At the 2007 AMS Annual Meeting the National Research Council, an arm of the National Academy of Scineces, released its just completed report on a “decadal survey” of NASA and NOAA Earth space missions that had been under way for two years. Among the 15 missions that the NRC recommended that NASA undertake was one called the “Precipitation and All-weather Temperature and Humidity” mission (PATH). A “MW array spectrometer” was identified as the presumed instrument payload for PATH. Such an instrument, called the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR), has been developed at NASA's Jet Propulsion Laboratory, and it is likely that it will be implemented for a space mission in the near future. GeoSTAR is a microwave sounder with the same capabilities as have been available on low earth orbiting (LEO) satellites for nearly 10 years with the Advanced Microwave Sounding Unit (AMSU) system – soon to be succeeded by the Advanced Technology Microwave Sounder (ATMS). Providing such a capability in geostationary orbit (GEO) has long been a goal for NOAA and NASA, since the GEO vantage point offers key advantages over LEO – such as a continuous view of the entire life cycle of storms and hurricanes. Due to the very large antenna aperture needed for a microwave sounder to provide the required spatial resolution, it has not been possible to develop such instruments for GEO. Only infrared sounders have been feasible, but they are severely hampered by clouds – which is not a problem for microwave sounders. GeoSTAR overcomes those difficulties by using a radically new approach to synthesize a large aperture, and the development of the GeoSTAR concept therefore makes a GEO microwave sounder finally possible. This was clearly viewed by the NRC as a very important breakthrough, and plans for a PATH/GeoSTAR mission are now under development. GeoSTAR will provide a number of measurements that are crucial for the monitoring and prediction of hurricanes and severe storms – including hemispheric 3-dimensional temperature, humidity and cloud liquid water fields, rain rates and totals, tropospheric wind vectors, sea surface temperature, and parameters associated with deep convection and atmospheric instability – everywhere and all the time, even in the presence of clouds. We discuss possible mission scenarios and some of the science applications.