Ominous, portentous, fateful, inauspicious, unpropitious basically mean having a menacing or threatening character or quality.
What is ominous has or seems to have the character of an omen, especially of an omen forecasting evil; the term commonly suggests a frightening or alarming quality that bodes no good, and it may imply impending disaster.
What is portentous has or seems to have the character of a portent; portentous, however, less often than ominous suggests a threatening character; it usually means little more than prodigious, monstrous, or almost frighteningly marvelous, solemn, or impressive.
What is fateful has or seems to have the quality, character, or importance decreed for it by fate or suggests inevitability, but the term often means little more than momentous or appallingly decisive.
What is inauspicious (compare auspicious under FAVORABLE ) is or seems to be attended by signs that are distinctly unfavorable. But inauspicious usually means nothing more than unlucky, unfortunate, or unlikely to succeed.
What is unpropitious (compare propitious under FAVORABLE ) carries or seems to carry no sign of favoring one's ends or intentions.
In its more common extended sense the term means merely unfavorable, discouraging, or harmful.