Numerical models are plagued by poor representations of Gulf Stream separation. Rather than moving to the deep ocean at Cape Hatteras, many model Gulf Streams 'overshoot' the separation, exiting the US east coast several degrees to the north of Cape Hatteras. This is responsible for the largest bias in ocean SST found in coupled climate models. Higher model resolution often helps, but even then correct separation is not guaranteed. We examine Gulf Stream separation in a regional model with a view towards understanding the dynamical controls on separation. Our results support the convergence of the isobaths northeast of the Charleston Bump as the key element. Initial tests of a theoretical explanation based on mean flow-topography interaction support the result.