In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, surface flashover across a magnesium fluoride (MgF2) window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. That is, neither electrode was favored with respect to flashover/breakdown initiation. A semiconductor-switched 32 kV pulse with 140 ns rise time was applied to the 8 mm wide flashover gap in air, nitrogen, and oxygen at atmospheric pressure. Fast voltage and current measurements along with nanosecond imaging revealed four stages of flashover development: (1) Onset of a cathode directed streamer with a charge on the order of 100 pC and traveling with a speed of 1 mm/ns at a macroscopic field level of ∼10 kV/cm associated with a slow current rise (on the order of 10-3 A/ns) temporarily augmented by (2) a 5 ns wide current spike at the moment when the streamer reaches the cathode followed by (3) a cathode directed streamer focused toward the center of the flashover gap with a slow rising current leading to (4) a sharp current rise (on the order of 10 A/ns) reaching roughly a circuit limited 45 A about 20 ns after the return strike meets an anode directed streamer. Although present in all tested gases, the current spike at the end of stage (2) is most different for all three gases and having the greatest impact in air.