Quasi-Stationary, Extreme-Rain-Producing Convective Systems Associated with Midlevel Cyclonic Circulations

This study identifies and examines the common characteristics of several nocturnal midlatitude mesoscale convective systems (MCSs) that developed near mesoscale convective vortices (MCVs) or cutoff lows. All of these MCSs were organized into convective clusters or lines that exhibited back-building behavior, remained nearly stationary for 6-12 h, and produced locally excessive rainfall (greater than 200 mm in 12 h) that led to substantial flash flooding. Examination of individual events and composite analysis reveals that the MCSs formed in thermodynamic environments characterized by very high relative humidity at low levels, moderate convective available potential energy (CAPE), and very little convective inhibition (CIN). In each case, the presence of a strong low-level jet (LLJ) and weak midlevel winds led to a pronounced reversal of the wind shear vector with height. Most of the MCSs formed without any front or preexisting surface boundary in the vicinity, though weak boundaries were apparent in two of the cases. Lifting and destabilization associated with the interaction between the LLJ and the midlevel circulation assisted in initiating and maintaining the slow-moving MCSs. Based on the cases analyzed in this study and past events described in the literature, a conceptual model of the important processes that lead to extreme rainfall near midlevel circulations is presented.