Surface flashover induced by a High Power Microwave fast rise-time pulse causes a significant drop in transmitted power, along with reflections that can damage the source. Momentum transfer collision rates in the range of 100s of GHz (for pressures exceeding 5 kPa) lead to low plasma conductivity, corresponding to absorption levels of up to 60 % of the incident power. A simulation algorithm was developed using the finite-difference time-domain (FDTD) method in order to model the growth and transport of the electron density near a dielectric surface, and the resulting interaction with the microwave pulse. The time-dependent plasma parameters are governed by empirical and simulated scaling laws for ionization and collision rates, along with diffusion coefficients; the resulting frequency-dependent plasma permittivity is transformed to a discrete algorithm to describe the spatially resolved plasma in the FDTD algorithm. A plasma thickness of up to 2 mm is simulated that compares with side-on ICCD imaging of surface flashover. Breakdown parameters, such as delay times and breakdown electric fields, in nitrogen, air and argon, are compared with experimental data on surface flashover across a polycarbonate window at atmospheric pressures; the simulated results correlate well with measured, and the model exhibits low computational complexity when simulating a pulse on the order of microseconds, making it a good alternative to standard particle-in-cell codes. The source is a S-band magnetron that produces a 2.5 MW peak power, 50 ns rise-time pulse with 3 μs duration at 2.85 GHz center frequency.