Climatic Aspects of the 1993 Upper Mississippi River Basin Flood

The 1993 record-breaking summer flood in the Upper Mississippi River Basin resulted from an unprecedentedly persistent heavy rain pattern. Rainfall totals for the Upper Mississippi River Basin were, by a large margin, the largest of this century for the 2-, 3-, 4-, and 12- month periods encompassing the 1993 summer. The totals for these periods are estimated to have a probability of occurrence of less than 0.005 yr−1 In addition, the number of reporting stations receiving weekly totals in excess of 100 mm (events identified in a previous study as being closely correlated with floods) was the largest in at least the last 45 yr. Other conditions contributing to the flood include above-normal soil moisture levels at the beginning of June 1993; large-sized areas of moderate to heavy rains; occurrence of rain areas oriented along the main stems of major rivers; a large number of localized extreme daily rainfall totals (greater than 150 mm); and below-normal evaporation. The large-scale atmospheric circulation patterns during the summer of 1993 were similar to the patterns associated with past heavy rain events, although much more persistent than past events.