if it thunders, there is a loud noise in the sky, usually after a flash of lightning
2 [intransitive always + adverb/preposition]CS
to run or move along quickly, in a way that makes a very loud noise:
The children came thundering downstairs.
Huge lorries thundered past us.
to make a very loud deep noise:
Guns roared and thundered all around us.
to shout loudly and angrily:
'You must be mad!' he thundered.