  We report a systematic strengthening of the local solar surface or
fundamental $f$-mode $1$-$2$ days prior to the emergence of an active region
(AR) in the same (corotating) location. Except for a possibly related increase
in the kurtosis of the magnetic field, no indication can be seen in the
magnetograms at that time. Our study is motivated by earlier numerical findings
of Singh et al. (2014) which showed that, in the presence of a nonuniform
magnetic field that is concentrated a few scale heights below the surface, the
$f$-mode fans out in the diagnostic $k\omega$ diagram at high wavenumbers. Here
we explore this possibility using data from the Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory and show for six isolated ARs,
11130, 11158, 11242, 11105, 11072, and 11768, that at large latitudinal
wavenumbers (corresponding to horizontal scales of around 3000 km), the
$f$-mode displays strengthening about two days prior to AR formation and thus
provides a new precursor for AR formation. Furthermore, we study two ARs, 12051
and 11678, apart from a magnetically quiet patch lying next to AR~12529, to
demonstrate the challenges in extracting such a precursor signal when a newly
forming AR emerges in a patch that lies in close proximity of one or several
already existing ARs which are expected to pollute neighboring patches. We then
discuss plausible procedures for extracting precursor signals from regions with
crowded environments. The idea that the $f$-mode is perturbed days before any
visible magnetic activity occurs at the surface can be important in
constraining dynamo models aimed at understanding the global magnetic activity
of the Sun.
