September 2009 was a month with near normal temperatures and below normal precipitation. An unusual weather feature was an extended dry period which affected much of central and southeast Illinois between Labor Day and the 19th. A stagnant weather pattern resulted in a large area of high pressure remaining in place over the Great Lakes. This provided a dry northeasterly flow to much of the Midwest and Great Lakes, keeping any rain-producing systems well to our south. The rain-free period lasted about two weeks and was the longest stretch without precipitation for most areas this year.
A change in the upper air pattern produced a more active final 10 days of the month. An upper level low pressure system tracked into Illinois from the south on Sunday, Sept. 20, and produced widespread rains along with some locally heavy rainfall. Isolated locations in west-central and southeast Illinois picked up between 3 and 5 inches of rain in a several hour period which resulted in some flash flooding. The highest reported total was 4.76", 2 miles south of Jacksonville, which fell in less that 4 hours. Below is an image of doppler radar estimated 24-hour rainfall. Note how localized the heavier rains were, with little falling along the I-57 corridor.
A few more weak to moderate strength systems moved through in the last week of the month, and pushed rainfall to near normal for areas west of I-55. Farther east, rain amounts were 25 to 75 percent of normal east of I-55 and north of I-70.
Temperatures were near normal, thanks to a lack of any extended stretches of hot or cold air. Only one record was tied at an official climate site, when Springfield dropped to 48 degrees on the morning of the 2nd.
Click the links below to view September's Monthly Climate Summary: