There is no thunder sounds or even thunder 
in thunderstorm mode versus rainstorm. On the other hand rainstorm doe have thunder sounds and some small flashes. I was expecting on thunderstorm to be like Forza Motorsport 7 where there is actually thunders in the sky and you can clearly see them even on car reflection. I don’t know if this is a bug or intended. @JetPartySalad if you want video i can provide.