In the interest of assessing airfoil performance in heavy rain, a Lagrangian particle tracking scheme for a general body-fitted coordinate system has been developed and linked with a thin layer incompressible Navier-Stokes code. As raindrops impact an airfoil surface, splashed back droplets are accelerated by the air flow field, de-energizing the boundary layer and leaving it more susceptible to separation. Using an iterative particle-source-in-cell approach, a two-way coupled two-phase flow scheme has been developed to model this phenomenon. A body-force-like momentum source/sink term is added to the Navier-Stokes equations to account for the effect of particle drag on the air flow field. Results show a rain-induced decrease in airfoil lift due to premature flow separation that is qualitatively similar to that observed in experimental investigations.