  We present panchromatic observations and modeling of supernova (SN) 2020tlf,
the first normal type II-P/L SN with confirmed precursor emission, as detected
by the Young Supernova Experiment transient survey with the Pan-STARRS1
telescope. Pre-explosion emission was detected in $riz-$bands at 130 days prior
to SN 2020tlf and persisted at relatively constant flux until first light. Soon
after discovery, "flash" spectroscopy of SN 2020tlf revealed prominent narrow
symmetric emission lines ($v_w < 300$ km s$^{-1}$) that resulted from the
photo-ionization of unshocked circumstellar material (CSM) shedded in
progenitor mass loss episodes in the final weeks to months before explosion.
Surprisingly, this novel display of pre-SN emission and associated mass loss
occurred in a RSG progenitor with ZAMS mass of only 10-12 M$_{\odot}$, as
inferred from nebular spectra. Modeling of the light curve and multi-epoch
spectra with the non-LTE radiative transfer code CMFGEN and
radiation-hydrodynamical (RHD) code HERACLES suggests a dense CSM limited to $r
\approx 10^{15}$ cm, and mass loss rate of $10^{-2}$ M$_{\odot}$ yr$^{-1}$. The
subsequent luminous light-curve plateau and persistent blue excess indicates an
extended progenitor, compatible with a RSG model with $R_{\star} = 1100$
R$_{\odot}$. Inferences from the limits on the shock-powered X-ray and radio
luminosity are consistent with these conclusions and suggest a CSM density of
$\rho < 2 \times 10^{-16}$ g cm$^{-3}$ for distances of $r \approx 5 \times
10^{15}$ cm, as well as a mass loss rate of $\dot M<1.3 \times
10^{-5}\,\rm{M_{\odot}\,yr^{-1}}$ at larger distances. A promising power source
for the observed precursor emission is the ejection of stellar material
following energy disposition into the stellar envelope as a result of gravity
waves emitted during either neon/oxygen burning or a nuclear flash from silicon
combustion.
