A full-wave Finite-Difference-Time-Domain (FDTD) model is developed to evaluate the propagation effects of lightning electromagnetic fields over mountainous terrain in the earth - ionosphere waveguide (EIWG). The obtained results are validated against simultaneous experimental data consisting of lightning currents measured at the Santis Tower and electric fields measured in Neudorf, Austria, located at a 380 km distance from the tower. The waveform of the lightning-radiated electric field at 380 km includes both a ground wave and a reflected sky wave. The presence of mountainous terrain mainly affects the time delays and amplitudes of the ground wave, while the skywave parts mostly depend on the effect of the ionospheric cold plasma characteristics.