A new WRF-3DVAR system has been developed. The final goal of our project is to investigate the impact of assimilating ground-based GPS Precipitable Water (GPS-PW) data using the WRF-3DVAR system on a heavy rain case. In the present study, we simulated a squall line observed during the International H2O Project (IHOP) that took place over the Southern Great Plains using WRF with 10km horizontal resolution. Firstly, as a preliminary study, a suitable cloud scheme, the Lin model, was selected by performing a number of the sensitivity tests of the WRF model to cloud microphysics and cumulus parameterization schemes. Secondly, we investigated the impact of GPS-PW assimilation with WRF-3DVAR for this case. The assimilation reduces the difference in PW between the model and observations, and decreases the overestimation of the precipitation amount through an interaction between adiabatic thermodynamic process and convergence of water vapor flux. The movement of the squall line is also slower than that without assimilation and in better agreement with observations (e.g., score is 5% better). However, in some areas the assimilation of GPS-PW increases the error when compared with observed precipitation.