This question is based on practical considerations. A forecast is called a real-time if the combined reaction- and execution-time of the forecast is shorter than the maximum delay that is allowed, in view of circumstances outside the forecast. In other words, a flood forecast including uncertainty analysis is computed as soon as all input data are available.

The two branches arising at this node could be combined, as all methods under this heading are examples of conditioning on data and could be applied for real-time forecasting or off-line analysis. The main difference is that methods which are quoted in the left leave usually treat data sequentially time step by time step, whereas methods in the right leave usually work on en bloc data set. However, even this distinction is blurred. Therefore the question is reasoned by preference based on extensive experience of the authors.