The most expensive tornado in U.S. history tore across 37 miles of central Oklahoma on May 3, 1999. The tornado was rated F5, with winds approaching 300 miles or 480 kilometers per hour. It killed 36 people and caused some 1 billion dollars in damage. Tornadoes are rapidly rotating winds blowing around a small area of extreme low pressure that develops within a severe thunderstorm. Often visible as a funnel-shaped cloud, tornadoes can develop with little or no warning. Doppler radar and other advanced technology now give advance warning of many tornadoes. The average lead time of tornado warnings has roughly doubled across the United States since the national Doppler radar network was completed in the 1990s. Prompt warnings with substantial lead times helped to reduce the death and injury toll in Oklahoma's 1999 tornado outbreak.