At about 16:00 BT 4 September 2016 the urban area of Hangzhou, West Lake and the surrounding area received a sudden convective shower, which caused great impacts on preparation related to “Hangzhou G20 Summit”. This paper analyzes the mechanism of the shower, and discusses limitations of shorttime forecasting and difficulties of nowcasting. The results indicate that the mainly focusing convection system at that time over Ningbo, eastern Hangzhou area, and Shaoxing did not directly affect Hangzhou City, and eastwardmoving synopticscale upper trough system did not affected this area either. However, under the weakly statically unstable conditions, the shower was produced by a shallow convective system which was triggered by an afternoon sea breeze front with a dry line in the northwest Hangzhou Bay. As this convective system had some characteristics such as small spatial scale, short life, rapid movement, low cloud top, weak reflectivity and intensive rainfall intensity, together with a large number of highlevel cirrus clouds which cannot effectively be discriminated from cumulus over Hangzhou and the surrounding area in the satellite images, the ability of weather radar and stationary meteorological satellite monitoring the system was weakened significantly, so that nowcasting the shower is also very difficult if only using these two types of observations. Therefore, it is crucial to use automatic weather station data with high spatiotemporal resolutions to analyze the environmental conditions for nowcasting the type of shallow convective weather systems.