When someone expects the worst possible scenario, regardless of there being evidence to support them. The poster will usually mention that they hope it doesn't happen. However, everything they say always reeks of pessimism.
Poster 1: this decade will be when WW3 starts
Poster 2: lol what?
Poster 1: trust me, I want to be wrong but it'll still happen
Poster 2: stop doomposting