Deadliest Tornadoes

DEADLIEST TORNADOES

PBS Airdate: April 11, 2012

NARRATOR: Every spring, tornadoes devastate communities, ripping homes and families apart.

SALLY FOWKS (Joplin Tornado Survivor): Everything's such a tangled mess, you can't tell what's what.

BARACK OBAMA (President, United States of America/Television News Clip): I've never seen devastation like this.

NARRATOR: But last year, 2011, stands out as one of the worst ever.

STACEY BARTOW (Joplin Tornado Survivor): I remember the things falling on me, and I was afraid something big was going to crush me to death.

NARRATOR: Over 550 lives are lost in a multi-billion-dollar trail of destruction.

STORM CHASER: Houses are gone; people are thrown, injured; tremendous casualties. It's massive.

NARRATOR: What can be done to prevent such disasters in the future?

ADAM WRIGHT (Joplin Tornado Survivor): Our, our house isn't even here anymore. We have to figure out a better warning system.

NARRATOR: But a better warning system requires a better understanding of tornadoes, still among nature's greatest mysteries.

HOWARD BLUESTEIN (University of Oklahoma): There's an urgency to try to, to try to crack the code, so we're going to have to get data as often as we can.

NARRATOR: And that data could lead to better prediction and save lives.

STACEY BARTOW: To see so far into the distance and there be nothing…like someone dropped a bomb.

NARRATOR: NOVA follows the scientific investigation into tornado season, 2011.

BRANDY PHILLIPS (Joplin Tornado Survivor): …something I don't want to have to live through again.

NARRATOR: We trace the events of one of the deadliest tornado seasons ever and follow the race to prevent future disasters…

JEFF SNIEDER: There's more supercells dotted down the line, all the way through North Texas.

NARRATOR: …by warning people earlier, in life or death situations.

GARY ENGLAND (KWTV, Oklahoma City): They, they love the weather here. Well, you know, if they don't pay attention, they die.

NARRATOR: On the anniversary of one of the worst tornado seasons ever, can we solve the riddle of the Deadliest Tornadoes? Right now on NOVA.

In a terrifying onslaught, over 1,600 tornadoes strike the United States. It's 2011, the worst tornado season since 1925, with 550 deaths. The most destructive incident is a single tornado that touches down on May 22nd.

Joplin, Missouri, 5:30 p.m.: It's just another Sunday evening, and then…

ADAM WRIGHT: And then all hell broke loose above us.

NARRATOR: …a massive tornado drops out of the black sky.

STORM CHASER: It's trying to come down here, it's coming into the city. Get the sirens going, get the sirens going, I'm telling you.

NARRATOR: The deadliest single tornado in the U.S., for six decades, rips through the south side of Joplin.

STORM CHASER: There it is. Oh, gosh, it's a monster tornado.

NARRATOR: While people run for shelter, their homes are hit head on.

ADAM WRIGHT: It felt like the whole earth was shaking, and, and you could just feel a, a, a something that just wanted to grab you.

NARRATOR: This powerful tornado brings death in its wake. People fear for their lives.

STACEY BARTOW: You could feel yourself rising up. And my 15-year-old was asking me, "Mom, are we going to die? Are we going to die?" And I'm telling her, "No, honey, we're not going to die," and then, in the back of my mind, thinking, "Yeah, probably."

NARRATOR: Spinning at over 200 miles an hour, the mile-wide Joplin tornado devours everything in its path.

BRANDY PHILLIPS AND DAKOTA RENTFROW (Joplin Tornado Survivors): Um, at that point in time, I was just scared, thinking we wasn't going to make it. Like, like a train was coming.

And the whole foundation in the house…

…was shaking…

Everything was shaking.

NARRATOR: The tornado kills more than 160 people and injures hundreds more, destroying thousands of homes, leaving whole neighborhoods in shock.

SALLY FOWKS: I don't think it dawned on us what happened, until we looked at the neighborhood, and it was like, like, you know, there was no comprehension whatsoever.

NARRATOR: People return to find their homes gone, knowing what would have happened had they remained.

ADAM WRIGHT: We wouldn't be alive. As you can see, we would have been caught in our, in our kitchen, and our, our house isn't even here anymore. So, whoever has the power, we have to figure out a better warning system.

NARRATOR: Tornado warnings are often too late to save lives, but a better warning system poses challenges for scientists.

GREG CARBIN (National Weather Service, Storm Prediction Center): Joplin…basically we saw this swath of severe weather potential, but it would be very hard to say well in advance that this part of Missouri was going to be at risk.

