The Air Force Research Laboratory High Power Microwave Division has developed a vector and parallel processing electromagnetic finite difference time domain (FDTD) computational Temporal Maxwell (TMax) solver. FDTD is a robust technique well suited for vector and parallel processing yielding a useful electromagnetic modeling tool. In this paper, TMax simulations of the Impulse Radiating Antenna (IRA) via the Cray X1 high performance computing machine with state-of-the art vectorizing FORTRAN compilers are shown to produce high resolution results for a sizable volume generating good agreement with experimental data thereby providing a way to assess different IRA configurations.