Models of volcanic ash (tephra) fallout are increasingly used to assess volcanic hazards in advance of eruptions and in near–real time. These models often approximate the wind field using simplistic assumptions of the atmosphere that do not account for four-dimensional variations in wind velocity. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) is used to improve forecasts of tephra dispersion. MM5 is a 3D model that can predict circulations in areas with sparse meteorological observations and complex terrain, such as volcanic plateaus. MM5 is applied to the 1995 eruption of Cerro Negro, Nicaragua. Validation of MM5 is achieved by comparing the simulated winds with rawinsonde observations. Estimates of diffusivity and particle settling velocities are used in conjunction with MM5-generated wind fields to forecast the major axes of the tephra dispersion. The predicted axes of dispersion derived from the MM5 winds approximate very closely the observed bilobate tephra accumulation and tephra plumes observed in satellite images. MM5 winds provide far more accurate spatial and temporal forecasts than do the wind assumptions that had been used previously to assess Cerro Negro tephra hazard.