This is not foreign to the Biblical text. Jeremiah 18 says exactly what God will do in these situations: If a nation turns from evil, God will not destroy her; if the nation does not turn from evil, God will destroy it.
Let us go back to Jonah 3. In 40 days God will destroy Nineveh, but Nineveh repented and God turned His wrath away. Prophecy works in different ways so we cannot assume that every time God says something it is related to His eternal decrees. We maintain that when God decrees something, no human action will change His plan.
There are two primary forms of prophecy:
a) Some prophecies are promises of what God will do. They are connected to a divine oath. They are tied to covenant promises: a) The prophecies of a Messiah, b) the promise of a Final Judgment, c) the recreation of the world, etc. These are not contingent on man’s response.
b) The second form of prophecy is relational or covenantal. God’s relationship to His creatures may change over time. We need to understand that prophecy is not future-telling; prophecy is ethical and evangelistic. If we approached all prophecy in a que sera sera manner, we might as well be Muslims. The God of Islam does not relate to His people, but the God of the Bible is a relational, covenantal God. So in Jonah 3 God is giving them a conditional threat that did not necessarily need to happen. There is an implicit condition in the threat.
Listen to Calvin’s words in the Institutes:
Who now does not see that it pleased the Lord by such threats to arouse to repentance those whom he was terrifying, that they might escape the judgment they deserved for their sins? If that is true, the nature of the circumstances leads us to recognize a tacit condition in the simple intimation. Institutes 1.17.14
There is a tacit condition implied. God does not operate like an unmoved mover, but He operates as a heavenly Father. Pastor Rich Lusk says:
God is passionate, involved and consistent, but you never know what He is going to do next. We are not to de-personalize God. God is jealous; He expresses joy and delight; he shows patience and wrath and even repentance.
Does God change His mind? What does Jonah 3:10 mean?
We are to learn two important truths:
a) God’s eternal decree does not change. God can’t be surprised or mistaken and He is not dependent on His creatures in anyway.
b) But on the other hand, God does change His mind in the sense that He is involved, engaging and He is in a personal relationship with us. He is intimate. In the end, we better hope that God changes His mind, because this is the heart of the gospel. This is the point of Nineveh where God turned wrath into grace; and God changed His mind about us as well. God changed His mind at the cross. God relented at the cross. He decided not to send us all to hell, but to give us eternal life through Christ our Lord.
 Sermon series on Jonah 3.
 Quote from Lusk.