NARRATOR: Meteorologist Greg Carbin coordinates warnings at the Storm Prediction Center, in Norman, Oklahoma.

GREG CARBIN: I've been in meteorology for almost 25 years, forecasting, and it is mind-boggling to sit back for a moment and think about the advances that have been made in just that short period of time. But the ability to forecast a tornado event with…prior to an actual thunderstorm forming, is an ability we still don't have.

NARRATOR: Although almost every tornado begins with a thunderstorm, not all thunderstorms produce tornadoes. The difficulty is predicting which thunderstorms will be most dangerous.

GREG CARBIN: It's, that's a double-edged sword in meteorology. You don't want to incite panic and, you know, talk doomsday scenario, but, then again, you don't want to be so careful and quiet that people don't get the word.

NARRATOR: But it's not easy. Thunderstorms can be predicted days in advance and detected within hours of forming, by satellites, weather balloons and radar, but tornadoes are born in minutes.

The most effective prediction tool today is Doppler radar. It works by firing microwave pulses at raindrops to reveal their distance, speed and direction. This distinctive hook shape often indicates that a thunderstorm has started to rotate and could spawn a tornado. This kick-starts the whole process of alerting communities at risk.

When a tornado warning is issued, the amount of time that elapses between the warning and when the tornado strikes, if it does, is called lead time.

HOWIE BLUESTEIN: By lead time, we mean that the tornado will hit your place in X minutes.

NARRATOR: The amount of lead time can be crucial.

HOWIE BLUESTEIN: Certainly, if you have an hour warning, you can avoid a traffic jam, you can get out, and you can drive away and, and be safe.

NARRATOR: But the average lead time that Doppler gives is only 13 minutes, and it's not failsafe. To avoid false alarms, people on the ground are needed to report what's happening locally.

STORM CHASER: Tornado's coming up north, coming up from the city. There he is.

NARRATOR: Gary England was the first TV meteorologist to use Doppler radar for tornado warning, in 1981, but he still counts on tornado chasers.

GARY ENGLAND: You can have the fanciest radar in the world, and you don't really know for sure. It's just a, it's just a piece of electronic equipment. You need the eyes on it. You have to have that person in the field that says, "Yes, I see a tornado." "No, I don't see a tornado."

NARRATOR: But once a tornado is on the ground, it may be too late to help people in its path.

GREG CARBIN: We are trying to anticipate the formation of these. Really, our challenge is to try to anticipate and forecast that development before it occurs.

NARRATOR: So the race is on to improve lead time. But like the weather itself, tornadoes are complex and varied. Still, scientists are coming to grips with the nature of these violent storms.

HOWIE BLUESTEIN: A tornado is a, a rapidly rotating column of air.

CHRIS WEISS (Texas Tech University): In connection with both the ground and also the, the base of the thunderstorm, cold air descends with rain and hail, and wraps around the circulation.

ROGER A. PIELKE, SR. (University of Colorado): And if they concentrate that circulation in certain areas, you have a tornado.

NARRATOR: This is a typical tornado, spawned from rotating winds, in a thunderstorm called a supercell. Most tornadoes are small and local, with wind speeds less than 110 miles an hour. The most extreme tornadoes are two-miles-wide, with 300 mile-an-hour winds, and travel hundreds of miles.

HOWIE BLUESTEIN: We certainly know what a tornado is. However, the big mystery is trying to discover why a tornado forms. We don't understand why some thunderstorms produce tornadoes and others don't. Only a very small fraction of them, 10 percent or perhaps even, even fewer storms go on to produce tornadoes.

NARRATOR: So how can we get better at predicting which thunderstorms will produce tornadoes?

At the University of Oklahoma, in Norman, Howard Bluestein has been asking this question for decades. His research has been key to increasing lead time, but he is determined to go for more. So every year, he is out looking for tornadoes, here in Tornado Alley, a wide swathe of land between the Rocky and Appalachian mountains.

It's April, 2011, the start of tornado season. Howie and his team are out on the plains of Oklahoma, the heart of Tornado Alley, testing their new mobile Doppler radar. So far, all is quiet. But 100 miles away, in another part of Oklahoma,…

STORM CHASER: It is on the ground.

NARRATOR: …a tornado touches down, in Stroud, the first of many in this area.

STORM CHASER: Oh, no. Oh, wow.

NARRATOR: The average tornado lasts two to three minutes. Today, some of these tornadoes are going for ten minutes and longer.

STORM CHASER: Whoa. That's violent. That's incredible.

