Using the Advanced Research WRF (ARW WRF) model and the Gridpoint Statistical Interpolation (GSI) three-dimensional variational analysis (3DVAR) system, the impact of assimilating ATOVS (Advanced TIROS Operational Vertical Sounder) radiance through the prototype Community Radiative Transfer Model (pCRTM) is evaluated on the forecasting of a heavy rainstorm occurring over the central Guangdong province in the southeast of China on 20-21 June 2005. A pair of comparison experiments (NODA and DA) for this case is conducted with multiple configurations, including nesting domains with 4-km and 12-km grid distances. The results showed that by changing the initial condition through data assimilation, a modified divergence and moisture field with the structure of dipoles has been added to the axis of the rainband with a southwest-northeast orientation. When more moisture carried by a southwesterly low level jet (LLJ) was converged into the northeast portion of the rainband around the observatory station of Longmen, the amplitude of moisture static energy (MSE) increased substantially at low levels much more than at middle levels, resulting in the enlarging of differences in MSE between 500 hPa and 850 hPa; the atmosphere became more unstable. Consequently, the convective rainfall increased in the northeast part of the province around the Longmen station, which was consistent with the observed distribution of rainfall.