An experimental and numerical investigation targeting the establishment of the controlling parameters of ignition in a high-speed reacting flow was performed. Nanosecond-gated Laser-Induced Breakdown Spectroscopy (n-LIBS) was used to quantify fuel concentration at multiple locations within a cavity-based flameholder and validate high-fidelity non-reactive hybrid RANS/LES simulations. The n-LIBS technique simultaneously provided an ignition source and was used along with the simulations to define an a priori ignition probability. The ignition probability based upon fuel concentration at the point of energy deposition provided a reasonable first level comparison to experiments. Inclusion of where the ignition kernel was being transported within the cavity based upon fuel concentration provided improved agreement to experiments. More controlling parameters, such as the velocity and shear at the energy deposition location and along the path taken by the ignition kernel need to be considered to better replicate the experimental results and provide a more robust ignition probability mapping.