Numerical simulations of supercritical coastal flows were performed with a three-dimensional, nonlinear, hydrostatic meso-&ggr;-scale model with a higher-order turbulence closure. The purpose was to resolve questions from previous studies related to terrain forcing. Idealized terrain, representing northern California, was used; this facilitated testing of hypotheses in a simple fashion. It was found that the angle by which the coast turns away from the flow regulates the acceleration in the expansion fan and determines how far downstream the maximum in the wind speed is located. Simulations with curved coastlines confirmed that the gradual curvature of the coastal mountains north of Cape Mendocino is sufficient to excite an expansion fan. A decreasing height of the terrain along the coast lead to an increased acceleration of the flow, largest when the slope of the terrain was confined to the change in coastline orientation. A cape perpendicular to the main coastal mountains significantly blocked the flow, even when as low as half the upstream marine atmospheric boundary layer (MBL) depth. Moreover, a realistic cape caused gravity-wave breaking. These simulations confirmed that the local terrain at Cape Mendocino is responsible for the collapse of the MBL in Shelter Cove. One striking feature in this study was that the flow started to accelerate far upstream of the change in coastline orientation, even in supercritical conditions. This phenomenon was most pronounced in the cases with the highest wind speeds. Using normalized wind speed and the Bernoulli function from calculated back-trajectories, it was concluded that this feature is not due to deviations from linearity or from the shallow-water concept. ¿ 2001 American Geophysical Union