programmatically create thunder sound that covers specific duration

In a snow-simulation that I implemented there are thunderbolts in the background.

I never before worked with any generic sound effects.

What I am trying to ask is for a way to create a thunderclap sound programmatically so that I can define the duration and intensity of the clap and get a sound that matches the flash.

The flashes are random (there start, duration and the pause inbetween).

Its done with javascript and runs in a browser (html5+canvas).

That shouldn't matter though as I am not looking for any ready-to-use code but more for a concept, algorithmic idea or just basic information (links) on how to actualy start with such a task.

Maybe it would be an option to take a thunderclap sound and squeeze/stretch it? No idea on how to start with that either though ^^

Any hints are welcome.

In advance: Thank you all very much :-)