What does augur badly mean?

Could you please clarify the meaning of the phrase 'augur badly'? It seems to suggest a negative forecast or omen, but I'd like a more precise explanation of how it's used in context. Is it often employed to describe economic downturns or political instability? And how does it differ from other similar-sounding phrases, such as 'forebode ill' or 'portend disaster'? I'm interested in understanding the nuances of this expression and when it might be most appropriate to use it.