GPS tropospheric travel-time delays can be used to determine precipitable water (integrated water vapor) at the GPS site, if the station pressure is known. A large number of GPS receivers have been deployed by geophysicists in order to track lithospheric plate motion and movement along faults; unfortunately, barometers are not routinely deployed at these sites, so they are not directly available for precipitable water measurements. To get around this problem, we have calculated station pressures from RUC model output. When combined with hourly GPS delay data, this gives us the ability to calculate precipitable water on an hourly basis and map it to a grid. The timeliness of this data and its high resolution make it a unique tool in operational forecasting.