The tornado that tore through Joplin, Mo., followed an even deadlier outbreak in late April across the South. Following that tragedy, experts weighed in on what factors went into the severe weather system.
A leading candidate: La Nina, the periodic ocean cycle that cools the waters off South America and can impact weather globally.
Emerging research suggests La Nina's impact "cascades downstream into thunderstorms," Russell Schneider, director of the U.S. Storm Prediction Center, told msnbc.com.
La Nina could be influencing where thunderstorms form in the tropics and what happens to the jet stream, he added.
In a La Nina, the jet stream tends to move north through the Pacific Northwest and Great Lakes, keeping cold air on the northern side while the southern side tends to have warm, humid air. Cold fronts that would dry out the atmosphere on the south side are blocked, which means wet storms there.
And when a northern front pushes into the south, twisters can happen.
"This deadly event occurred because we had an unusually strong jet stream winds digging into the deep South with ample heat and moisture in place," said NBC News meteorologist Bill Karins. "The winds were also changing in direction with height (wind shear), giving any strong thunderstorm a significant chance of producing a tornado."
Experts are also studying whether the transition out of a La Nina cycle adds to the impact. "We are emerging from a strong to moderate La Nina," Schneider noted. "It is unclear from current research if the transition phase is in part responsible for the statistical evidence of enhanced tornado activity."
The biggest recorded outbreak prior to Wednesday was on April 3, 1974 — 148 twisters touched down in 13 states, killing 310 people and injuring nearly 5,500. It, too, happened during a La Nina, adding weight to that argument.
Given the limited data and the fact that reliable tornado records go back only to 1950, Schneider wasn't ready to buy into another potential factor: global warming.
Some climate experts believe more intense storms can be expected as temperatures warm, and they point to warming Gulf of Mexico waters as a potential factor in severe storms along the Gulf and in the South.
But Schneider said the evidence is not there to "draw a distinct link between climate change and tornadoes."
Karins agreed. "As a rule no single weather event can be linked to or apart from climate change," he notes. "With that said a recent study did attribute a slight uptick in significant weather events, with climate change the likely culprit. But it's so slight that the average person would never know it."
Grady Dixon, a Mississippi State University geosciences assistant professor, believed climate change likely has no effect. "I will even go so far to argue that it is irresponsible to suggest such a thing because tornado outbreaks, even multiday events, are not new and they do not happen regularly enough to imply a trend or pattern," he says.
"As for tornado numbers, those are increasing regularly over the past 30-plus years, but tornado researchers agree that is because of increased population, improved technology and awareness, and better education and reporting.
"Another measure is the number of days that experience conditions conducive to tornadoes, and that is not increasing over time," he added. "In fact, as active as this month has been, we have had only 20 tornado days, which barely places 2011 within the top 25 percent of the past 60 years. The record for April, which has happened 3 times, is 23 days. Several years have seen May and/or June experience 30 or 31 tornado days nationally."