On 5 August 2001, Shanghai was struck by a torrential rainfall coming together with a tropical depression reentering the sea. The rainfall intensity is the strongest in recent 50 years. A set of meso-scale re-analyses data is used to understand the physical mechanism of such a heavy rain. Results show that the outburst of a southerly jet in low atmosphere triggered the explosive development of cyclonic vertical vorticity in the region with steep potential temperature surfaces in the front of a moist and warm air mass. That was slantwise vorticity development. While in the upper atmosphere, cyclonic vorticity increased notably as the air flew from west to east and entered a region with small vertical stability. The simultaneous sharp development of cyclonic vorticity in both low and upper atmosphere should be the main cause for the torrential rainfall.