Ominously tells you that someone did something, or something happened, in a way that makes you feel trouble is coming. 'She smiled ominously' means the smile was not kind; it made you worry. 'The sky rumbled ominously' means the sound felt like a warning. Same idea as ominous, just built to sit next to a verb. It is a little formal and a little story-like. In everyday talk you might just say 'in a scary way' or 'that did not sound good.'

不祥地、預兆不好地。