NARRATOR: Over several long hours, Alabama, Arkansas, Mississippi, Oklahoma and North Carolina are all struck by tornadoes.

It's typical this time of year in Tornado Alley, but why?

It starts with cold winds coming in.

HOWIE BLUESTEIN: During the springtime, we have air coming in at high levels in the atmosphere. It comes in, and it goes up and over the Rocky Mountains, and it subsides and it warms, and that makes southerly winds out over the central part of the United States. And those southerly winds bring in a relatively warm and moist air off the Gulf of Mexico and overspreads the plains area.

NARRATOR: This creates huge thunderstorms, then winds coming in from different directions produce spin.

HOWIE BLUESTEIN: The winds turn with height, and they become a lot stronger with height, so we have a source of rotation within the, the storm.

NARRATOR: This is what tornado chasers record beginning on April 14: strong storms over a wide area, starting to rotate. This one, over Tushka, Oklahoma, becomes a tornado, killing two people. But that's just the beginning.

Over 52 hours, 155 tornadoes touch down in 16 states. Thirty-eight people lose their lives, from Texas to North Carolina. This is a shocking start to tornado season, described as one of the largest single system tornado outbreaks in U.S. history. Scientists are asking, "Is it an isolated incident or part of a pattern?"

GREG CARBIN: We saw a number of these events. We saw, around the 16th of the month, a major tornado outbreak in North Carolina. The questions became, "Can we predict that this pattern will continue?"

NARRATOR: To answer that question, Greg compares the April, 2011, outbreak to previous seasons.

GREG CARBIN: We have, actually, a system that will take a forecast and it will compare that forecast to historic weather events of the past, and so there was a good analogue for this event that had occurred in the past to the forecast pattern that was coming up in the days ahead.

NARRATOR: When Greg examines his data, the result is not reassuring.

GREG CARBIN: The match that came up was a Veterans Day event of November, 2002—nearly 40 fatalities associated with that event—kind of an unusual time of year; not the spring, but actually what we call the second season of activity.

NARRATOR: Fall, 2002, saw 76 tornadoes sweep through 17 states. Greg is concerned that the position of the jet stream, the river of air that circles the earth high in the atmosphere, is affecting the weather. It's shown here in blue.

GREG CARBIN: This is, this is the November jet stream pattern that was in place in the year 2002. We can see the similarities with this event in 2011: jet stream diving across the Rocky Mountains, driving intense thunderstorms across the southeast Tennessee Valley and Ohio Valley.

I sent an email out to the National Weather Service—publicly available through our website—talking about the fact that the upcoming event showed signs of similarity to the outbreak that we saw in November, 2002.

NARRATOR: There's cause for concern. Some meteorologists believe it could be even worse than the 2002 Veterans Day outbreak. A tantalizing clue lies off the coast of Peru.

The eastern Pacific, July, 2010: Ocean buoys record unusually cold sea surface temperatures. This is called "La Niña," and for centuries, Peruvian fishermen have been aware that it not only affects their fishing but also the weather.

Scientists can now measure the effect. Here's the way a La Niña looks on a satellite's thermal imaging camera, showing the cooler sea temperature off the coast of Peru in green. Scientists discovered that the huge expanse of cool La Niña water could affect the surrounding atmosphere and the jet stream, shifting severe weather into new areas, and intensifying it in places like the southern United States.

ROGER PIELKE: We had a very strong La Niña in the wintertime that set up a strong jet stream that provided the wind speed energy that was necessary to generate thunderstorms, but we also, then, had very humid and moist air in the southeastern part of the United States that provided the fuel for these thunderstorms. And the combination of those two provided an environment that was more conducive to large tornadic outbreaks than you might have in other years.

NARRATOR: By spring, 2011, the newly intensified jet stream was already contributing to rainfall and floods across the south, plus droughts and raging wildfires in Texas, warning signs of historic weather extremes.

ROGER PIELKE: We were worried, because when you have a La Niña, as our research has shown, there tends to be more family outbreaks of these tornadoes in the southeast of the United States.

NARRATOR: In fact, one of the worst tornado events in history, the "Super Outbreak" of April, 1974, also took place in a La Niña year.

A hundred-forty-eight twisters touched down, in 13 states, from Mississippi all the way up to New York, killing 330 people and injuring thousands.

But will the pattern hold in 2011?

STORM CHASER: It's crossing the interstate right where we were.

NARRATOR: On April 25th at 7:25 p.m., violent storms erupt in Vilonia, Arkansas, the start of 52 hours of deadly tornadoes.

