A quantitative estimate of any property of the tsunami hazard that is made in advance. Properties that may be forecast include the time of initial wave arrival, the time of maximum wave arrival, the amplitude of the maximum tsunami waves, and the duration of the tsunami hazard. Forecasts are primarily produced by warning centres using the output of numerical models. These may include travel time models, propagation models, and inundation models. All models rely on assumptions, primarily regarding the tsunami source that may or may not be accurate and can contribute to errors in the forecast. Most models can be constrained with observations of the tsunami as they become available, thus make the forecast more accurate. Tsunami forecasts may be issued at forecast points, for sub-blocks geographically or according to geopolitical jurisdictions within a country in order to provide detailed advice on the tsunami threat.