My dog is barking at the lightning, how can I calm him?

He’s generally an outdoor dog (he has plenty of shelter, Garage and porch), and storms haven’t noticeably bothered him in the past, but now there’s pretty heavy rain and loud thunder and every time it goes off he runs out of the garage, growling and barking, then returns quickly and sits behind my leg. If I go inside he whines at the window. I know he’s fine but he’s never been this distressed about anything before. He’s a yellow lab. Can anyone help or explain why he’s suddenly so nervous?