A new forecasting model for water cooling in the Gulf of Bothnia and Lake Vänern is presented. The model elements consist of a transient Ekman model, where the turbulent exchange coefficients are calculated with a two-equation model of turbulence, a heat flux package for calculating the net heat loss at the air/sea interface, and geometries based upon area/depth distributions for the Bothnian Bay, the North Bothnian Sea, Lake Vänern/Dalbosjön, and Lake Vänern/Värmlandssjön. The model system can handle up to five different forecasts of maximum 30 days each simultaneously. It is constructed for easy and rapid interactive handling by letting the forecaster answer questions on a terminal screen. During the winter navigation period of 1983/84 the model was tested in routine. The results from that test period were most satisfactory, and it is believed that the new forecasting model can serve as a reliable adviser for effective winter navigation planning during coming seasons