The fastest flow (lowest static pressure) is over the first 10-20% of the upper surface.
But the shock wave forms, I believe, from the area rule effect (cross-section of the wing + fuselage increasing around the center section). When sitting on the shady side of the fuselage, you can often see the shadow of the shock wave on the wing surface. It dances around a bit in light turbulence. I think there are photos of same somewhere in pprune.