precursory

Anything that's precursory comes before something else, often in an ominous or inauspicious way, like a precursory rumble that proceeds the eruption of a volcano.

While some synonyms of precursory include preliminary and introductory, it's most common to use this adjective with a certain threatening sense of warning. So you might describe the precursory growl of a tiger just before it attacks, or the precursory stillness in advance of a tornado's descent. Precursory comes from Latin roots, prae, or "before," and currere, "to run."