Abstract: Under the control of a Western Pacific subtropical high, a local short-lived severe precipitation occurred in Shanghai in the afternoon of August 19, 2016 that lasted 3 hours in a range of 20 to 40 km. Based on 2-minute average ground automatic meteorological observations, the ageostrophic Q vector diagnostic method, which was only based on a single layer data, is used to diagnose the cause of the precipitation. The main results are as follows: (1) Analysis of surface air temperature overlapped with winds shows that the heat contrast between the "urban heat island" and the adjacent of Yangtze River leads to alongshore-wind, which transports moist and cold air above the river banks to the urban land, which meets with the dry and warm air there, causing the local short-lived severe precipitation to appear. The occurrence of precipitation then cools the surface air temperature, weakens the "urban heat island" effect, dampens land-water temperature contrast and thus weakens the alongshore winds, which directly cripples the dynamical and thermal forcing conditions conducive to precipitation and then results in its decline. (2) Analysis of ground convergence of Q vector divergence field overlapped with the difference of dew-point shows that during the developing stage, the convergence of Q vector divergence forces relatively strong vertical ascending motion, while the moisture condition in the ascending air is weak. During the enhancing stage, the convergence of Q vector divergence and air moisture not only becomes strongest, but also the ascending region is coincided with the high-humidity region. During the decaying stage, the air moisture remains high while the convergence of Q vector divergence is weak. Analysis of Q vector divergence superposed with the difference of dew-point reveals the inherent factors which contribute to the development and evolution of precipitation, and their coincidence region is a good indicator for the precipitation area. Finally, there is a discussion on the potential application of the convergence of Q vector divergence to nowcasting of local short-lived severe precipitation in the future.