STORM CHASER: Here comes the rain. Whoa.

NARRATOR: The worst day is the 27th of April, starting at 2:30 p.m.

RADIO (Radio News Clip): A large tornado is now down on the ground.

NARRATOR: A powerful tornado touches down in Philadelphia, Mississippi, and kills three people.

Thirty minutes later, Hackleburg, Alabama is hit, killing 18 people. Then Cullman, Alabama, leaving 19 dead.

4:45 p.m., Tuscaloosa, Alabama: Classic anvil-shaped clouds form supercells, thunderstorms where rotation has begun, as Greg Carbin observes.

GREG CARBIN: I came out into operations, during the, during the late afternoon, seeing incredibly well-formed supercells, the likes of which was truly just stunning.

NARRATOR: A distinctive hook echo, the radar signature of a fiercely rotating storm, heads toward Tuscaloosa.

GREG CARBIN: You knew that with every one of those hook echoes you had a violent tornado on the ground.

STORM CHASER: This is a large, violent tornado coming up on downtown Tuscaloosa. Get in a safe place right now!

NARRATOR: A mile-and-a-half-wide tornado cuts through the heart of Tuscaloosa, leveling entire blocks and tossing trees and power poles around like toothpicks, tornado scientist Chris Weiss recalls.

CHRIS WEISS: The storm that affected Tuscaloosa actually initiated back in Mississippi, actually travelled for a good hour and a half and then produced its tornado; stayed on the ground all the way up into Birmingham. The storm itself actually lasted seven and a half hours, because of the various dynamics, with the storm producing tornadoes along a good chunk of that length. You see regions all across northern Alabama, into northwestern Georgia, even up into western North Carolina. Tennessee was also affected, so a tremendous number of tornadoes, for this outbreak.

NARRATOR: The 52-hour onslaught produces 343 tornadoes, the most ever recorded in a single outbreak. Families in trailer homes and timber-framed houses lose everything.

MIKALI MUHUMMED: Most of this is sentimental 'cause it's my mother's, and I've been had it for 40 years. And I can't get any of it back; I can't get any of it back.

NARRATOR: Even brick houses couldn't withstand the force of this tornado. Whole blocks are virtually flattened.

SURVIVOR: It is unreal; it looks like a third world country or place that has been strategically hit by war.

NARRATOR: In Oklahoma City, TV meteorologist Gary England follows the path of the storms on radar.

GARY ENGLAND: We watched them from here at the radars, and you could see the tornadoes developing, you know, massive supercell thunderstorms, big, big rotations inside. The rotation sometimes becomes the entire tornado, and that's what was happening down there, and it looked like a fleet of them coming across.

NARRATOR: Oklahoma is lucky to escape this time, but the impact is devastating. Although disaster investigator Tim Marshall sees it often, he is always shocked.

TIM MARSHALL (Haag Engineering): I have always been surprised by the power of tornadoes. I mean, after all, all it is air and water, so how dangerous could that be?

NARRATOR: Investigators like Tim use a system called the EF scale to measure the strength of tornadoes by rating the damage they do.

TIM MARSHALL: Typically, every year we get 1,500 tornadoes in the U.S. An EF-0 is damage to tree limbs, some shingles off of a roof or so. And then EF-1 is more substantial damage, like some roof decking.

NARRATOR: An EF-1 can be powerful enough to overturn a mobile home.

TIM MARSHALL: EF-2 the roof has gone.

NARRATOR: The aftermath of even an EF-2 can look like a bomb exploded.

TIM MARSHALL: EF-3 is basically the outer walls of the house are down, and only the interior walls remain.

NARRATOR: An EF-3 releases the same amount of energy as 10 tons of T.N.T., like the tornado that hit Haleyville, Alabama, on April 27.

TIM MARSHALL: EF-4 is basically all the walls are down, with just a pile of debris left on the foundation.

NARRATOR: Very little is left standing after an EF-4 tornado. This EF-4 hit Cullman, Alabama on April 27.

TIM MARSHALL: And EF-5 is complete sweeping clean of the foundation of the house, of all the belongings, such that there's only a little perimeter left in the ground where the house once was.

NARRATOR: And an EF-5 is equivalent in damage to the atomic bomb dropped on Hiroshima. On April 27, an EF-5 tornado strikes Smithville, Mississippi.

TIM MARSHALL: To have an EF-5 go through a major metropolitan area is rare. Less than one percent of all tornadoes get to be that strong and get to produce that kind of intensity.

