“Aquinas is emphatic that God’s will is unchangeable. And so words ascribing change to God, or words ascribed to God which entail change:

have a metaphorical turn according to a human figure of speech. When we regret what we have made we throw it away. Yet this does not always argue second thoughts or a change of will, for we may intend in the first place to make a thing and scrap it afterwards. By similitude with such a procedure we refer to God having regrets, for instance in the account of the Flood, when he washed off the face of the earth the men whom he had made; to speak of God as repenting is to use the language of metaphor.… The conclusion to this argument is not that God’s will changes, but that he wills change.

Thomas Aquinas, Summa Theologiae, trans. Thomas Gilby (London: Spottiswoode, 1966), 1a.19.7.”