Metadata is a integral component of Hydro-NEXRAD, a software system that aims to make rainfall intensity and accumulation products derived from NEXRAD data available to hydrologists and others scientists. Hydro-NEXRAD computes and manages well defined, radar-centric, basin-centric, and point (for selected set of rain gauge locations) metadata. Basic data quality control is incorporated into definition of metadata. Users can very efficiently select data subsets that meet their criteria. Metadata do not have to be very accurate to be extremely useful. However, fast computation is important so that data ingest process are not slowed. In this paper the authors discuss the technical challenges of computing metadata and compare different approaches to address these challenges.