Tropical Storm Isaac dropped a lot of rain on Illinois over the Labor Day weekend. As noted in an earlier post, tropical storms are rare in Illinois but they do happen. However, this one was especially interesting because the heavy rains fell over areas in drought across the state. In a normal year, such heavy rains would have produced widespread flooding in low areas, streams, and small rivers. But not this year. I have not heard any reports of significant flooding. This suggests that the dry soils absorbed most of the rainfall. And what little runoff was produced, ended up in streams that were much below-normal. More on this in another post.
The first map shows the 7-day rainfall totals from Tropical Storm (TS) Isaac. The areas in red had over 4 inches of rain. Areas north of Interstate 80 saw little of the rainfall.
The second map shows the 30-day rainfall totals. Parts of Illinois already had substantial rainfall before TS Isaac. With the addition of the rainfall from Isaac, much of central and southern Illinois had accumulated over 4 inches of rain. In fact, some areas had over 6 inches of rain (areas shaded in yellow or red).
The third map shows the 30-day rainfall departures from normal. Areas in green and blue are above normal. The areas in the two shades of blue are 2 to 4 inches above normal. The areas in purple and violet (?) are 4 to 8 inches above normal. Clearly, areas with much above normal rainfall will have a significant impact on the drought.