Corridors of warm season precipitation 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 mesoscale convective systems (MCSs) follow similar paths lasting several days. The total cumulative rainfall within a corridor can be substantial while precipitation at nearby regions may be below normal. Understanding the nature of the corridors and the environmental factors important for their formation thus has important implications for quantitative precipitation forecasting 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 United States (95 degrees-100 degrees W), a region where convection often intensifies and becomes highly organized. It is found that similar to 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 maximum rainfall ranges from 8 to 50 cm, underscoring the fact that corridors can experience excessive rainfall. Combining radar with Rapid Update Cycle model kinematic and thermodynamic fields, 5-yr composites are presented and stratified according to the environmental conditions. While the corridors show the expected association with areas of enhanced CAPE and relatively strong northwesterly/westerly shear, the strongest association is with the northern terminus region of the nocturnal low-level jet (LLJ). Furthermore, the relative intensity of the rainfall is positively correlated with the strength of the LLJ. The LLJ is thought to play a role through enhanced convergence and lifting, moisture transport, and frontogenesis. In the five years analyzed, the large-scale environment varied considerably, but the role of the LLJ in the formation of corridors remained persistent.