Noah was six hundred years old when the floodwaters came on the earth.

Genesis 7:6=8:19
Noah was six hundred years old when the floodwaters came on the earth. And Noah and his sons and his wife and his sons’ wives entered the ark to escape the waters of the flood.  Pairs of clean and unclean animals, of birds and of all creatures that move along the ground, male and female, came to Noah and entered the ark, as God had commanded Noah. And after the seven days the floodwaters came on the earth.
The waters flooded the earth for a hundred and fifty days.
But God remembered Noah and all the wild animals and the livestock that were with him in the ark, and he sent a wind over the earth, and the waters receded.
The waters continued to recede until the tenth month, and on the first day of the tenth month the tops of the mountains became visible.
Again a background of judgment, the story focusses on God's saving act. Noah, the only good man of his Tim, the one human being who lived in fellowship with God, will not go down with the rest God has a detailed rescue plan. He will provide a place of safety and refuge. A lifetime of knowing and trusting God has prepared Noah for this moment. And he does exactly as God tells him.
Why?
It is God's judgment came upon this world, only Noah and his family and some animal were saved from flooding.
The only good man who obey the word of the Lord could be saved.
40 days rainfall on the earth will be a great disaster now either.
How?
God is the creator and terminator of this world.  My God is an awsome God.