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Spectroscopic detection of narrow emission lines traces the presence ofcircumstellar mass distributions around massive stars exploding ascore-collapse supernovae. Transient emission lines disappearing shortly afterthe supernova explosion suggest that the spatial extent of such material iscompact, and hence imply an increased mass loss shortly prior to explosion.Here, we present a systematic survey for such transient emission lines (FlashSpectroscopy) among Type II supernovae detected in the first year of the ZwickyTransient Facility (ZTF) survey. We find that at least six out of ten eventsfor which a spectrum was obtained within two days of estimated explosion timeshow evidence for such transient flash lines. Our measured flash event fraction($>30\%$ at $95\%$ confidence level) indicates that elevated mass loss is acommon process occurring in massive stars that are about to explode assupernovae.

A large fraction of hydrogen-rich supernova progenitors experience elevated mass loss shortly prior to explosion