The 24 h extreme heavy rain or heavy rain event forecast over Southern China was issued by National Meteorological Centre at 06:00 BT on 22 April 2016. But the corresponding observations show that the accumulated precipitation only reached light rain grade. Using the routine sounding and surface observations, ERA-Interim reanalysis data and ECMWF forecast products, we investigated the cause of the false prediction of this event, and compared it with another heavy rain event occurred in same area. The results indicate that with respect to the common synoptic pattern characterized by the high-altitude trough moving eastward and the developing of the southwest jet at front of the trough when Southern China experiences heavy rain, there were some dynamic and moisture conditions that is not conducive to the occurrence of heavy rainfall from 08:00 BT 22 to 08:00 BT 23 April 2016, including weak divergence in the low-level, low specific humidity, shallow wet layer, lack of consistent ascending motion throughout up and down levels, and low height of water vapor flux convergence. After a short-period severe convective weather event in Southern China from 20:00 BT 21 to 08:00 BT 22 April 2016, energy conditions such as low K-exponent, convective available potential energy (CAPE) and Si index, and northerly concentrated θse (pseudo-equivalent potential temperature) isoline area and weak convective instability all are not conducive to the occurrence of heavy rainfall. Compared to observation, the forecast from the numerical models to this event showed a large error. Especially, the low relative humidity above 700 hPa is not conducive to the occurrence of extreme heavy rain, but the precipitation area can be to some extent revised by it. Analysis of comparison event shows that if weak divergence appears in the mid- and low-level, strong precipitation can also occur so long as the moisture condition is good enough. Therefore, environment conditions, such as specific humidity of 2-3 g·kg-1 more than the mean in the mid- and low-level, reasonable triggering conditions in boundary layer, deeper updraft and strong convective instability, all should be considered when forcasting heavy rain in the warm sector in spring in Southern China.