The temporal evolutions of the most important sea state characteristics during storms are presented together with the corresponding uncertainties. Storm data from the Northern North Sea are used for this purpose. The influence of the uncertainties associated with an extreme storm on the foundation safety is demonstrated for a gravity-based structural system founded on a clay. The stability is analyzed with respect to failure in large cyclic displacements accumulated during the storm. The failure probability is estimated by a first-order reliability method and the most important variables concerning their contribution to the failure probability are indicated. In most practical foundation safety investigations, an idealized deterministic storm profile is adopted. The adequacy of this profile is considered by calibrating a “load factor” to be used in connection with the idealized profile.