  In environments with high dense neutrino gases, such as in core-collapse
supernovae (CCSNe), the neutrinos can experience collective neutrino
oscillation due to their self-interactions. In particular, fast flavor
conversion driven by the crossings in the neutrino angular distribution can
affect explosion mechanism, nucleosynthesis, and neutrino observation. We
perform the numerical computation of nonlinear flavor evolution on the neutrino
angular distribution with tiny crossings expected to be generated in the
preshock region. We demonstrate that the fast instability is triggered and a
cascade develops under a realistic three-flavor model considering muon
production and weak magnetism in the SN dynamics. The tiny crossing excites
specific spatial modes, and then the flavor instability propagates into other
modes which otherwise remain stable due to the nonlinear effects. Our results
indicate that fast flavor conversion can rise in the preshock region and have a
sufficient impact on the flavor contents.
