What is the best synonym for "ominously"?

The word "ominously" conveys a sense of foreboding or a hint of something bad or threatening that may happen in the future. The synonym "threateningly" aligns closely with this meaning, as it also suggests an atmosphere or manner that implies danger or harm. Both words evoke a sense of unease and are often used to describe situations, actions, or tones that are menacing or foreboding.

In contrast, the other choices suggest positive or neutral emotions. "Joyfully" and "cheerfully" both convey happiness and positivity, which starkly contrasts with the threatening implication of "ominously." "Uncertainly" deals more with doubt or lack of clarity and does not carry the menacing connotation inherent in "ominously." Thus, "threateningly" is the most appropriate synonym given its shared connotations of danger and forewarning.