A small squall-line developed rapidly on the south side of the MCS. Concurrent with this rapid development were observations of a sharp vertical gradient in radar reflectivity across the melting layer, in an area with low relative humidity. This rapid development was also concurrent with an intensification of the forward and downward flow of the rear inflow jet, as revealed from the quad-Doppler data. Microphysical data from the 2D-C and 2D-P probes during a spiral descent across the rear inflow jet revealed that ice was present down to 8 °C. Strong latent cooling may have resulted in negative buoyancy, thus enhancing the downward flow of the rear inflow jet. In this paper, the radar, kinematic, thermodynamic and microphysical structure and evolution of this MCS will be presented.