In this study we investigate mechanisms responsible for heavy precipitation within a long-lived convectively generated mesoscale vortex (MCV) using analyses from the Rapid Update Cycle (RUC) and cloud-resolving numerical simulations the PSU/NCAR mesoscale model (MM5). The MCV case occurred over the south central plains and southeastern United States during a period of several days (27-29 May 1998) and produced multiple episodes of flash flooding.

Heavy precipitation that was associated with the MCV occurred within a conditionally unstable environment that possessed large lower-tropospheric relative humidity and weak vertical wind shear. Deep convection within the MCV circulation followed a distinct diurnal cycle, with individual MCSs during the multiday event initiating during the early evening within approximately 100 km of the MCV center, on its downshear side. For each cycle, convection reached maximum organization and intensity near sunrise, before dissipating by the following afternoon. The highly organized nocturnal convection near the center of the vortex is crucial in reinvigorating the MCV, thus allowing it to persist for several days in the presence of differential advection by the ambient shear. Surface heating appears to play role in the initial formation of convective systems at the edge of the MCV-induced midtropospheric cloud shield, while moisture transport by the nocturnal LLJ may influence the longevity of the individual convective systems. Throughout its lifecycle, the quasi-balanced lifting, arising from the interaction of the vortex with the ambient vertical shear, determines where within the MCV circulation long-lived organized convection occurs.