The EF-5 is a category reserved for only the fiercest and most devastating of tornadoes, and it's based on what little is left rather than the force of what swept through.
James LaDue, a meteorologist at the Warning Decision Training Branch, says EF-5s are worse than hurricanes and in terms of damage potential equal the tsunami that struck Japan in March.
"It is capable of completely sweeping away one- and two-story houses, leaving nothing left but the basement itself," said LaDue. "It's also capable of turning vehicles into missiles."
According to the National Weather Service, one of the tornadoes that killed hundreds in the South was an EF-5 tornado, the first to hit Mississippi since 1966. The tornado, which hit Smithville, was a half-mile wide and packed winds of 205 miles per hour. It was on the ground for almost three miles, killing 14 and injuring 40.
The Smithville tornado has been preliminarily categorized as an EF-5 based on photos, but a complete survey of the damage is required before the classification is confirmed. In order for a tornado to be eligible for EF-5 classification the twister must have winds of 200 miles per hour or greater for about three seconds.
"Any tornado has the potential of doing EF-5 damage, you just don't know when or where," said LaDue.
EF-5 tornadoes are extremely rare, but LaDue believes that once more of the damage is surveyed it is likely that multiple EF-5s touched down in the South during the tornado outbreak.
"The last time ... where more than one EF-5 was reported in a day was back in 1990," said LaDue. "And if we have more than two EF-5s out of this then the last time before that was probably in 1974."
The chances of surviving the most powerful tornado possible without a concrete reinforced safe room or a basement are "pretty slim" LaDue told ABC News.
"People are not trained and prepared," LaDue said. "There is very little collective knowledge or history, and certainly most people don't have a memory of what these things can do."
The Associated Press contributed to this report.