Why large tornadoes repeatedly strike the same location - WALB.com, South Georgia News, Weather, Sports

Why large tornadoes repeatedly strike the same location

TYLER, TX (KLTV) -

It's been a deadly tornado season all across the Midwest.

It's been two weeks since the EF-5 tornado in Moore, OK killed at least 24 people.

I asked KLTV meteorologist Grant Dade to explain why the same places keep getting hit over and over with large tornadoes.

But first he explained, step by step, what causes tornadoes to form.

Dade says you need two things to form a tornado - winds that change with height and speed in the atmosphere - that's called wind shear - and a lot of energy in the low-levels of the atmosphere - that's called buoyancy.

"A tornado is a perfect balance, if you will, of winds coming to and from. And as long as it's in perfect balance, and keeping it under the updraft of the thunderstorm, which is where the air rises in the thunderstorm, that tornado will continue," said Dade.

Just outside Moore, the storm's winds shifted and the outflow wind became the dominant wind, which is how the tornado died out.

"So once we have outflow winds the dominant wind, more dominant than the inflow, it pushes the tornado off the updraft, basically, and then we call that the rope-out stage," Dade said.

And Dade says we see more large tornadoes now than at any other time of the year.

"Of course, they can happen at any time, but the ingredients across Oklahoma and Kansas come together more often in late May into the first week of June," he said.

Grant told me violent tornadoes form in the same places mainly because of the country's topography.

"There are certain areas of the country where all the ingredients come together to produce what we were talking about. The wind shear, the buoyancy. We need to have the Gulf of Mexico moisture available," Dade said. "And it just so happens that the central plains is an area of the country, from Kansas down to Oklahoma, where those ingredients come together the most."

That's because storms with colder air will come out of the Rocky Mountains, while at the same time, warm, moist air from the Gulf of Mexico moves into the area, mixing with the cold air and creating buoyancy.

"Remember, we have that low pressure in Colorado bringing the winds off the Gulf into the Plains states. That's a southeasterly wind at the surface. Just above the ground, the winds are racing out of the west. So we get that changing of wind of height," Dade said, creating the perfect opportunity for larger tornadoes to form.

But even though we're close to Oklahoma, as Dade points out, no EF-4 or EF-5 tornadoes have happened in our viewing area of East Texas since 1950.

"That's not to say it can't happen. Just the odds of those combinations coming together are so unlikely compared to what it is just a few hundred miles to the northwest," he said. "The further west you go, the drier the air mass, and that dry air plays a huge role in the severity of the storms."

Our topography plays a large part in keeping the larger tornadoes away, too.

Things like our hills and trees obstruct the winds moving in the lower levels of the atmosphere, which is where violent tornadoes form.

Copyright 2013 KLTV. All rights reserved.

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