Different high resolution versions of the UGAMP GCM are used to forecast the position of the stratospheric vortex for a period in January 1992. The sensitivity of these forecasts to changes in the vertical resolution, gravity wave drag parametrization scheme and horizontal diffusion are presented. The forecasts are compared to the ECMWF 5 day forecast and ECMWF analyses. For this case period, all runs produce a good 5 day forecast. Increased vertical resolution and reduced horizontal diffusion improve the 7 day forecast. The forecasts are also sensitive to the parametrization of gravity wave drag. ¿American Geophysical Union 1994