Rashi explains: When G-d began the Flood, He first made it rain mercifully – as regular rain – so that if the generation would repent, the falling water would become normal, seasonal rain.
When they still did not repent, the rain became a flood. Although G-d had warned the people about the impending Flood 120 years earlier, and seeing Noah build the ark this entire period was a constant reminder of G-d’s warning, the people still did not repent during that entire span of time.
Even after watching Noah enter the ark and witnessing the accompanying miracles, they still did not repent. And yet, if they had repented at this point – even after the rain started – not only would they have been spared the Flood, they would have been showered with rains of blessing.
This demonstrates the incredible power of repentance, which has the power to transform all misfortune into good, even in a split second.
—from Daily Wisdom3