Ominously is a word to describe how something threatening appears. If you're golfing and dark gray clouds ominously appear, you'd better call it a day.
Originating from the Latin word ominosus, meaning "full of foreboding," the adverb ominously indicates when an action is done in an ominous manner, possibly indicating danger or evil is in the future. You might want to reconsider that moonlit hike if a wolf ominously howls just as you are about to begin. If gigantic waves are ominously crashing into the shore, it would be wise to devote the day to building a sand castle rather than snorkeling.