Comparison of the 2011 Mississippi River Flood to the MR-T Project Design Flood

During the period of April 19 to May 4, 2011, an unusually large amount of precipitation fell across the middle to lower portion of the Mississippi River basin and resulted in record flooding in many locations. The greatest amount of moisture, with rainfall totals near or in excess of 508 mm (20 in.), was located throughout northern Arkansas, southern Missouri, far southern Illinois, and western Kentucky. This study determined, using the procedures outlined in the Manual on Estimation of Probable Maximum Precipitation of the World Meteorological Organization (WMO), the extent of how much more precipitation could have occurred in the same region from the same storms if the maximum possible amount of moisture had been available. A detailed examination of the synoptic weather conditions from mid-April through early May in 2011 indicated that two distinct systems affected the region with heavy rainfall during that period. National Weather Service radar and surface observation data from each storm was combined and maximized in accordance with the WMO procedures and compared with the precipitation depths used by the U.S. Army Corps of Engineers for the currently accepted project design flood (PDF). The calculations performed by this in-place maximization study produced values, calculating that the first system could have been 22% greater than observed, whereas an increase of 34% was possible for the second system. The result of maximizing both storms that occurred in the April 19 to May 4, 2011 time frame across the mid-Mississippi River Valley were shown to not exceed the rainfall used for the current Mississippi River and Tributaries (MR-T) Project PDF and demonstrates that the current PDF does not need to be changed on the basis of the observations of this event alone.