The present progressive can refer to a future situation that has been arranged before the present time. The arrangement continues through the present until the situation occurs:
Emma is seeing Luke tomorrow, (They arranged this meeting yesterday)
Note that only the context or co-text (in the last example the word tomorrow) can tell us which time-period a present progressive verb form refers to.
Also note that it is simply not possible to make arrangements for some future situations. It is therefore not normally possible to say XIt is raining tomorrow. X