High Power Microwave (HPM) induced surface flashover is currently being investigated in order to gain a better understanding of the underlying processes involved and reduce the limitations it places on transmittable pulse lengths. The present experimental setup is designed to produce a flashover on the high pressure side of a transmission window without the influence of a triple point. A 2.5 MW magnetron produces a 900 ns pulse at 2.85 GHz with a 50 ns rise time. The experimental setup allows for the control of several parameters including gas pressure, gas composition, and external UV illumination of the window. Diagnostic equipment enables the analysis of incident, reflected, and transmitted power levels with sub-nanosecond resolution. A previously developed Monte Carlo simulation is used to model the processes involved in the flashover discharge formation. This Monte Carlo code is upgraded to account for the occurrence of field induced electron detachment from negative ion clusters within the high field region, >10 kV/cm, near the window. The code has also been expanded to include the occurrence of photoelectrons, emitted from the window while under UV illumination. Such illumination of the transmission window was experimentally shown to reduce the time to flashover by over 100 ns in air at 155 torr, and thereby the total pulse energy that can be transmitted. In addition, UV illumination also reduces the variation in flashover delay times from shot to shot, up to 67% in air at 155 torr. The simulation will determine if the observed reductions in delay time and variation can be explained by the addition of initiatory electrons via UV illumination of the surface.