A recent outbreak of tornadoes in the American southeast has left some experts scratching their heads; what caused this outbreak of storms to become deadlier than any other series of violent weather in the last 86 years?

“That’s a fairly complicated question of why particular outbreaks are more deadly than others,” said Keith Miller, assistant professor of geology.

The explanation for the violent storms is as complicated as the question. The first cause is simple misfortune.

“You have to have very particular conditions to have a major outbreak of tornadoes. You need the warm moist air out in front of a cold air front, and you get that strong contrast in air temperatures and those conditions set up for a dangerous outbreak,” Miller said.

But a simple collision of warm moist air with a cold front is not enough to cause the kind of storm that ripped through Alabama last week, leaving hundreds dead and thousands injured. Jet streams, fast flowing currents of air in the atmosphere, can intensify already ferocious storm fronts.

“If a jet stream is in a particularly good position, the speed of that jet stream adds to the energy of the storms and also helps them to rotate and spin, and in order to have tornadoes you need to have super cells that are rotating,” Miller said.

What separates this recent outbreak in the southeast from other storms is its location and timing, Miller said. The most intense outbreaks tend to occur later in the tornado season and in the heart of tornado alley.

“The center of tornado alley goes from Oklahoma through eastern Kansas and then up into Missouri and Iowa, kind of across the middle of the country, and the real peak of tornado season is actually May, not April,” Miller said. “This was a big outbreak of tornadoes that you normally expect to occur in May and in a different location.”

The recent storm, which caused damage in Alabama, Mississippi, Georgia, Virginia and Arkansas, caused an inordinate amount of fatalities. Not since 1925, when 747 people were killed in a storm that spanned seven states, have so many casualties been caused by an outbreak of tornadoes. The reason behind the deaths has perplexed some experts.

“If you read the discussions on weather.com and things like that, there’s a lot of puzzlement as to why there were so many fatalities,” Miller said. “You have to go back to the 1920s to find a higher death toll, and that was a time when there weren’t any tornado warnings. There were no satellites, sirens, radar, none of the things that we use now to track and predict tornadoes, so obviously a lot of fatalities occurred because there was no warning.”

“But here, in this storm, there were warnings sent out, and tornadoes and storms were predicted,” Miller said.

Part of the high tally may be attributed to the population of the southeast region being unfamiliar with correct tornado safety protocol. Many of the homes may not have been outfitted with storm shelters or basements, Miller said, so residents would have been vulnerable to the storm even if a warning was provided.

The number of tornadoes reported during the storm was also unusually high. According to the National Weather Service, at least 11 tornadoes with EF3 ratings – which are given to tornadoes with wind speeds between 65 and 85 miles per hour – were recorded. One particular tornado was estimated to be three-quarters of a mile wide.

“It caught a lot of people by surprise, just the magnitude of it. It didn’t slightly break previous records, it smashed previous records in terms of the number of tornadoes and fatalities,” Miller said. “The 1925 outbreak was not bigger, it was just deadlier because there were no warning systems. In terms of number of tornadoes, I think this is probably the highest number of tornadoes from a single event on record.”

The magnitude of the storm has raised some questions about the potential effects of climate change on storm severity. Though Miller said it has been verified that tornadoes have become more common since the 1950s, there is little research that definitively ties increased storm violence to climate change.

One study by Ashton Robinson Cook, a researcher at the National Weather Service Storm Prediction Center, does suggest that tornado outbreaks in the United States are stronger and more frequent after winter periods of moderate tropical Pacific sea surface temperatures. Warmer winter temperatures can cause more moisture content to linger in the air, which can fuel an increase in tornado frequency, according to the study.

However, Miller does not believe that research has arrived at a final conclusion.

“I haven’t seen any definitive research on that one way or the other. There is a consensus that the intensity of hurricanes has increased due to climate change, but there is not a similar scientific consensus about tornadoes,” Miller said.

Regardless of the cause of the outbreak, experts remain astounded by the shear magnitude of the storm.

“It’s a pretty astounding and shocking event. It was just an extraordinary outbreak,” Miller said.