"The National Weather Service's (NWS) weather products are a vital component of the Federal Aviation Administration's (FAA) air traffic control system. In addition to providing aviation weather products developed at its own facilities, NWS also provides on-site staff at each of FAA's en route centers - the facilities that control high-altitude flight outside the airport tower and terminal areas. NWS's on-site staff is called a center weather service unit. For several years, NWS and FAA have been exploring options for improving the aviation weather services provided at en route centers. GAO agreed to (1) determine the status of the agencies' efforts to restructure aviation weather services, (2) assess the agencies' progress in establishing performance baselines in order to measure the effect of any changes, and (3) evaluate plans to address key challenges. To do so, GAO evaluated agency progress and plans and compared agency efforts with leading practices"--Highlights.