In this paper, Global Positioning System (GPS) derived slant path residual values are studied with respect to a rainfall event. Slant path residuals can be defined as the difference between the slant path wet delay at a particular elevation angle and the zenith wet delay projected to that elevation angle. The slant path residual values are corrected for multipath using multipath stacking algorithm. The corrected residuals show a good correlation with a rainfall event. It is observed that the standard deviation (SD) of the residuals is higher during a rainy time period. The slant path residuals can be used as an added feature in the existing algorithms to improve the rainfall prediction results.