A mathematical model for the Baltic Sea has been introduced in an operational system for real-time calculations and forecasts of water temperatures. The model divides the Baltic Sea into 13 sub-basins and treats each sub-basin as a one-dimensional boundary layer with vertical mean velocities based upon in- and outflows from surrounding basins. The operational scheme uses the model in two modes. The first one, the real-time mode, fits the model to meteorological and oceanographical data collected in the recent past. The second one, the forecast mode, starts from the real-time mode, and calculates forecasts on the basis of the ECMWF weather model and statistics. The results from about 8 months of pre-operational tests, illustrate that the real-time mode reproduces observed sea surface temperatures quite satisfactorily. However, more efforts must be paid to the obtaining of oceanographic data in real-time. The results from the forecast mode were also satisfactory, but the quality was, of course, dependent on the meteorological forecasts.