Abstract: Using a variety of unconventional observations,such as regional weather stations,island stations,wind towers,wind profiles and Doppler weather radars,and NCEP/NCAR 0.25×0.25° reanalysis data,on the west coast of Guangdong in the morning of June 22,2017,a comprehensive diagnosis was made for the cause of the local heavy rain event in which the forecast failed,focusing on the impact of local sea-land breeze and topography (mutual) on the event. The results show that:(1) The rainstorm occurs under the weak weather environment. There is no frontal nor high-altitude trough activity in South China. The ultra-low-level southward jet in the boundary layer provides unstable energy for the rainstorm. The thermal difference of the underlying surface with different properties leads to the mesoscale convergence line of the sea-land breeze (southerly) and northerly winds in front of Tianlu Mountain,triggering the initial convection in the key area. (2) When the warm and moist air in the south is blocked to the north,it develops into MCS under forced uplift of Tianlu Mountain topography. The difference in thermal friction between the underlying surface may exist between the mesoscale vertical shear and the precipitation forced by the piedmont. Positive feedback phenomenon extended the life history of β mesoscale convective system. (3) Convective cloud clusters are excited continuously on mesoscale convergence line,forming train effect of precipitation,which leads to this rare local heavy rain event,highlighting the important role of sea-land breeze circulation in this heavy rain in warm sector. (4) Before the occurrence of this event,all the objective numerical forecasting models in service did not predict significant precipitation. The numerical model is difficult to make short-term prediction precipitation with grade above storm in warm zones,especially under the weak weather conditions. Current monitoring and short proximity warnings are the main tools for forecasters.