In addition,
be going to is used to predict the future when we have already got some evidence that something is certain or likely to happen. When this evidence is not present, or at least is not as concrete,
will tends to be used.
- It's going to rain later (I can see the clouds building up)
- I think it will rain later (It often does at this time of year)
Source: learnenglish.org.uk
. . . the weather forecast says
it's going to rain.
