It's a passive form. Active: People suppose/expect/predict that it will rain. Passive: It is supposed to rain. In this case the subject of the passive sentence is the impersonal pronoun 'it' that we usually use for weather. You can also use it with other subjects. For example: 'I'm supposed to start work at 7am tomorrow, but I'm feeling really tired. I'll ask my boss if I can start at 8 am instead.'