Water vapor is a greenhouse gas that is found to increase in the stratosphere. Here we present observations of the stratospheric water vapor mixing ratio inside, outside, and at the edge of the polar vortex measured by the FLASH-B Lyman-? hygrometer during the LAUTLOS campaign in Sodankylä, Finland, in January and February 2004. Analysing the measurements with the semi-lagrangian advection model MIMOSA, water vapor profiles typical for the polar vortex' interior and exterior have been identified, and laminae in the observed profiles have been correlated to filamentary structures in the potential vorticity field. Applying the validated MIMOSA transport scheme to specific humidity fields based on the ECMWF T106 model, large discrepancies from the observed profiles arise. Although MIMOSA is able to reproduce weak water vapor filaments, the simulations reveal a dry bias of about 1 ppmv in the lower stratosphere above 400 K, accounting for a relative difference from the measurements in the order of 20 %. The large dry bias in the models' representation of stratospheric water vapor in the Arctic implies the need for future regular measurements of water vapor in the polar stratosphere to allow the validation and improvement of climate models.