The be going to form used for predictions:

This form can express the speaker’s feeling of certainty. The time is usually not mentioned, but the action is expected to happen in the near future:

e.g. Look at the clouds! It’s going to rain

It can be used in this way after such verbs as be sure / afraid, believe, think:

e.g. How pale that girl is! I am sure / I believe / I think she is going to faint.

Comparison of be going to (used for prediction) with will (used for probable future):

will is a common way of expressing what the speaker thinks, believes, hopes, assumes, fears, etc. will happen:

e.g. It will probably be cold / I expect it will be cold

e.g. Tomatoes will be expensive this year / I’m sure tomatoes will be expensive

will and be going to are therefore rather similar and often either form can be used:

e.g. It will take a long time  to photocopy all the documents = It is going to take a long time to photocopy all the documents

But there are two differences:

1. be going to implies that there are signs that something will happen, will implies that the speaker thinks / believes that it will happen.

2. be going to is normally used about the immediate / fairly immediate future; will doesn’t imply any particular time and could refer to the remote future.

e.g. The lift is going to break down (implies that it is making strange noises or behaving in a strange way)

e.g. the lift will break down (implies that this will happen some time in the future, perhaps because we always overload our lifts, perhaps because it is an XYZ Company lift and they don’t last)