The main limitations affecting visible-light and IR astronomy are related to visibility, atmospheric humidity, and spatial resolution. High-resolution imaging is fundamentally limited by optical turbulence (OT) in the atmosphere. OT is induced by dynamical turbulence developing within a stratified temperature field and depends on weather conditions. The resulting degraded telescope resolution is called ‘seeing.’ At the best ground-based observatories, characterized by a seeing of order 0.5 arcseconds, the resolution of a telescope with an aperture diameter of 10m is no better than 20cm (at ground level) at optical wavelengths. At the present time, when large and expensive telescope projects are increasingly becoming the norm, observatory managers rely on a proper assessment and understanding of weather conditions for optimal scheduling of scientific operations. Flexible rotas (i.e., planning optimal use of the available instrumentation) are now frequently used at the largest astronomical observatories. Site testing in preparation for new observatory developments is another important challenge. For site characterization for the European 42m (diameter) Extremely Large Telescope (E-ELT)1 or the Antarctic ‘Dome C’ project,2 we designed the DIMM (differential image-motion monitor) and SSS (single-star scidar)3 optical monitors. Since monitoring requires the availability of specific instruments and logistics, and it is limited in place and time, we are investigating other ways to retrieve atmospheric OT conditions frommeteorological measurements or forecasts. Atmospheric conditions are defined by the speed, temperature, and composition of the constituent elements. The corresponding state of the atmosphere is governed by the Navier-Stokes equations prevalent above and around the site. For astronomical seeing prediction, one must obtain the local (microscopic) atmospheric conditions on the metric OT scale. Several models can be used, but most are computationally time Figure 1. Seeing forecasting scheme. OT: Optical turbulence. Cn2 (Cn): OT index. P.T.U.: Pressure, temperature, humidity.