  We present UV/optical/NIR observations and modeling of supernova (SN)
2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time
("flash") spectroscopy of SN 2024ggi within +0.8 days of discovery shows
emission lines of H I, He I, C III, and N III with a narrow core and broad,
symmetric wings (i.e., IIn-like) arising from the photoionized,
optically-thick, unshocked circumstellar material (CSM) that surrounded the
progenitor star at shock breakout. By the next spectral epoch at +1.5 days, SN
2024ggi showed a rise in ionization as emission lines of He II, C IV, N IV/V
and O V became visible. This phenomenon is temporally consistent with a
blueward shift in the UV/optical colors, both likely the result of shock
breakout in an extended, dense CSM. The IIn-like features in SN 2024ggi persist
on a timescale of $t_{\rm IIn} = 3.8 \pm 1.6$ days at which time a reduction in
CSM density allows the detection of Doppler broadened features from the fastest
SN material. SN 2024ggi has peak UV/optical absolute magnitudes of $M_{\rm w2}
= -18.7$ mag and $M_{\rm g} = -18.1$ mag that are consistent with the known
population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of
radiation hydrodynamics and non-local thermodynamic equilibrium (nLTE)
radiative-transfer simulations suggests a progenitor mass-loss rate of $\dot{M}
= 10^{-2}$M$_{\odot}$ yr$^{-1}$ ($v_w$ = 50 km/s), confined to a distance of $r
< 5\times 10^{14}$ cm. Assuming a wind velocity of $v_w$ = 50 km/s, the
progenitor star underwent an enhanced mass-loss episode in the last ~3 years
before explosion.
