- When it comes to tornadoes, the United States is a truly freakish outlier receiving around 75% of all known tornadoes on earth.

And the Great Plains has long been the country's hotspot, a region known as Tornado Alley, but scientists have discovered that tornado alley is moving.

- This is definitely a trend.

This is real.

- And this matters for a particularly disturbing reason.

The places gaining tornado risk are also places where tornadoes are far more deadly.

- Disaster potential is growing exponentially in those regions, the deck is stacked against them.

- Before I could even get up, I was sucked through the window.

Nothing like that's ever happened around here.

- So where exactly is tornado alley shifting to?

What do we know about why it's moving and why is this change making tornadoes so much more deadly?

- Oh my goodness.

Oh!

- It becomes very striking.

When you see the visual representation of a map or look at the risk through time at a specific location, you start to say, wow, this is definitely a trend.

This is real.

- This is Victor Gensini.

He's the atmospheric scientist who first discovered tornado alley's shift, but the idea didn't occur to him in the lab.

- I first started to think about tornado alley moving after several years of storm chasing in the Great Plains where we had very low frequency years.

And I was thinking, you know, this is supposed to be the greatest frequency location on earth.

- So he went back to his lab to crunch the numbers.

- And lo and behold, when you start plotting the trend at any given city, what you start to notice is that places in the Great Plains have been declining over the last 40 to 50 years.

- But the overall number of tornadoes across the country wasn't declining, which meant they had to be going up somewhere else.

- Everybody was telling me like, when you get ready to get home, you know, be careful, be careful.

And my daughter had told me, you know, be careful.

When I came outside, it was like really warm, really calm, and it wasn't storming or raining or anything.

So I just drove on home like any normal day.

And I got home, I turned the TV on just to see how the weather was, and I guess I just dozed off of sleep and then that's when it came.

- This is Heather Chambers.

She lives in Wingo, Kentucky and didn't expect the night of December 10th, 2021 to be particularly memorable.

- I woke up like the house felt like it was like moving almost like taking a deep breath and letting it out.

It's almost what it felt like and I sat up and then immediately I heard the sound of glass breaking and I, before I could even get up, I was sucked through the window.

- Heather survived the deadliest December tornado on record.

Multiple weather patterns came together to create at least 66 tornadoes across the Midwest and Ohio Valley.

But here's the thing, Wingo, Kentucky where Heather lives isn't in traditional tornado alley, but it is squarely in the region where tornadoes are on the rise.

To understand exactly where this new risk is moving.

We first need to look at why tornado Alley exists in the first place.

It all basically comes down to a geographical fluke.

North America is the only continent in the world that extends all the way from the Arctic to the tropics without any major mountain ranges cutting across from east to west, leaving the central part of the continent wide open to colliding air masses.

So Tornado Alley is the meeting place of three different types of air masses.

Warm, moist air from the Gulf.

Cold dry air from Canada.

And warm dry air from the desert Southwest.

Which meteorologically speaking makes this region incredibly unstable.

- So when we're trying to make a tornado in the atmosphere, one of the things that we look for is instability.

That's the air to naturally wanna rise and create clouds and thunderstorms.

We also look for a lifting mechanism, which is something to kick that air and push it upward fast to develop not just clouds, but thunderstorms.

- And then the final crucial ingredient is wind shear.

- And that's how wind changes with speed and direction as we go up in the atmosphere.

- Wind shear is what allows storms to strengthen, stay organized and begin to rotate because it separates the storm's updraft from the downdraft.

- Wind shear is that changing direction with height, and so what happens is as precipitation is lifted through the cloud, it'll lift on one side of the cloud and then fall on the other side.

If we don't have wind shear rain lifts and then it falls right on top of itself and we lose the updraft, it'll all rain and then the clouds go away and we're done.

- So to recap here, the main ingredients needed for tornado formation are moisture, instability, lift, and wind shear.

- So when we look at how tornado environments are changing, we can look at how changes in those individual ingredients are changing over time as well.

- Scientists believe the key to this eastward shift might come down to an atmospheric feature known as the cap.

This is a layer of hot dry air that parks itself right above the warm, humid air from the Gulf, suppressing storm development below it.

- And you allow all of that energy to store up just like you're boiling water on a stove.

And then eventually if the updrafts, if the storms can become strong enough, they break through that lid, they break through that cap, and that's when you get the explosive development that we see with supercells.

