ABSTRACT
An analysis of the atmospheric circulation patterns associated with twenty-one catastrophic floods in the conterminous United States demonstrates that each flood can be linked to anomalous patterns of circulation. Extreme regional floods over broad areas evolve from different types of large-scale anomalous behavior: uncommon locations of typical circulation features, unusual combinations of atmospheric processes, rare configurations in circulation patterns, and exceptional persistence of the same circulation pattern. Extreme local flash floods over small drainage areas can be classified into synoptic categories of existing flash-flood forecasting schemes, and in addition, these small-scale catastrophic events exhibit sensitivity to large-scale circulation anomalies. Blocking configurations in the upper-air flow pattern are important features during catastrophic flooding episodes. A clustering of catastrophic events in time is evident and may be related to the frequency of blocking or the existence of alternate states of equilibria in the atmosphere. This episodic behavior has important implications for geomorphology, especially in terms of recovery times between recurring catastrophic events, and the probability of occurrence of channel-forming sequences of extreme floods.
