Radii of the forward (solid lines) and reverse (dashed lines) shocks versus time in the NW (red lines) and SE (blue lines) hemispheres for model W15-IIb-sh-HD-leta. The shock positions are averaged over solid angles (as seen from the explosion center) of 10° around the NW and SE directions. The vertical dotted line marks the approximate age of Cas A; the shaded rectangular box marks the time of remnant-shell interaction. The grey bands under the colored average curves indicate the range of shock positions at each time in the solid angles examined.