For parabolic equation modelling of forward scatter over a rough surface, we are concerned with the average field, corresponding to specular reflection. Most applications involve a non-homogeneous propagation medium, a situation which is extremely difficult to treat. Although some progress has been made on rough surface scattering in idealized ducting environments, there is no rigorous solution in general. Moreover in the case of propagation over the rough sea surface, which constitutes the most important application, insufficient information is available on the complex boundary layer phenomena which cause both the refractive effects and the rough surface formation. Further complication is added by the fact that it is usually very small grazing angles which are of interest for long-range radiowave applications.