Prophet of doom
Definition 1 of 1
Pro Tip 1/3Negative Connotation
Used to describe someone who often predicts negative outcomes; it implies a sense of annoyance or disapproval. 
At family gatherings, Uncle Joe is the prophet of doom, always talking about impending crises.
Pro Tip 2/3Not Literal
The phrase is figurative, not describing an actual prophet, but someone who behaves like one in spreading gloomy forecasts. 
Every time there's a dark cloud, he acts like a prophet of doom, saying it'll ruin the weekend.
Pro Tip 3/3Emotional Impact
Can cause irritation or anxiety among listeners, as continuous negative predictions may be unsettling. 
Her prophecy of doom about the company's future left her colleagues feeling uneasy.