Weather BulletinThe National Severe Storms Laboratory in Norman, Oklahoma dispatches “storm chasers” to track, follow, and observe the behavior of storms in a predicted tornado outbreak. The chasers manage to document the complete development of a tornado in Union City, Oklahoma on film and on an experimental Doppler radar system; for the first time, large-scale cloud rotation at high altitude is observed on radar prior to the appearance of a funnel cloud, a key discovery in tornado prediction. This phenomenon, called the Tornadic Vortex Signature, is a precursor to virtually every radar-detected tornado.