This study examines the radar-indicated structures and other features of extreme rainfall events in the United States over a three-year period. A rainfall event was defined as 'extreme' when the 24-h precipitation total at one or more stations surpassed a given threshold. Using the National Weather Service (NWS) high resolution rain gauge network, and eliminating bad rainfall data, this definition yielded 193 such events from 1999 to 2001 in the area east of the Rocky Mountains, excluding Florida.

Two-km national composite radar reflectivity data were used to examine the structure and evolution of each extreme rainfall event. While these data reveal a wide variety of storm types, several patterns emerge. In the northern and Great Plains states, almost all of the extreme rain events are associated with mesoscale convective systems (MCSs); in the northeast, synoptic-scale weather systems play a much greater role. In total, approximately half of the total number of events are associated with MCSs.

Of the MCS cases, two patterns of organization were observed most frequently, and each is conducive to large point precipitation accumulations. The first is a linear MCS in which the motion of the convective cells is approximately parallel to the convective line, while the line itself remains nearly stationary. The second type of system is a line of convection with an area of stratiform rain that is parallel to the line and is advected downstream, while new convective cells form upstream and ``train'' over a particular area.

The seasonal cycle of these extreme rain events generally agrees with previous studies of precipitation systems. Extreme rainfall-producing storms in the northern part of the country are confined to the months of April through September. Farther south, the distribution is less dependent on season.