Quasi-stationary, extreme-rain-producing convective systems associated with midlevel cyclonic circulations

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.