What happen in Tornado Alley in May 1999?

On May 3-4 of 1999 there was a very large tornado outbreak across the Great Plains. In those two days 114 tornadoes claimed 47 lives. The worst of these tornadoes was a mile wide F5 that tore through the Oklahoma City area, killing 36 people and causing $1 billion in damage. Mobile doppler radar measure winds in this tornado to 302 mph, the strongest ever recorded on earth.