If the disaster promises to disrupt travel and services -- but you don't have anywhere to go and can comfortably survive the duration without the grid, staying put is often the right move.

If the disaster is that a Hellmouth is opening and will swallow the city up entire, the earlier you get out, the better.

Most disasters fall somewhere in between. I advised my mother to evacuate in advance of Hurricane Irene. She scoffed and wouldn't be budged. Fortunately, the effects were much less than predicted where she lives. Based on what we knew at the time, I still think she should have spent the weekend with friends inland -- she had no way of knowing that there wouldn't be widespread flooding.

With regard to urban environments, the problem is that they're hard to leave. Public transportation is incapable of moving a large percentage of the population out of the city, and highways are unable to handle the day-to-day load of commuters without major delays. Hordes of frightened evacuees can and do increase those delays tremendously. Leaving early may let you avoid the worst congestion, and may also let you find the most commodious shelter outside of the disaster-struck area. Wait too long and you run the risk of being unable to leave in time.

I live in the suburbs and it's easier for me to clear out than it is for most city dwellers. I would not hesitate to leave early if we had warning of something that could make staying home untenable.