Is the Southeast really tornado alley?
Published 11:15 pm Wednesday, November 18, 2015
Congress earlier this year authorized a research initiative on tornadoes in the southeastern United States called VORTEX-SE. The project extends VORTEX I and II, which focused on tornadoes in the Plains. The movie “Twister” featured a Hollywood version of VORTEX I. A planning workshop for VORTEX-SE was held in Huntsville this month, bringing together one hundred atmospheric scientists, National Weather Service forecasters, and social scientists like myself.
The risk posed by tornadoes in the southeast justifies such a project. Alabamians are well aware of this risk after the April 27, 2011 tornado outbreak. Statistics and analysis confirm the lethality of southeastern tornadoes, which predates 2011.
For example, Alabama has had the most tornado fatalities since 1950, while Mississippi ranks first in fatalities per million residents. Forty eight percent of tornado fatalities nationally since 1990 have occurred in seven southeastern states, Alabama, Mississippi, Florida, Georgia, Tennessee, North Carolina, and South Carolina. Fatalities exceed what we would expect given that 20% of all tornadoes occur in these states.
I have published two books and many papers on the impacts of tornadoes, coauthored with Kevin Simmons. We found that the southeastern tornado problem has two layers. The first involves regional concentration of several vulnerabilities. Residents of manufactured homes are at least ten times more likely to be killed in a tornado than persons in site-built homes. Tornado winds that might only damage the roof of a permanent home can do catastrophic damage to a manufactured home. Furthermore, tornadoes after dark, and particularly after midnight, are more deadly, largely because people may not receive a warning in time to shelter. And tornadoes in the fall and winter months are also more deadly, perhaps because people don’t expect twisters during the “off-season.”
Each of these factors are more prevalent in the southeast. Almost 13% of residents here live in manufactured homes, more than double the percentage in the rest of the nation. Forty five percent of all tornadoes between November and February occur here, and nearly 40% of overnight tornadoes.
A second and more puzzling layer of vulnerability exists: southeast tornadoes are just more deadly. That is, tornadoes here are more deadly even when controlling for mobile homes, time of day, month of the year, the Fujita scale rating of the tornado, and a host of other factors. A state tornado casualty index we constructed based on statistical analysis ranks six of the seven southeastern states among the eight most vulnerable states; Alabama ranks 6th. This pattern of lethality is evident even back to 1950.
Several atmospheric and physical differences between tornado environments in the southeast and the Plains may explain the lethality of southeastern tornadoes. The two factors affecting tornado formation are shear, meaning winds moving in different directions at different levels of the atmosphere, and instability, resulting from warm, humid air at the surface and cooler air aloft. Tornadoes in the southeast occur often in high shear, low instability environments. By contrast, the more extensively studied Plains tornadoes typically require significant instability.
Other physical differences exist. Tornadoes can be spawned either by super cell thunderstorms or along squall lines of thunderstorms. Squall lines, called quasi-linear convective systems by meteorologists, are responsible for a larger proportion of southeast tornadoes. But even super cells seem to behave differently. In the Plains, the entire supercell will rotate, and then eventually turn out a tornado. In the southeast, rotation is more often confined to the lower portion of the storm, which Doppler weather radars have difficulty observing. Finally, thunderstorms seem more able to maintain intensity after dark in the southeast, contributing to the problem of overnight tornadoes.
What will we learn from VORTEX-SE? If we knew, we would not need to conduct the research. The first projects will take place in 2016, and Congress may fund additional research. We cannot guarantee that secrets to solving the southeastern tornado problem will be discovered in 2016, or even that any exist to be discovered. Reducing the lethality of tornadoes in our region will almost surely require contributions from both atmospheric and social scientists.
Daniel Sutter is the Charles G. Koch Professor of Economics with the Manuel H. Johnson Center for Political Economy at Troy University and host of Econversations on TrojanVision. To learn more about the VORTEX-SE project, visit http://www.nssl.noaa.gov/projects/vortexse/.