Why does Unapisthm anticipate the disaster ?

Utnapishtim explains that long ago he was once the king of city called Shuruppak, a city that was situated on the Euphrates. The gods Anu, Enlil, Ninerta, Ea, and Ennugi met in council and decided to bring a flood to destroy the city. Ea whispers to Utnapishtim through his house walls to abandon his belongings and instead build a boat. Ea gave him dimensions for this boat. It would have six decks and be of an enormous size, about 180 feet high. On this boat, Ea said, Utnapishtim should bring his family and the seed of every species