An event of record breaking extreme precipitation produced 3000 mm/day of accumulated rainfall over southern Taiwan in August 2009. Theinteractions between Typhoon Morakot and the prevailingsouthwesterly (SW) monsoon are the primary mechanism for this heavy precipitation during 5-13 August 2009. This extreme precipitation could be produced by the abundantmoisture from SW monsoon associated with the interaction between typhoon and monsoon wind fields, and then leads to severe damages. The accurate mapping of extremeprecipitation caused from the interaction between monsoon and typhoonis a critical for early warming in Taiwan. In this study, the heavy rainfall event is simulated based on Weather Research and Forecast system model (WRF) with three nested domains configuration. By means of data assimilation with virtual meteorological field using 3D-Var system, such as wind field to alter SW monsoon strength in the initial condition, the impacts of intensified convergence and water vapor content on the accumulated rainfallare analyzed for quantizing the intensification of typhoon rainfall potential (TRaP). The results showed the positive correlation between the enhanced precipitation and the intensity of low-level wind speed, convergence as well as water vapor content. For the case study of typhoon Morakot, it could cause the rainfall for approximately 2×104 mm at 6 hours intervalinsouthern Taiwan area when 10×10-6 s-1 convergence intensified at 850hPa level around the southern part of Taiwan Strait. The results suggested that the low-level wind speed, convergence and water vapor content play the key roles in the rainfall potential of typhoon coupled with SW monsoon.