A record-breaking wave of tornadoes begins across the southern and midwestern United States on this day in 2003. By the time the wave is over, more than 500 tornadoes are recorded for the month, shattering the previous record by more than 100.

The amazing spate of twisters followed an unusually quiet month of April, when tornadoes are usually most frequent. March and April also saw a lack of significant thunderstorms, another highly unusual situation. But the residents of the South and Midwest were not in the clear. In 2003, the moist, warm air necessary for the formation of tornadoes did not arrive from the Gulf of Mexico until May and, when it did, weather conditions in these regions changed suddenly. The first 10 days of May brought an incredible amount of destructive weather to the central United States, including more than 300 tornadoes.

The best available records regarding tornadoes in the second half of the 20th century show that the previous high for tornadoes in a month was 399 in June 1992. May 2003 had a remarkable 516 recorded twisters. However, the 38 people killed in May 2003 were far fewer than the record 163 dead in May 1953. The death toll was lower because 2003 saw no F5 tornadoes (the highest intensity with winds in excess of 260 mph) and very few measured as high as F4 (with winds in excess of 207 mph).

Illinois alone had 74 recorded tornadoes in May 2003, almost 50 percent more than its previous monthly high. Missouri suffered through 71 twisters in the month, which dwarfed the pervious high of 29 in December 1982.

The worst stretch of tornadoes over a small stretch of time was recorded April 3-4, 1974, when 148 individual tornadoes touched down across the Midwest in an 18-hour period. During a single hour in the middle of this vast storm, 20 tornadoes were recorded at the same time. More than 300 people died in this single storm.