During the warm season, precipitation traveling eastward across the central U.S. is often confined to relatively narrow latitudinal corridors for periods lasting several days. Because of the persistence of the corridors and their relatively slow N-S drift, cumulative precipitation in such regions often exceeds the climatological mean by several fold. Thus, understanding the nature and forcing mechanisms of the corridors has important implications in quantitative precipitation forecasts.

Using the WSI corporation NOWrad national radar composites and the RUC (Rapid Update Cycle) model analysis for the 1998-2003 warm seasons, the nature of the precipitation corridors are documented. The corridors typically presist 5-7 days and produce 15-30 cm of total rainfall. In some instances a clearly defined synoptic scale front can be associated with the precipitation corridor, but in many cases the synoptic conditions are quite weak (particularly in July and August) and no obvious forcing features are evident. By analyzing the radar and RUC data in reduced dimensions time-latitude or time-longitude format (Hovmoller plots) and averaging over several seasons, kinematic and thermodynamic signatures in the RUC fields that can be related to the precipitation become evident, even in weak synoptic conditions.