A pessimistic person, who always says that bad things are going to happen, is sometimes called a Jeremiah.
In the Jewish and Christian religions, Jeremiah is recognised as being a 6th Century BCE Hebrew prophet who said that Jerusalem would be defeated and that God would become angry with the Jews and punish them. The Book of Jeremiah in the Old Testament of the Bible contains his prophecies ( prophecyprophecy).
[Longman Dictionary of Contemporary English; modified]
Jeremiah ... 2: person who is pessimistic about the present and foresees a calamitous future
Jeremiah (and God) often get a rather bad press; the state of the nation was inviting disaster. And the name Jeremiah itself in Hebrew means 'God will exalt!'