(Program not available for streaming.) On May 3, 1999, one of the most powerful tornadoes ever recorded carved a path of complete destruction near Oklahoma City. To scientists, the supertwister held sobering lessons about the future for rapidly expanding cities in tornado-threatened areas. Most tornadoes form suddenly and with little warning. But now meteorologists are on the verge of a breakthrough that may solve the puzzle of how these killer storms spawn and where they are likely to strike. NOVA follows stormchasers as they probe the tornado's deadly secrets.