During the recent ASHOE (Airborne Southern Hemisphere Ozone Experiment) and SESAME (Second European Stratospheric Arctic and Middle-Latitude Experiment) field campaigns we produced a number of 10-day chemical forecasts of the stratosphere to aid in flight planning and interpretation of the observations. A general circulation model was integrated at high resolution to produce a 10-day meteorological forecast. These forecast fields were then used to specify the circulation in an off-line three-dimensional chemical transport model to produce a medium-range chemical forecast of the stratosphere. The chemical model has a detailed stratospheric chemistry scheme, including heterogeneous reactions on polar stratospheric clouds and sulfate aerosols. The NASA ER-2 high-altitude research aircraft made in situ measurements of chemical tracers in the lower stratosphere during the ASHOE campaign, which took place in four deployments from March to October 1994. This paper presents examples from the chemical forecasts made during the second and fourth deployments. We show that the ability to indicate the likely composition of the atmosphere is an important improvement over the standard meteorological forecasts, especially in air perturbed by chemical processing on polar stratospheric clouds. Of particular interest was the ability to forecast the chemical composition of filaments, which had been eroded from the vortex edge. Furthermore, immediate post-flight comparison between observed data and synoptic model data gave useful chemical interpretation of the observations.¿ 1997 American Geophysical Union