  There are different ways for fluid flow to become turbulent, but usually many
instabilities take place before that. Among these instabilities the transition
from laminar flow to vortex shedding carries significant practical importance.
Here we study a flow, where at high enough flow rates, vortices are generated
by a fixed obstacle (cylinder), which penetrates a flowing, quasi-two
dimensional soap film.
  We present experimental results that demonstrate that the transition from
laminar flow to a periodic K\'arm\'an vortex street can be hysteretic, i.e.
changing the mean flow rate $\bar V$ vortices can survive at velocities lower
than the velocity needed to generate them. This is an unexpected result since
3D experiments are well described by the Hopf equation, which is incompatible
with hysteresis. Beyond that, our data cannot be satisfactorily fitted by the
generic model of hysteresis, i.e. the 5th order Landau equation. It is found
that a phenomenological model describes our experimental findings very well.
  Evidences are presented that wetting properties of the rod, mechanical
instabilities (i.e. vibrations) of the setup, and the effect of the surrounding
air are not the cause of the hysteresis. To reduce three dimensional effects,
the rod was replaced by a disk having a thickness roughly eight times that of
the film. The replacement of the rod by a disk increases the gap width and the
shedding frequency. Behaviour of the system (for instance the unstable
trajectory) in the hysteretic gap is investigated by means of transient
measurements.
