http://ozarkspub.vo.llnwd.net/o37/KSMU/audio/mp3/sense-community-science-behind-what-causes-039tornado-alley039_38302.mp3
Reporter standup: “Good afternoon. I’m Jennifer Moore. For this Sense of Community report, we’re gonna look at the science behind what makes one geographical area more prone to severe thunderstorms and tornados than another. I’m standing And to find out what causes Joplin to be in what’s referred to as “Tornado Alley,” we’re gonna consult with the experts at the National Weather Service.”
“Tornado Alley is a term that’s been defined by the media. And it depends on what criteria you use,” says Dave Geode, the Science and Operations Officer with the National Weather Service in Springfield.
Geode says it’s hard to say that Joplin is more prone to tornadoes than Springfield or even south central Missouri without scouring historical data—that’s because data comes in so many categories: reported tornadoes, the number of tornadoes, severity of tornadoes, deaths caused by tornadoes, and more.
“Overall, it’s generally the area between the Rocky Mountains and the Appalachians that see the most tornadoes within the United States each year. Now, in terms of the stronger tornadoes, they occur primarily from about northwestern Texas up through the plains of Kansas and Nebraska, and into Iowa and Minnesota. I guess you could say that Joplin, and even the Springfield area, could be defined as being on the eastern edge of that ‘Tornado Alley,’” he said.
Many Ozarks residents are used to seeing the green, or sometimes red and pink radar line stretching from northeast Oklahoma into southwest Missouri. It often forms a diagonal line on the map, with the storms moving northeast. Geode says there’s a reason for that pattern, too.
“That has a lot to do with the position of the jet stream,” he said.
Normally, he says, when you get an upper-level low pressure system coming across this area, the jet stream out ahead of it is oriented in a southwest-northeast direction. So, when the thunderstorms develops along that front, they tend to move in that direction.
“People will say the tornadoes follow the interstate, which, I-44 just happens to be in a southwest-northeast direction across this area here. And so this is why you get a lot of the thunderstorms moving in that direction,” he says.
Also in the Ozarks, we are in a geographical area that’s prone to the five ingredients that make a tornado. The first one is moisture. You need a lot of moisture to generate the clouds and the rain and the other precipitation of the thunderstorm. Here in the Ozarks, our warm, moist air sails up on the jet stream from the Gulf of Mexico.
The second ingredient is an unstable air mass. Again, Dave Goede.
“And by unstable, I mean that once that air mass is given a push upward, it continues to rise upward. You can kind of think of that in terms of a hot air balloon. As you heat that air up inside that balloon, it gets less dense and more buoyant until finally, it’s able to lift the weight of the balloon in the basket and the person flying the balloon,” Goede said.
Third, you need a lifting mechanism—something to give that air an initial push to get it to rise into the atmosphere. In most cases, Geode says, that’s going to be a front: a warm front, or a cold front. But it can even be a downdraft from a previous thunderstorm.
Reporter standup: “Another ingredient needed to form a tornado is what’s referred to is a “cap.” And to illustrate this, I’m standing in my own kitchen. I’m standing in front of my gas range. It’s fired up, and I have a pot of water at a fast boil. What happens with the cap is, it acts like a lid, which I’m going to put on right now…[clank]…and the cap acts as a boundary between warm, moist air underneath and cool air on top. This cap is now preventing the warm, air underneath from rising, even though it’s less dense and it wants to rise, it can’t…and this leads to more instability.”
“As the air heats up at the surface during the day, it eventually builds up enough heat, enough lift, or enough buoyancy to break that cap,” Goede says.
When that happens, you get the explosive development of thunderstorms.
The fifth and final ingredient for a tornado is wind shear.
“All wind shear is just a change in the wind speed with direction and height within the atmosphere,” Geode said.
That allows the updraft in that thunderstorm to develop a rotation. And it’s that rotation that leads to tornados.
This unique combination of ingredients is what turns the Midwest Plains area—including the Ozarks region we call home--into a ‘Tornado Alley,’ if you will.
For KSMU’s Sense of Community Series, I’m Jennifer Moore.