A local heavy rainfall event in Ningshan, Shaanxi Province on 28-29 August 2003 is diagnosed. The results show that the event occurred in a weak convection condition with convergence at high and low-level against strong convection. Vortex and inverted trough at 850hPa to 700hPa supplied plentiful vapor and powerful updraft. Several air currents from different way with different characters encountered in Ningshan, strong vapor convergence is a main factor of this event.