From The Openness of God:

But the biblical descriptions of divine repentance indicate that God’s plans are exactly that – plans or possibilities that he intends to realize. They are not ironclad decrees that fix the course of events and preclude all possible variation. For God to will something, therefore, does not make its occurrence inevitable. Factors can arise that hinder or prevent its realization. Consequentially, God may reformulate his plans, or alter his intentions, in response to developments.