NARRATOR: There are four EF-5s on April 27. The large Tuscaloosa tornado is rated an EF-4.

TUSCALOOSA TORNADO SURVIVOR: We're alive, and our neighbors are alive, and our son is alive, and so we're okay.

NARRATOR: It killed 64 people. Many more could have died, had it been an EF-5. Overall, the April, 2011, death toll hits 369.

President Obama visits Tuscaloosa to support the shattered community.

BARACK OBAMA: (Television News Clip) I've never seen devastation like this. It is heartbreaking. Some residents here were lucky enough to escape alive, but they lost everything.

NARRATOR: The death toll is a stark reminder of the need to increase lead time to get people to safety.

GARY ENGLAND: There's a tremendous circulation in parts of North El Reno.

NARRATOR: But it used to be worse,…

PILOT: There's a clear wedge tornado, Gary, about a quarter-mile-wide tornado.

NARRATOR: …as Gary England recalls.

GARY ENGLAND: You know, when I first came here, in 1972, the lead time on how early could we get a tornado warning out, it's probably about minus-two minutes. The warnings in those days were just absolutely terrible. Equipment weren't too…the radars were nice, but just nothing to go with them, no computers. And I can only warn you, because it blew someone else's house away, down the street. That's how bad the warnings were.

NARRATOR: This 1950s government information film on tornadoes illustrates how limited tornado warnings were.

1950s FILM FOOTAGE: I'm going to keep watch on the southwest side. It's where most of them come from.

Do you think it's likely?

No, the odds are way against it, even in weather like this.

NARRATOR: Forecasters had to rely on ground observations and weather balloons to tell them if storms were coming. In the 1960s, satellites were launched to observe cloud formations and give readings of Earth's temperature.

But when Doppler radar was introduced, in 1973, scientists could clearly see the hook echo, signaling that rotation had begun. Powerful computers that could analyze vast amounts of data helped get the tornado lead time to today's 13-minute average. This has saved many lives, but could it be better?

Most forecasters believe that a breakthrough will come only by unlocking more detail on exactly how a tornado forms.

Howie Bluestein and his team intend to do just that. They've developed a new Doppler radar to carry on their truck. If they catch a tornado, it could give them enough data to create a computer model they can use to evaluate future storms.

HOWIE BLUESTEIN: What people are trying to do is to take weather data and put it into a numerical model and then let the numerical model produce tornadic thunderstorms. So, then, you can issue a forecast and say, "There's a 20 percent chance that in your neighborhood, four hours from now you might get a tornadic thunderstorm.

NARRATOR: The main question concerns rotation, as Chris Weiss explains in a tornado simulator at Texas Tech.

CHRIS WEISS: To get a tornado, we need that storm to acquire supercell attributes. And that just means that the wind is coming from different directions and speeds, with height.

NARRATOR: The supercell storm starts with air clashing and spinning, mostly in a horizontal direction. To turn into a tornado it needs to go vertical.

CHRIS WEISS: We need to have an updraft, an area of very quickly moving air, pulling the air upwards very quickly. And what that does is it takes the spinning air, and it stretches it in the vertical. So you can imagine that, say, you had one of those Chinese finger trap toys, and you pull on it on both ends, it constricts that access of rotation. It makes it spin faster. That helps us explain most of how tornadoes form, but we don't have a good handle, necessarily, on, on the mechanisms that create that spin near the ground, though. That's where a lot of the research is focused at the moment.

NARRATOR: So, vertical spin is only part of the picture. What else can turn a rotating thunderstorm into a tornado? If scientists can work out other possible factors, like wind speed, temperature and pressure, they may be able to, in effect, "reverse-engineer" a tornado.

HOWIE BLUESTEIN: This is a supercell that's moving to the southeast. I cannot discern any rotation visually, but we need to keep a, keep an eye on that.

NARRATOR: Out on the plains of Oklahoma, in late April, Howie is hoping to get close to a tornado with his new mobile radar. Earlier models scanned the sky about every two minutes; this new radar does it every two seconds. It also scans in minute detail, capturing the actual size of raindrops, hail and debris.

HOWIE BLUESTEIN: There's a funnel cloud due west of us; doesn't appear to be very intense.

NARRATOR: His new radar could significantly advance tornado science, but first, Howie needs a tornado.

HOWIE BLUESTEIN: There are also a cluster of three cells to our northwest, and they look fairly good on radar, not great. So, we're just going to sit here and wait.

NARRATOR: Although it's been a very active tornado season, today there are none on the horizon, but the team shouldn't have long to wait. Historically, May is even worse than April.

