  The convection velocity of localized wave packet in plane-Poiseuille flow is
found to be determined by a solitary wave at the centerline of a downstream
vortex dipole in its mean field after deducting the basic flow. The fluctuation
component following the vortex dipole oscillates with a global frequency
selected by the upstream marginal absolute instability, and propagates obeying
the local dispersion relation of the mean flow. By applying localized initial
disturbances, a nonzero wave-packet density is achieved at the threshold state,
suggesting a first order transition.
