Warm season precipitation corridors in the central United States

During the warm season in the central United States there often exists a corridor of precipitation where a succession of MCSs follow similar paths lasting several days. The total cumulative rainfall within a corridor can be substantial while nearby regions may be below normal. Understanding the nature of the corridors and the environmental factors important for their formation thus have important implications for QPF and hydrological studies. In this study a U.S. national composite radar dataset and model analyzed fields are used for the 1998-2002 warm seasons (July-August) to understand the properties of corridors and what environmental factors are important for determining when and where they develop. The analysis is restricted to a relatively narrow longitudinal band in the central U.S. (95o-100o W longitude), a region where convection often intensifies and becomes highly organized. It is found that ~68% of MCSs were members of a series and that corridors typically persist for 2-7 days with an extreme case lasting 13 days. Cumulative radar derived rainfall range from 8 cm to 50 cm, underscoring the fact that corridors can experience excessive rainfall. Combining radar with RUC model kinematic and thermodynamic fields, five-year composites are presented. While the corridors show the expected association with areas of enhanced CAPE and relatively strong northwesterly/westerly shear, the strongest correlation is with the northern terminus region of the nocturnal low-level jet (LLJ). Furthermore the relative intensity of the rainfall is positively correlated to the strength of the LLJ. In the five years analyzed, the large-scale environment varied considerably, but the role of the LLJ in the formation of corridors remained persistent.