  Recent observations of a large fraction of Type II supernovae show traces of
dense circumstellar medium (CSM) very close to the progenitor star. If this CSM
is created by eruptive mass loss several months before core-collapse, the
eruption itself may be visible as a precursor, helpful as an early warning of a
near-future supernova. Using radiation hydrodynamical simulations based on the
open-source code CHIPS, we theoretically model the emission from mass eruption
of a red supergiant star. We find that for a modest mass eruption the
luminosity is typically on the order of $10^{39}$ erg s$^{-1}$, can last as
long as hundreds of days until the star explodes, and is mainly bright in the
infrared (from -9 to -11 mag around peak). We discuss observational strategies
to find these signatures from Galactic and local Type II supernovae.
