Tornado Alley is traditionally thought to include parts of Texas, Oklahoma, Kansas and Nebraska. More tornadoes are documented in this geographic region than in any other spot on Earth.
But meteorologists and researchers question whether the Great Plains' version of Tornado Alley actually poses the greatest threat.
"Dixie Alley," a lesser-known version of Tornado Alley, has about the same number of strong and violent tornadoes but more killer tornadoes.
Dixie Alley is defined as an area stretching from Arkansas and Louisiana to Middle Tennessee and Georgia, from Tennessee's northern boundary to the Gulf Coast. Alan Gerard and John Gagan of the National Weather Service office in Jackson, Miss., and John Gordon of the NWS office in Louisville Ky., considered the differences in "A Comparison of Tornado Statistics from Tornado Alley and Dixie Alley." A slide presentation of their findings is available online at: www.srh.noaa.gov/jan/dixieAlley/.
Dixie Alley, they reported, experiences tornado threats most of the year while the threat in the Great Plains Tornado Alley occurs mainly April through June.
"The country's most vulnerable region for tornado-related fatalities and killer tornado events basically stretches from Little Rock (Ark.) to Memphis to Tupelo (Miss.) to Birmingham (Ala.)," Ashley said in an NIU press release.
He offered five reasons why tornadoes here are so deadly.
First, 44 percent of all fatalities during tornadoes occur in mobile homes and the Southeast has the highest percentage of mobile homes in the nation. Mobile homes are the most vulnerable structures in a tornadic situation, Ashley said.
Second, the Southeast has a higher likelihood of nighttime tornadoes. Tornadoes occurring between midnight and sunrise are 2.5 times more likely to take lives than daytime tornadoes.
Third, the Mid-South has more forestland. It's harder to spot tornadoes when you're looking through trees.
Fourth, tornadoes could happen just about anytime of the year in the Southeast. Persons may be caught off-guard if tornadoes develop outside of the traditional spring season.
Fifth, the lack of a focused tornado season may lead persons to become complacent.
Ashley followed up that study with another looking specifically at killer tornadoes. The new study found that only 27 percent of all tornadoes in the United States occurred at night, but 39 percent of all tornado fatalities and 42 percent of killer tornado events happened at night.
Ashley and two other NIU researchers determined that nighttime tornadoes - those occurring between midnight and sunrise - are 2.5 times more likely to take lives than daytime twisters.
Unfortunately, the statistics also show that 45.8 percent of Tennessee's tornadoes between 1950 and 2005 occurred at night. That's the highest percentage in the nation.
Arkansas is second with 42.5 percent; Kentucky, third with 41.5 percent; and Mississippi, fourth with 39.6 percent.
Nighttime tornadoes, obviously, are more dangerous because most people are asleep and less likely to take cover. But, the NIU study also attributed the public's increased vulnerability to the fact that:
* Tornadoes are difficult to see at night, even by trained spotters.
* People are more likely to be in houses, mobile homes or manufactured homes that have not been built to withstand tornado-force winds.
* Warning sirens, which are designed to notify persons outdoors, may not be heard by persons who've turned in for the night.
"Because most people go to bed after the late-evening news, they are sleeping and unaware of televised weather alerts," Ashley said in the press release. "And warning sirens give us a false sense of security. They're not designed for warning people who are already indoors."
The study also found the cool and spring-transition seasons from November to April have the highest nocturnal fatality rates, despite having relatively few tornado events, the press release said. Daylight hours are at a minimum during these months. Also, persons may be caught off-guard by storms occurring before the national severe storm season peaks in May and June.
"Nocturnal tornadoes are dangerous anywhere, but the danger is enhanced in the South," Ashley said. "There are more nocturnal events in the South than in the Great Plains. And the mobile-home density is much greater n the South as well. It's a combination of factors."
Richard Okulski, warning coordination meteorologist for the National Weather Service in Memphis, agreed.
"There is an area from western Kentucky to northern Mississippi which is quite vulnerable to tornadoes," he said. "The area has been dubbed 'Dixie Alley.' This area has a longer severe weather season (November to May) and nocturnal tornadoes (10 p.m. to 6 a.m.) are common."
In addition to the factors Ashley noted, Okulski said many homes in this area do not have basements or safe rooms to protect occupants.
"We looked at tornado-related deaths from 1997 to 2006," Okulski said. "(Weather Forecast Office) Memphis' four-state service area (northeast Arkansas, Missouri Bootheel, western Tennessee and northern Mississippi) had the highest number (around 70) during that time period."
The increased risk of deadly twisters in the Mid-South is something Dyer County Emergency Management Director James Medling wants residents to understand. It's important to be prepared for storms.
Medling and Ashley, the meteorologist, both advocate for increased usage of NOAA weather radios. Some models come with alarms that sound when severe weather watches and warnings are issued for specific areas.
During Severe Weather Awareness Week, Medling asked schools, the hospital and nursing homes to hold tornado drills. He conducted tests of the Dyersburg tornado warning sirens, emergency generators and radios and paging equipment.