Flash-flood-producing convective systems associated with mesoscale convective vortices

This study identifies and examines the common characteristics of several nocturnal midlatitude mesoscale convective systems (MCSs) that developed near midlevel 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 significant 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. None of the MCSs formed along a front or other preexisting surface boundary. Instead, lifting and destabilization associated with the interaction between the LLJ and the midlevel circulation assisted in initiating and maintaining the slow-moving MCSs.