  We use classical field simulations of the homogeneous Bose gas to study the
breakdown of superflow due to vortex nucleation past a cylindrical obstacle at
finite temperature. Thermal fluctuations modify the vortex nucleation from the
obstacle, turning anti-parallel vortex lines (which would be nucleated at zero
temperature) into wiggly lines, vortex rings and even vortex tangles. We find
that the critical velocity for vortex nucleation decreases with increasing
temperature, and scales with the speed of sound of the condensate, becoming
zero at the critical temperature for condensation.
