A joke I misunderstood

I first saw BTTF II when I was maybe 7. When they arrive in the future, Doc and Marty have this exchange.

Doc: "get out and change your clothes" Marty: "what, right now? It's pouring rain!'

Doc looks at his watch

Doc: "wait five more seconds."

Weather clears.

"Right on the tick. Too bad the Post Office isn't as efficient as the Weather Service!"

As a child, I thought Doc had some kind of device that controlled the weather. I've rewatched this movie many times and I did not revisit this idea.

I realized last night that the joke is that the joke there is that the weather is simply being predicted accurately but the Post Office is still a slow mess.

Well, now I know.

EDIT: What I have learned from this post and the response is that there are several interpretations of this scene out there 🤣