The most important variable is repentance , where a people change their behavior or beliefs in a way that enables heaven to alter the consequences that would have otherwise come. This is why the warning of pending events is so important, for it can stir in mankind the necessary broken heart and contrite spirit to prevail upon God to both turn away the otherwise inevitable results and to bestow upon the people a new heart so that they might move beyond the tendency to repeat the pattern. The event does not necessarily have to unfold in order for people to learn the lesson that needs to be learned. The most notable example of this phenomenon is with Jonah and Ninevah. The best prophecies are those whose predictions do not come about because the people choose the path of repentance.
Timing is another variable.
Impending calamity can be accellerated or postponned, besides just being increased or diminished. I am of the belief that the severity of destruction that could have awaited this people has been significantly diminished because of the Lord's magnificent righteousness manifest in a good number of people who have stepped forward to influence the world for better. The Ghandis and Mother Theresas of the world, seen and unseen, impart a divine leavening. The mighty intercessory prayers of the humble on behalf of others do not go unheeded. So while some prophecies are sure, such as the coming forth of a new heaven and a new earth, and the punishment of those who refuse to repent, how these actually come about is up to us.