We consider the spatio-temporal aspects of the transition to turbulence in a boundary layer above a flat plate exposed to free-stream turbulence. Combining results from the receptivity to free-stream turbulence with the observation of a double threshold from transition studies in e.g. pipe flow we arrive at a physically motivated prediction for the spatial distribution of nucleation events in boundary layers. We use a cellular automaton to implement a complete model for the spatial and temporal evolution of turbulent patches and show that the model reproduces the statistical features of the boundary layer remarkably well. The success of the modeling shows that bypass transition occurs as a spatiotemporally activated process, where transition is triggered by critical fluctuations imported from the free-stream turbulence.