Multi-vortex Joplin tornado was 8th deadliest in U.S. history

This article first appeared in the St. Louis Beacon, ​May 24, 2011 - WASHINGTON - The powerful multiple-vortex tornado that devastated Joplin this week ranks as the deadliest twister of the past 60 years and probably the 8th deadliest such storm to strike this country in recorded history, experts say.

With the death total in Joplin at 122 on Tuesday, this year's interim total of tornado-caused deaths has surpassed 486 -- the highest number of such fatalities since 1953. Only five months into the year, 2011 already ranks as the 8th worst year for twister deaths in U.S. history, despite vastly improved storm forecasting. (The news is not totally bleak - two more survivors were found Tuesday afternoon, bringing the total to nine people found by search and rescue teams.)

The tornado section of the National Weather Service's Storm Prediction Center lists 1,189 tornadoes so far this year, with a record 875 of those touching down in April. Over the previous three years, the average annual number of U.S. tornadoes was 1,376.

"It's definitely an unusual year," said Ernest M. Agee, who heads a leading tornado research group at Purdue University in Indiana. "The width and intensity of the Joplin tornado was extremely devastating, with a complicated flow field" that featured "multiple vortices" -- that is, twisters that spin off from the wider wind funnel.

Agee told the Beacon that the Joplin tornado, with wind speeds estimated at about 198 mph, was "in the extreme upper level of the EF-4 range" -- the second-strongest tornado rating under the National Weather Service's "Enhanced Fujita Scale" -- with an EF-5 being the most devastating twister. "With another 2 mph, Joplin would be an EF-5, and it may eventually [after exact measurements] be labeled as an EF-5 tornado." And, in fact, the tornado was upgraded to an E-5 Tuesday night.

While the Joplin tornado will go down in history as one of the nation's worst, its death total is far below that of what the Weather Service lists as the deadliest storm -- the Tri-State tornado of March 18, 1925, that killed 695 people in eastern Missouri, Illinois and Indiana. That day's death total was 747, when other tornados are taken into account.

St. Louis and the Metro East also were struck by what the Tornado Project lists as the third-deadliest tornado in the nation's history, which roared through Lafayette Square and other parts of St. Louis, crossed the Mississippi River and then devastated a wide swath of East St. Louis, killing a total of 255 people on May 27, 1896. That twister caused the equivalent of $3.8 billion in damages, measured in 2011 dollars.

While it pales in comparison to the worst twister years, the relatively high tornado-related death toll in 2011 was unexpected because advances in early-warning systems, made possible by the development of Doppler radar systems, give many residents 20 minutes or more of warning before a tornado hits. 

"We never thought there'd be another year of deaths like this, with all our warning systems," tornado historian Thomas P. Grazulis told the New York Times.

Joplin Tornado Had Several Spinoff Twisters

While some scientists contend that the worsening weather patterns are related to global climate change, Agee and others say the jury is still out. He believes, however, that this spring's above-average spate of twisters may relate to the La Nina cycle.

A cyclical drop in temperatures in the Pacific Ocean, La Nina tends to last five or six months and repeat every three to five years. This spring, such a pattern is forcing the North Americans jet stream eastward and southward, which alters prevailing winds. The jet stream's high, cool air then pulls upward more humid, warmer air -- creating the "super cells" of thunderstorms.

"But [the cause] is not all La Nina," Agee said. "It's very normal to have abnormal weather." He said that global warming trends would lead to more heated moisture, but any direct connection to climate patterns is unclear. "You have these [tornado] episodes, and then you don't have them. They hit in rural areas, and then they don't."

Agee, who was honored this month with the University of Missouri's Distinguished Graduate Alumni Award, is widely recognized for his research into the development of tornadoes, which in turn has helped increase safety systems. He told the Beacon that the Joplin tornado ranks close to the most powerful and violent twisters.

"The Joplin tornado had multiple vortices," or spinoff funnels, Agee explained. "It's a two-cell vortex, meaning that there's a down-draft in the center that creates a circulation that, on a smaller scale, is much like a hurricane."

Agee said an audio recording made in Joplin during the tornado clearly demonstrated the roar-silence-roar pattern that is typical of a multi-vortex twister. "You could hear the storm approaching and then it passed and it got quiet and then people heard it come again. That was probably a second vortex."

Agee, whose lab built the world's largest tornado simulator to study the fluid dynamics of twisters, said that multiple-vortex tornados often produce from two to six such smaller vortices. "As the swirl increases, you go to what's called a vortex breakdown. It spreads apart aloft and becomes turbulent and a downdraft starts to develop from above. That structure has a wineglass shape.

"As the swirl keeps increasing, that downdraft goes all the way to the surface -- and that's your 'wedge tornado'" -- which is unstable and has the potential to develop multiple vortices, or funnel clouds, that strike in a wide area.

Missouri, Illinois Among Top 10 Tornado States

While Missouri and Illinois are not included in the National Weather Service's definition of "Tornado Alley" -- all or parts of Texas, Oklahoma, Kansas, Nebraska as well as western Iowa and far eastern Colorado -- they rank in the top 10 tornado states.

According to Weather Service tornado archives going back to 1950, as analyzed by the Tornado History Project, Missouri has had 1,942 tornados and Illinois has had 2,102.

Those twisters have killed 262 people in Missouri -- before this week's Joplin tornado, which killed an additional 122 so far -- and 227 people in Illinois. The average tornado intensity in Missouri (on a scale of 1 to 5, with 5 being the worst) has been 1.05, and there have been 0.03 twisters a square mile. In Illinois, the average intensity has been 0.89, and there have been 0.04 twisters a square mile.

Based on those statistics (with tornadoes per square mile weighted as twice the average severity level), the Daily Beast site on Tuesday ranked Illinois as the 8th and Missouri as the 10th worst tornado states. The worst tornado state was Oklahoma, with 3,290 twisters since 1950 and 442 deaths.

Deadliest Tornado Years

*as of the evening of May 24