A tiny subset of a medieval city's million-person population is aware of an imminent magical cataclysm via dreams or visions. They are unaware of each other.
Magic in the world is rare and limited. Some of these individuals might have magical power, but it's destructive in nature - no mind control, illusions, anything like that. It's tightly controlled and rare enough to the point of many not even believing it exists, making it even harder to convince people.
The city is the capital of an empire. Those ruling it will value stability and work against any 'unfounded' rumors that would cause panic.
What's the best way for them to convince the most people to evacuate in the span of one or two weeks?