A severe convective case, occurring in Shanghai on 30 July 2005, is examined with the composite datasets from WSR88D Doppler radar, automatic weather stations and mesoscale numerical model WRF. It is found that the occurrence of mesoscale convergence lines was mainly induced by the southward intrusion of weak cold front and the preexisted local mesoscale baroclinic instability. The growth of these mesoscale boundary layer disturbances within the convergence lines was favored by the dynamical cooperation among high gradient of moisture, strong vertical wind shear, and energy front within the lower troposphere, resulting in the abrupt growth of the convection. Mechanisms for the simultaneous occurrence of hail, heavy rainfall and gale during this severe convection are also analyzed.