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How do you modify a thunder strobe to make it realistic?

I have  a Cannon Thunder Strobe w/sound effects.  I would like to make it act more like a real thunder/lightening storm.  The flash from the strobe and then the roll of thunder.  Right now, out of the box, it just flashes normally and the effect plays.  It does not look like a real lightening flash with thunder.  It needs to have a major pause between lightening flashes so the thunder can roll, then silence and darkness.  I have seen it done in Halloween Spirit stores, but the employees did not know how it was done.  It was fixed up by someone else.

Any help will be appreciated.

Jeff