Two well-developed sea-breeze cases in the La Plata River region, selected from a 5-month summer period, are studied using local observations, satellite images, and hydrostatic boundary-layer model simulations. Both the northern and southern coast cases are characterized by offshore regional flow that help develop stronger sea breezes due to enhanced horizontal convergence by the opposing flow. The study shows that the sea-breeze propagation accompanies changes in the three-dimensional circulation within the boundary layer. The inland propagation speed of the cloud bands evident in satellite imagery is simulated relatively well by the model’s progression of maximum vertical motion in both cases. Local coastline features affect the inland penetration of the sea-breeze fronts. The inland propagation speed of the surface sea-breeze front estimated with the model simulations is greater than the speed of the cloud bands aloft.