According to my grammar, when we talk about predictions, "will" can be used when we are certain something is going to happen and "going to" for evidences. I think that when we are talking about evidences, we are certain something is going to happen. Am I not right?
When the forecasters forecast the weather, I believe they use "will". Well, they are almost 100 sure and they say this because they have some evidence about this because of the satellites and stuff. So why not "going to"?
When I am 100 % sure about predictions, I use "going to" and when about true facts I use will.
Look at the sky. It looks as if it's going to rain.
His birthday will fall on August 12th.