The Federal Aviation Administration FAA Integrated Terminal Weather System ITWS is supporting the development of weather products important for air traffic control in the terminal area. These products will take advantage of new terminal area sensors, including Terminal Doppler Weather Radar TDWR, Next Generation Weather Radar NEXRAD, and the Meteorological Data Collection and Reporting System MDCRS. Some of these ITWS products will allow air traffic managers to anticipate significant short-term changes in ceiling and visibility. This report focuses on the scientific data requirements for supporting prototype model-system development and diagnostics. Model diagnostics can include case studies to determine the most important physical processes that were responsible for a particular ceiling and visibility event, providing the insight necessary for the development of effective ceiling and visibility product algorithms. In time such case study diagnostics could also include careful off-line failure analyses that may affect the design of the operational system. General ceiling and visibility test beds are discussed. Updated reports will be released periodically as the ITWS ceiling and visibility project proceeds.