In addition to the possibility linked to the surety prediction you advocated in the previous replies, you might also argue that you will never know with a 100% confidence that the flight will actually leave at 6pm today.

If the airline is taking a train at 6pm tomorrow (like the rest of the

flights, the flight schedules to make it available) the technically correct usage would be – “The flight is scheduled to leave at 6pm today”. “As

per the schedule, the flight departs at 6pm tomorrow.” “But

for all purposes of common usage, the sentence you quoted in the question suffices for audience communication.

If there is a situation in which Earth is raining, it would sound appropriate, coming from a soothsayer, an oracle or an oracle, or a psychic. My question would be the same. What would you suggest to them? I guess it is within their business obligations to use such sentences to sound mighty-sure and give themselves an aura of invincibility against nature’s vagaries.

Would I be at a disadvantage to see or hear such sentences?