Harbinger
Etymology
The surprise is in the roots: it comes from Old French herbergere, "one who provides lodging," from herberge, "lodging" (a Germanic word related to "harbor"). A harbinger was originally the servant sent ahead of a royal party to arrange rooms for the night. From "one who goes before to prepare the way," the meaning shifted to "one who announces what is coming."
noun · /HAR-bin-jer/
A person or thing that signals the approach of something else.
What it names
The word for an advance sign of what is coming — a harbinger of spring, a harbinger of trouble — announcing an arrival before it happens.
Example
The first crocuses pushing through the snow were the year’s harbingers, promising a spring still weeks away.
Related idea
Precursor is neutral about what follows and stresses coming first in sequence; harbinger actively signals or foreshadows the thing to come.