Around October 15, 1993, an unusual polar low formation took place over the Norwegian Sea. The evolution is studied by means of the HIRLAM forecasting model at the Danish Meteorological Institute, satellite imagery and observations. The investigation, to some extent based on generalized forms of the equations for omega and geopotential tendency, reveals that the disturbance forms initially by baroclinic instability along a frontal zone within a maritime polar air mass and in close proximity of an intense extratropical cyclone. During its growth to a polar low, air-sea interaction seems to work in conjunction with the baroclinic process. A parallel forecast with the latent heat set to zero shows that heating of the atmosphere, associated with precipitation, is necessary for the polar low formation.¿ 1997 American Geophysical Union