Forward (black) and reverse (red) shock velocities versus the position angle in the plane of the sky for model W15-2-cw-IIb-HD+dec at the age of Cas A. The dashed horizontal lines mark the median values of the respective velocities. The shaded area marks the position angle where the reverse shock appears to be stationary or even moving inward in observations of Cas A (e.g., Helder & Vink 2008; Sato et al. 2018).