Abstract: A large hail-producing supercell developed ahead of a severe squall line around Shanghai, China on 5 June 2009. The supercell and its interaction and relationships with the squall line are analyzed using the conventional weather observations, Doppler radar data, wind profiler data, and automatic weather station data. The results show that the supercell initiated and developed in the warm sector ahead of the squall line, in a strong unstable environment which was caused by strong cold advection at high levels over warm advection at low levels. The storm intensified while passing through a surface convergence zone induced jointly by an urban heat-island effect and a sea-breeze front. Some characteristics of a classic supercell were found, such as low level hooks and pendant echoes, the weak echo region, and a meso-scale cyclone, etc. A Three Body Scatter Spike (TBSS) representing large hail in the sky was detected extending downrange behind a reflectivity (above 60 dBz) core. Descending and ascending large hail areas are detected based on the velocity of TBSS. An upper trough of a cold vortex in northeastern China brought strong cold advection and pressed the freezing level to lower. This provided a suitable ambient temperature structure for the hail embryo enhancement. Dual-wind profiler observation comparisons show that the ambient wind field of the supercell could produce more low level vertical wind shear and storm relative helicity. On the other hand, the mesoscale convective system (MCS) might change the ambient wind structure. As a result of the interaction of the MCS and the environment, a positive feedback effect made the MCS into a long lived system. The supercell’s motion implied the motion of the squall line with a leadership mechanism of an “arrow” relative to a “bow” suggested. After arriving at the supercell’s cold air pool, some parts of the squall line decayed gradually. Finally, the supercell weakened and its supercell characteristics disappeared as it moved over the East China Sea. The squall line combined eventually with the remains of the supercell storm to become a line echo wave pattern (LEWP), and the bow echo of the LEWP changed its motion direction from the northwesterly to the northerly. A supercell ahead of a squall line might provide some useful operational nowcasting guidance for squall line motion and evolution.