'Will' or 'going to'?

Subject line said 'write a catchy title here'. I ignored it. Now the answer.

Question- Which is correct? 'Will'or 'going to' or both?

It'll probably rain later.

It's probably going to rain later.

Answer:

Both.

Why?

Well, you can use both of them for predictions. There's really no difference between them here, but 'going to' makes it sound like you've got some particular evidence for it. Please make a comment if you want to get really pedantic about it.