fell over a large portion of southwestern Missouri. Temperatures in the 90s were found only from southeastern Missouri through Indiana, Kentucky and Ohio. Over the remainder of the region temperatures were close to normal.

SDH