am going to sneeze...........reality(seen evidence)
you are to...............
We use be to + infinitive when discussing formal or official arrangements or to give formal instructions or orders and your examples illustrate these aspects very well. Further examples might be:
- The Prime Minister is to make a further visit to Devon next week.
- We are to receive a pay rise in line with inflation in September.
- I don't mind her going to Ruth's party but she's not to be back late.
- You are not to leave this house without my permission. Is that clear?