Both the basin characteristics and the precipitation characteristics were reviewed. One of the common features of the basins involved was the small size. The average basin size was less than 50 square kilometers. Urbanization was another very important contributor to the flash flood threat, especially in the central U.S. where relatively gentle basin slopes do not contribute greatly to rapid runoff. In general, the condition of previously saturated soils may be more important in the central and eastern U.S. where deeper soils and greater coverage of vegetation would typically result in greater rainfall infiltration rates compared to the thin, rocky soils in many parts of the West.
Atypically high precipitation rates were very important for enhancing the flash flood threat in many of the events. This value can vary depending on the local climate, with the average peak rainfall rate for western events being ~125 mm/hr, and an average peak rainfall rate ~200 mm/hr for central and eastern events. Thus, recognizing atmospheric and topographic features that contribute to anomalous precipitation efficiency can help distinguish the most significant threats in many of the cases. This typically involves some maritime, tropical characteristics of the atmosphere and a region of enhanced low-level flow transporting high theta-e air toward a boundary (either meteorological or terrain boundary). In the central and eastern U.S., enhanced duration of convective complexes along with enhanced rainfall rates was common in many of the cases and lead to excessive storm total accumulations. In the West, especially the Southwest, enhanced duration was not as typical, and flash flood threats are often the result of excessive short-duration rainfall rates over small, fast response basins.