- Now, here's why the map is shifting.

The American southwest is getting hotter and drier, and this is making the cap stronger and suppressing storm formation in the western part of traditional tornado alley in places like Texas, but further east in places like Wingo, it's a different story.

A warming Gulf of Mexico is pumping extreme moisture and instability into the Mid-South, providing the kind of explosive fuel needed to punch through that strong cap and shifting the danger zone right to Heather's front door.

- And then you start to look at the numbers in places like Memphis, right, some of the Mid-South of the US, and you start to see an increase in the number of frequency of tornadoes and strong tornadoes there.

- Now, this doesn't mean the Great Plains are no longer the nation's main tornado hotspot.

They still are.

The point is that the data show a robust downward trend in parts of the traditional Great Plains and an upward trend farther east.

- Tornado alley is the area where tornadoes are most frequent.

Kansas, Oklahoma, Texas, that's where we know tornadoes are most likely to occur.

However, if we look at where tornado deaths and fatal tornadoes are most likely to occur, it's in the Mid-South.

It's Alabama, Mississippi, Georgia, Tennessee, Arkansas.

- Which makes sense because there are a few key differences between traditional tornado alley and the Southeast tornadoes.

The first is that they're really fast.

- In Kansas, you could sit out in the middle of a field and see a storm moving five miles per hour.

That's not the case in central Alabama.

When the storms get there, they're moving 60, 70 miles per hour.

- But the next factor is part of the reason that Heather's story is so intense.

After Heather was pulled through her window by the tornado, she landed a few hundred feet from her house where her neighbor found her under piles of debris, - But it was just pitch black and couldn't see anything.

It's amazing that he'd even found me.

- Heather experienced a nighttime tornado, which is incredibly dangerous, but in traditional tornado alley, tornadoes mostly occur between the afternoon and early evening, typically between April and June.

- Conversely, in the southeast United States, there's pretty much no annual cycle and there's very little diurnal cycle as well.

So the story here is that in the Southeast you can have tornadoes anytime of the year at any time of the day, - And all this means more tornadoes at night.

Nocturnal tornadoes are about twice as deadly as their daytime counterparts.

During the day, a tornado may be visible from miles away.

At night, it can be effectively invisible until it's right on top of you, which is exactly what Heather experienced.

These nocturnal tornadoes are actually more likely in the Southeast because of its proximity to the Gulf of Mexico, which supplies warm moisture reliably throughout the year and around the clock.

- While people go to sleep.

The storm's still chugging along because it has plenty of moisture and heat to work with, and then at 2:00 AM here comes the tornado moving 70 miles per hour in the Southeast.

- But there's another reason tornado alley shift is so dangerous and it has nothing to do with weather or climate change.

- Dorothy running to the shelter with a tornado dancing behind her in the middle of the field.

That's not a common sight anymore.

Now that tornado's dancing through a brand new subdivision.

- Tornado alley is moving to more populated places, so that means damage to more homes, roads, and neighborhoods, simply more chances for a tornado to hit something or someone.

After the night of December 10th, Heather's life totally changed.

- You couldn't have told me like "a storms coming."

I'm like, "oh, well storm."

But now it's like they're saying, "on Tuesday of next week, we may have storms."

Like I'm preparing today for a storm that may be coming next Tuesday.

Think everybody should take it seriously, like always have some kind of plan in progress.

- I asked our experts how to prepare for a tornado, and they told me the most important thing was to do exactly what Heather has done, plan ahead, but they had a few more simple yet crucial recommendations.

- You do not want to be in a vehicle.

We know that if you are in a vehicle.

You are very likely to become a casualty.

Your outcome will certainly depend mostly on the type of structure that you're in.

- Put as many walls in between you and the outside as possible.

A smaller interior room is going to be better than a larger interior room.

Stay away from windows.

- Get to the lowest floor possible.

Just get to an interior room and put as many walls between you and the tornado as possible.

Even the strongest tornadoes are very unlikely to kill you as long as you take those precautions.

- These are from Eastside Concrete.

They're actually for septic systems, but the night of the tornado, people had actually got into these to seek shelter and houses were damaged all the way around, but these were still intact.

I've had six people and a dog safely in here, taking shelter from the storm.

It's like peace of mind almost to me, knowing that I have somewhere to go.

- That shift from assuming it probably won't happen to you where you are to preparing like it could happen tonight, maybe one of the most important adaptations of all.