A severe thunderstorm, including rain, hail, and strong winds, developed in north central Oklahoma on June 12, 2002. As part of the International H20 Project (IHOP), networks of global positioning system (GPS) stations were operating to observe this convergence and organization of water vapor. These stations included networks operated by the NOAA Forecast Systems Laboratory, SuomiNet, and seven supplementary stations installed and operated by CNRS/INSU from France. We have analyzed these GPS data to derive precipitable water and slant water estimates. With these estimates, it is possible to observe the localized convergence of water vapor. Total column amounts of water vapor exceed 5 cm, with variations as large as 2 cm over horizontal scales less than 100 km. The GPS provides a unique observational tool to monitor the water vapor before, during and after this strong convective storm.