Why does John believe the ridiculous story Nicolas tells him about the impending flood? When he hears about the threat, what is his first concern?

Nicholas informs John that, in studying astrology, he has discovered that the following Monday there would be a flood as big as the one God sent in Noah's time. Apparently, everyone will drown in less than an hour. John expresses sorrow over the possible death of Alisoun, and asks if there is no way to prevent it.