Coherent perturbations in a turbulent boundary layer subjected to a highly adverse pressure gradient are considered. The governing equations are the RANS equations with Boussinesq-type closure for the perturbation of background Reynolds stresses. Stability analysis of experimental velocity profiles in a boundary layer over a concave wall demonstrated agreement of theoretical and experimental data. A theoretical model taking into account the interaction of the coherent perturbations and the mean flow via the coherent Reynolds stresses is suggested. It is shown that coherent perturbations affect the mean velocity profile in such a way that the flow becomes more stable.