In this paper, the problem of dangerous conditions induced by variations of restoring in waves is addressed, with particular attention to parametric roll. An analytical model for the study of roll motion in irregular longitudinal long crested waves is presented. Stochastic stability boundaries in the first parametric resonance region are provided analytically. Nonlinear/non gaussian roll response above threshold is determined numerically and compared with experiments. A semi- empirical methodology is proposed to extend the classical deterministic static stability assessment to a rational probabilistic framework taking into account hydrostatic restoring variations in waves and different environmental conditions. Examples of application are provided.