JOHN YANG: Good evening, I'm John Yang.

The ominous weather forecasts and warnings came true for a large section of the country as a massive line of severe weather wreaks havoc from the deep south all the way north to the Great Lakes.

At least 21 people have been killed and an estimated 85 million people were in the path of the storms.

From two twisters and a wide-open Iowa field.

NO NAME GIVEN: Dude stop.

JOHN YANG: To a funnel captured up close by storm chasers.

The outbreak of severe weather was intense.

The system brought hail and even snow.

There have been multiple deaths in Tennessee to the east of Memphis and in the small town of Wynne, Arkansas which took a direct hit from a tornado.

Residents were left stunned.

NO NAME GIVEN: I'm speechless.

This is no word.

We can sign that.

This never happened down here like this.

For it to happen, and it's not words.

JOHN YANG: In the small town of Sullivan, Indiana, the mayor said parts of the community are unrecognizable.

Cities and Towns big and small were hit hard.

On Friday a tornado made its way through Little Rock Arkansas, population 200,000, sirens blared, warning some residents in the capital city to seek shelter.

Others relied on a signal from nature.

JOANNA MCFADDEN, Little Rock Resident: All the way we knew the tornado was coming, the leaves were swirling.

That's all the way we knew.

It looked like it was standing still.

Someone said run to the back.

Man, these two ladies here.

We ran to the back.

We huddled together and we pray for our lives.

JOHN YANG: Drone footage over parts of Little Rock revealed the scope of the damage.

Roofs ripped off, entire buildings flattened.

Arkansas Governor Sarah Huckabee Sanders spoke to reporters in front of one of the city's fire stations itself in the tornadoes path.

GOV.

SARAH HUCKABEE SANDERS, (R) Arkansas: I've had the opportunity over the last couple of hours to speak with both the Homeland Security Secretary as well as President Biden who have offered a tremendous amount of support, anything that Arkansas needs.

They have assured us that those resources will be here and on the ground.

And we really appreciate their willingness to help Arkansas out.

JOHN YANG: Daniel Cline and his family took cover minutes before the tornado hit.

DANIEL CLINE, Little Rock Resident: We went into a small closet close the door.

Then we heard some distant crashing and then the house started to shake and rumble.

Once we felt it was close, it was maybe 15 to 20 seconds kind of various chaos and then then it was dead quiet.

JOHN YANG: Cline said the damage in his neighborhood seemed random.

DANIEL CLINE: Lots of people were home and they seem to be from what I can gather mostly unhurt.

A lot of close calls of people, trees going through various parts of people's houses and they're missing getting hit by feet.

JOHN YANG: In Belvidere, Illinois the severe weather caused the roof of the Apollo Theater to collapse during a heavy metal concert.

Official said one person died and at least 40 others were injured.

And as the cleanup begins, more than 200,000 people across five states remain without power.

For more on these storms, Walker Ashley of Northern Illinois University.

He's an atmospheric scientist and a certified consulting meteorologist.

I understand you were doing some firsthand research last night, a little storm chasing, what did you see?

WALKER ASHLEY: Well, got on some good supercells in the central part of the state.

Luckily, they didn't produce any tornadoes in the area that I was in.

But unfortunately for the communities, they did get take some pretty big hail, but compared to some parts of the Midwest, I think we made out okay.

JOHN YANG: You've got a recent study about the future of supercells in the United States.

First of all, what is a supercell?

WALKER ASHLEY: Great question.

Supercells are rotating thunderstorms.

They're actually relatively small.

But they pack a heck of a punch.

I always liken them to peanuts.

You know, peanut is a relatively small piece of food, but boy, it's got a lot of calories, a lot of protein, a lot of fat.

And these supercells, although relatively small, produce the lion's share of tornadoes and significant hail that we experienced in the United States.

JOHN YANG: And some of the biggest tornadoes in recent years Moore, Oklahoma, Joplin, Missouri, the southern outbreak in 2011.

You, as I understand your study, you modeled various levels of carbon in the -- over the years to see what would happen, and what did you find?

WALKER ASHLEY: Well, it's just not carbon, we looked at various sort of future projections of the greenhouse gas, right, emission scenarios.

Both this intermediate and pessimistic warming reveal a dramatic increase in the number of supercells across the Mid-South, the Ozark Plateau, the lower Ohio Valley.

So that area of -- let's call it, northeast Texas, eastern Oklahoma, Arkansas, Missouri, and stretching over to Mississippi and Alabama, I take sometimes a doubling of the number of supercells.

If you double the number of supercells, the number of unfortunately, the hail and tornadoes could also see a doubling.

Somebody uniquely, though, we actually see a decline into the areas of the western Great Plains.

Some people like to call that colloquially sort of tornado alley, tornadoes, and supercells are still going to go through this area, but maybe at some slider, or maybe less rates as we move into the 21st Century.

JOHN YANG: And to these recent storms that we had, big storms last weekend again this weekend, did they suggest that the future is here?

WALKER ASHLEY: That's a great question.

Climate change is occurring right now.

It is going to accelerate based on the emission scenarios that we're seeing.

There's certainly things that we can do to mitigate that.

And tornadoes are becoming more frequent East.

And that's certainly what our model projections suggest.

JOHN YANG: In the past I've heard scientists say that tornadoes are such small events compared to a hurricane.

It's hard to make the connection with climate change.

Did these projections help us toward making a connection?

WALKER ASHLEY: I think the question is, a lot of times we get individual tornadoes, and people are always wondering, what was that caused by climate change.

And that's really framing it wrong.

What we really want to look at is the contribution to that event by climate change.

And we're not there yet.

On the individual tornado or even the outbreak scale.

We're not there quite yet.

I think we'll get there as to what the relative contribution of climate change is to that individual event.

What we're looking at is kind of climatological trends.

And so we're looking over 15-year periods.

What is the macro sort of statistics suggest, and the evidence is revealing in that we are modifying the basic fundamental ingredients necessary for not only the storms, but the tornadoes, particularly moisture, and something we call instability, or the gasoline needed for these storms.

Those two things are increasing and are projected to go increase even more throughout the 21st century.

So like baking a cake, if you increase the amount of sugar, it's going to be a sweeter cake.

JOHN YANG: Sweet cake that we're not necessarily looking forward to.

Walker Ashley of Northern Illinois University, thank you very much.

WALKER ASHLEY: Thank you.