IPA: /ˈduːmseɪər/
KK: /ˈdumˌseɪər/
A person who predicts disaster or negative events, often in a pessimistic way.
The doomsayer warned everyone about the dangers of climate change.
Doomsayer → It is formed from "doom" (from Old English *dom*, meaning judgment or fate) and "sayer" (from Old English *secgan*, meaning to say or tell). A doomsayer is someone who predicts or proclaims doom or disaster.
Think of someone who 'says' ('sayer') about 'judgment' or 'fate' ('doom') — that's why a doomsayer is someone who predicts disaster.
No commonly confused words.