  The flow pattern and evolution of the medium created in ultrarelativistic
heavy ion collisions can have significant influence on the energy loss of hard
partons traversing the medium. We demonstrate that within a range of
assumptions for longitudinal and transverse flow which are all compatible with
the measured hadronic single particle distributions, the quenching power of the
medium can vary within almost a factor five. Thus, the choice of the medium
evolution is one of the biggest uncertainties in jet quenching calculations and
needs to be addressed with some care.
