  We consider Prandtl's equations for the impulsively started disk and follow
the process of the formation of the singularity in the complex plane using the
singularity tracking method. We classify Van Dommelen and Shen's singularity as
a cubic root singularity.
  We introduce a class of initial data which have a dipole singularity in the
complex plane. These data are uniformly bounded in $H^1$ and lead to an earlier
singularity formation. The presence of a small viscosity in the streamwise
direction changes the behavior of the singularities.
