Midwest Weekly Highlights - August 8-14, 2008

---

Cool, Dry, and Quiet

Upper level trough over eastern U.S. kept most of the Midwest in strong northwesterly flow this week (Figure 1). This resulted in cool, dry weather for most of the region. The exception to the dry weather was along the extreme southern and western periphery of the cool air mass. For most of the region, precipitation ranged from less than 10 to 50 percent of normal (Figure 2). However, in southwest Missouri and western Minnesota rainfall was two to five times normal for the week. Rainfall was also well above normal in the "thumb" of lower Michigan. The August 12 U.S. Drought Monitor depicted an expansion of dry conditions over Minnesota, and a small area of Moderate Drought remained over the east-central portion of the state (Figure 3).

The persistent northwest flow washed away the muggy weather of the first week of the month, and conditions were more like early fall than early August across the region. Average daily temperatures ranged from 5°F to 6°F below normal in the Ohio Valley to near normal in western Minnesota, with a majority of the Midwest at least 3°F below normal (Figure 4).
 

Scattered Storms and Severe Weather

The strong push of cooler, drier air made its way through the Midwest August 9-10, and the heavy rain in Michigan and Missouri occurred with this system as it pushed through the Midwest. Later in the week, showers and thunderstorms were mostly limited to the western and southern portions of the region near the boundary of the cooler, drier air mass. On August 12 showers developed from Minnesota south through Iowa as the surface high pressure system moved to the east and warm air began to push back into the Midwest. That resulted in only a few severe storms, and there were only five reports of severe weather in the Midwest through August 12. A tornado was reported near Fort Dodge, IA (Webster County) on August 9, with the other reports coming from Wisconsin and Michigan. A more concentrated area of severe storms occurred in southwestern Minnesota and northwestern Iowa on August 13. A brief EF0 tornado caused some damage to buildings in Dickinson County, IA, and 3.00 inch hail was reported just west of Edgerton MN (Pipestone County).
 

Low Water Levels on Mississippi River

Only a little more than a month after record and near-record flooding on the Mississippi River, low water levels are causing problems for tow operators and boaters. The combination of a lack of rain and deposits of sand and sediments resulting from the flooding have created low water levels in the main navigation channel in the river. Two barge tows ran aground this week north of St. Louis, and the U.S. Army Corps of Engineers has begun dredging operations two weeks earlier than normal. In some areas, the river has been dropping as much as a foot a day.

SDH