  We derive a simple algebraic form of the nonlinear wavenumber correction of
surface gravity waves in deep water, based on temporal measurements of the
water surface and the spatial Zakharov equation. This allows us to formulate an
improvement over linear deterministic wave forecasting with no additional
computational cost. Our new formulation is used to forecast both synthetically
generated as well as experimentally measured seas, and shows marked
improvements over the linear theory.
