It is being conduced in Brazil the CHUVA (Cloud processes of the main precipitation systems in Brazil: A contribution to cloud resolving modeling and to the Global Precipitation Measurement – GPM) experiment which aims to map the main precipitating systems in the country. So far, experiments have been performed for two sites with various measures including those with a dual polarization radar, lidar, microwave radiometers, disdrometer, radiosonde, rain-gauge and various other instruments. During the experiments it was observed that the operational forecast did not show good results. It is also known that some developments have being made in the concern of radar data assimilation and many papers point out some good improvements on forecasting the precipitation amount. We have made some experiments using 3D-Var for assimilating radar reflectivity and radial velocity and we have gotten some improvements on forecasting the precipitation amount. Now, in this work, we aims to assimilate the dual polarization radar data from CHUVA campaign (among other conventional data like rain-gauges and radiosonde) for a specific squall line that have passed over Belem in order to improve the analysis of precipitation and compare with previous work. We propose to use the WRF model with 4D-VAR to achieve the best analysis for precipitation and other dynamical and thermodynamical variables. The Fractional Skill Score (FSS) is applied to compare quantitatively our results against the one from operational forecast and observations from other instruments. The results show improvements on forecast with radar data assimilation when compared with both the control one (i.e., without assimilation) and with the 3D-Var experiments. Among other aspects, these results are also important for providing input information to other CHUVA groups interested in hydrological and radiative models.