The term "gathering clouds" is often used as a metaphor to describe a sense of impending danger or trouble. However, there are several synonyms that can be used to convey a similar feeling. One such synonym is "looming storm," which implies a threat that is lurking on the horizon. Another is "darkening skies," which suggests an ominous atmosphere that is slowly encroaching. "Thickening fog" and "growing shadows" are two other synonyms that can be used to depict a sense of danger or uncertainty. Ultimately, the choice of synonym will depend on the context and tone of the piece in which it is used.