Military mission rehearsals and virtual prototypes increasingly rely upon combat simulations to predict feasibility and success. The current US force projection doctrine requires these operations to be conducted anywhere, anytime, and under any conditions. Standard scenarios provide accurate location data, but generally lack realistic weather conditions that can easily reverse the fortunes of war. This paper describes an approach to bring realistic weather to combat simulations by merging historical archives, engineering-level mobility and detection models, and simulation databases into a robust synthetic natural environment. The conduit is the Simulation-Weather interface Module (SWIM), a Java-based tool set connecting simulations and their managers via the World Wide Web. This work allows analysts and planners to accurately determine and plan for weather conditions in any part of the world at any time. It also enables reliable what-if scenarios for mission rehearsal by forecasting operational weather, which can then be customized by the planner. The Web-based interface allows scenarios to be revised via the Internet from anywhere in the world.