The potential of the concept of combining video data analysis and numerical simulations for numerical fire forecasting is illustrated for the case of a burning sofa in an ISO room. The fire is monitored by means of a video camera. The temporal evolution of smoke layer height, flame height and flame width are obtained from the real-time video data analysis. The fire heat release rate, estimated from the flame height and width, serves as input for the numerical simulations. The two-zone model approach is adopted, because the calculations are very fast. This is necessary for forecasting: time scales in fire development are in the order of seconds (minutes), not hours (which are typical calculation times in CFD simulations). Data assimilation with real-time adjustments according to sudden changes in the fire development as observed, improves the predictions by the two-zone model and allows to make a forecast of the fire development and possible subsequent hazards, in terms of evolution of smoke layer height and temperature.