A vertically integrated barotropic model for the Baltic Sea is described both in dynamic and numerical 1terms. The model has been ron operationally since March 1993 on a 5 x 5 km grid and is forced by interpolated 10 m winds and surface pressure from the 50 x 50 km HIRLAM atmospheric mode!. The water volume is not conserved but depends on inflow from rivers and in- or outflow through the Belt Sea and the Öresund.