  At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to
date, with a luminosity intermediate between the bulk of the population and
iPTF16fnl. Its proximity allowed a very early detection and triggering of
multiwavelength and spectroscopic follow-up well before maximum light. The
velocity dispersion of the host galaxy and fits to the TDE light curve indicate
a black hole mass $\approx 10^6$ M$_\odot$, disrupting a star of $\approx 1$
M$_\odot$. Comprehensive UV, optical and X-ray data shows that the early
optical emission is dominated by an outflow, with a luminosity evolution $L
\propto t^2$, consistent with a photosphere expanding at constant velocity
($\gtrsim 2000$ km s$^{-1}$), and a line-forming region producing initially
blueshifted H and He II profiles with $v=3000-10000$ km s$^{-1}$. The fastest
optical ejecta approach the velocity inferred from radio detections (modelled
in a forthcoming companion paper from K.~D.~Alexander et al.), thus the same
outflow may be responsible for both the fast optical rise and the radio
emission -- the first time this connection has been observed in a TDE. The
light curve rise begins $29 \pm 2$ days before maximum light, peaking when the
photosphere reaches the radius where optical photons can escape. The
photosphere then undergoes a sudden transition, first cooling at constant
radius then contracting at constant temperature. At the same time, the
blueshifts disappear from the spectrum and Bowen fluorescence lines (N III)
become prominent, implying a source of far-UV photons, while the X-ray light
curve peaks at $\approx 10^{41}$ erg s$^{-1}$. Assuming that these X-rays are
from prompt accretion, the size and mass of the outflow are consistent with the
reprocessing layer needed to explain the large optical to X-ray ratio in this
and other optical TDEs, possibly favouring accretion-powered over
collision-powered outflow models.
