Might not be exactly what you are looking for, but I think I remember doomsday having a white light flash after the rocket explodes.
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I'm finding an env_screenoverlay, with two logic relays that fire to the env_screenoverlay as well as an env_tonemap_controller. What I'm assuming is happening is that they are using the tonemap_controller to do the fade in and out to the white screenoverlay...so I don't know if it will be fast enough for a lightning flash, but you could probably try (the current duration is something like 0.2 seconds, so you could reduce by half to made achieve the affect you are looking for).
There is a lot of particularly technical stuff in there that I don't fully understand, but if you have time you can try playing with the settings.
These can again be found in sd_doomsday.
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As for the sound, I bet there is some sound file around somewhere of thunder that you'd just set to play at whatever time after the lightning flash through another logic_relay. (Come to think of it there is probably something from HL2 (so just checking vdc found some HL2 lightning sprites and sounds, but that again is HL2:
https://developer.valvesoftware.com/wiki/Lightning))