September 24-30, 2011
Midwest Weekly Highlights - September 24-30, 2011
A Cool, Damp, and Windy End to September
A large upper level low pressure system meandered across the Midwest the last week of the month, producing widespread cloudiness and abundant rainfall in some areas. A strong cold front swept through the region on September 29th producing strong winds across the northern half of the region.
Temperatures this week were well below normal in the central Midwest, with above normal temperatures in the far north and east (Figure 1). Average daily temperatures ranged from 2°F to 5 °F below normal from Missouri northward through Iowa an Indiana. Temperatures were 3°F above normal in far eastern Ohio, and 3°F to 5°F above normal in northwestern Minnesota. There were a few record lows and record low maximum temperatures set during the week. On September 29th over a dozen locations in western and southwestern Minnesota set new maximum temperature records in the mid 80s to low 90s.
Precipitation was much above normal east of the Mississippi River, with the largest departures along the Ohio River. The persistence of the upper level low produced frequent and at times heavy rain, and the rainfall pattern reflected the presence of the low (Figure 2). Rainfall this week ranged from 200 to 500 percent of normal in the eastern two-thirds of the region. West of the Mississippi River, rainfall was much below normal. Severe drought still is occurring in parts of the central and western Midwest, but the rain this week did bring improvement to Indiana, Kentucky, and parts of Illinois (Figure 3)
Days of Dreary Weather
The week began with the upper level low already cut off from the main jet stream, centered over northern Illinois, and dominating the circulation from Lake Superior to the Gulf of Mexico (Figure 4). It resided over the Midwest almost the entire week, and was finally pushed east by another strong wave heading southeast out of the Northern Plains on September 29th (Figure 5). This weather system provided several days of cool, dreary, but interesting weather for areas under its influence. On September 24th the cold unstable air aloft interacting with the relatively warmer waters of the Great Lakes produced conditions favorable for the development of waterspouts. A number of waterspouts were sighted over Lake Michigan (Figure 6) during the day.
Series of smaller waves rotated around the low during the week, bringing rounds of showers and some thunderstorms. The heaviest rain during the week occurred on September 26th as the low parked over northwestern Illinois (Figure 7). Heavy rain fell in northern Illinois, and along the frontal boundary in eastern Ohio (Figure 8). Two to three inches of rain fell in north central Illinois, more than three inches fell in parts of northwestern Ohio, and two inches in eastern Ohio. The heavy rain northwestern Ohio caused the Blanchard River to crest over flood stage prompting the issuance of a flood warning. Flash flooding also occurred early in the period in southern Indiana and Kentucky from heavy thunderstorms.
There were scattered severe thunderstorms this week as well, all of them in the eastern third of the region (Figure 9). Weak tornadoes touched down in southern Indiana and Kentucky on September 25th, in northern Indiana on September 27th, and in extreme southeastern Michigan on September 28th, all resulting in at least minor damage.
Strong Cold Front Kicks Low East, High Winds Cause Problems
On the morning of September 29th a strong cold front was marching across the upper Midwest, and by the morning of September 30th had moved as far east as the Appalachian Mountains (Figure 10). Cooler, drier air was ushered into the Midwest on strong northerly winds. Winds gusted to in excess of 50 mph across Minnesota, Wisconsin, Michigan, Iowa, and northern Illinois and Indiana, and high wind warnings were in effect for most of this area. In Iowa, numerous fires were started in very dry farm fields and quickly spread in the strong winds. At least one house and several buildings were destroyed in Clay County, where firefighters asked farmers to stay out of the fields because they could not keep up with the numerous fires. Semi-trailers were banned from the toll roads in northwestern Indiana on September 29th. Waves up to 25 feet were generated on southern Lake Michigan the night of September 29th. Tree and power line damage was common across the affected areas. On the night of September 29th 27,800 ComEd customers were without power in the Chicago area. In Indiana 16,000 customers of Northern Indiana Public Service Co. lost power due to the winds. A 53-year old man was killed on the morning of September 30th when he was struck by a tree toppled by the high winds on a golf course in southwestern Michigan.
-SDH-
The Iowa State Climate Office also contributed to this report.