North Carolina residents might be excused for breathing a premature sigh of relief when Hurricane Florence, once a Category 4 storm, was downgraded to Category 1 before making landfall. But those numbers don’t tell the whole story–and what they leave out can have life-and-death consequences. “Florence is an excellent example of a storm that is a lower category than it was and yet is still extremely dangerous,” Bill Lapenta, director of the National Centers for Environmental Prediction at the National Weather Service, tells TIME. Florence especially highlighted one key shortfall of the system known as the Saffir-Simpson Hurricane Wind Scale: while it measures a hurricane’s wind speed, it doesn’t take into account the speed at which the storm itself is moving.