To maximize the efficiency of using Grid resources, resource discovery and resource scheduling have been emphasized. It is desirable to provide meaningful resource performance information to Grid middleware, especially to a resource scheduler. We have developed an on-line Disk I/O performance monitoring and forecast system for storage resources. This includes a low-level performance monitoring system in real time and an on-line performance forecast system for delivering processed forecast information to an information consumer. This abstract briefly describes the Grid storage performance forecaster, called Storage Weather Service(SWS).