Three weeks into the month, it's eerily quiet.

GREG CARBIN: We do know that May is usually the most active month for tornadoes. Fascinatingly, this particular year, right after the events of April 27, these active, storm tracks broke down. After a record April, as far as tornado events, we were headed for a record May, as far as the fewest tornado events on the record, and then Joplin.

NARRATOR: On May 22, a large thunderstorm heads towards Joplin, Missouri.

STORM CHASER: Oh, my gosh. Oh, gosh.

NARRATOR: Suddenly, a lone and enormous EF-5 tornado touches down.

The tornado emerged from some unique weather conditions, as, Tim Marshall, explains.

TIM MARSHALL: The thunderstorm that produced the Joplin tornado began in Kansas.

NARRATOR: Forming at 2:30 p.m.

TIM MARSHALL: Southeast winds came in from the surface, and up aloft there was southwesterly flow. And this action, here, produces a spin.

NARRATOR: Then, higher than normal ground temperatures produce an updraft of hot winds.

TIM MARSHALL: And that updraft tilts that into the vertical and produces this counter-clockwise rotation.

NARRATOR: At 4:15 p.m., rain and hail begin to fall.

TIM MARSHALL: Cold air descends with rain and hail and wraps around the circulation. And as it crosses the state line between Kansas and Missouri, just west of Joplin, a tornado was born.

NARRATOR: The tornado is spotted on the ground at 5:34 p.m.

STORM CHASER: It's a massive tornado. Just massive destruction. Oh, these poor people. Joplin's gone. It's like this big EF-4….just gone. There's people in there.

Hello? Come here. Come here, sweetie. Honey.

SALLY FOWKS: Everything's such a tangled mess, you can't tell what's what. It's really hard to think about all the hard work that's gone in…just been gone.

JOPLIN TORNADO SURVIVOR: It tried to lift us off the ground. Then we'd settle, then we'd lift, you know? And so, it was over in, I don't know, I'm going to say two minutes.

NARRATOR: With over 160 deaths, Joplin is ranked as the seventh deadliest tornado in U.S. history, the worst since 1947.

GARY ENGLAND: The Joplin situation, as far as I'm concerned, was a disaster waiting to happen. Not many tornadoes go through the suburbs there. They're not used to that.

NARRATOR: Joplin hasn't been hit by a major tornado for decades, and development has placed more people in harm's way.

JOPLIN TORNADO SURVIVOR: It's amazing that this window stayed intact, 'cause, had it not been intact, we would have had debris falling in on us.

NARRATOR: It's a painful reminder that in a tornado-prone region, residents can never let down their guard.

STACEY BARTOW: I remember the things falling on me, and I was afraid something big was going to crush me to death. I remember laying there, feeling, like, pieces of wood hitting me. And how I didn't get some of these pieces of wood with the nails and stuff…oh, I know.

KERRY SACHETTA (Principal, Joplin High School): When I first drove up on the scene, it was just utter devastation. I first looked at the school, and my first thought was, when I was younger, the Oklahoma City bombing. That's what it looked like from the outside.

NARRATOR: Joplin High is ripped apart by thousands of pieces of glass slicing through everything in their path. Timing saves the students' lives, because the EF-5 tornado struck on a Sunday.

Tim Marshall flies into the immediate aftermath. His goal is to understand the tornado by studying the damage it left behind.

TIM MARSHALL: You know, it's hard to absorb all this damage, no matter how many times I see it. It's numbing in a way, every one of those little blobs, down there, of debris was a house, and there was a family there.

NARRATOR: From the air, Tim can see where the tornado started, just outside of town, and how its path rapidly widened.

TIM MARSHALL: This tornado track tells me, in its intensity, that there was no safe place, really, above ground. And it's sort of like a big saw blade, shredding house after house.

NARRATOR: On the ground, he studies the debris for any evidence the winds left behind.

TIM MARSHALL: I look for clues that indicate how strong the winds were. So that weighs many hundred pounds, there. The heavy weight of this concrete parking curb, being moved like that, tells us that the low-level winds were very strong.

NARRATOR: The Joplin tornado is so powerful it twists the local hospital four inches off its foundation. Most houses suffer much greater damage.

TIM MARSHALL: The tornado is only in contact with the house for a very short period of time. I mean, this all happens in 30 seconds to a minute, so the heavier your building, the better, and the more apt that you're able to survive it. If you're inside a car, and you're close to a window, you can be sucked out of the vehicle by the differential pressure.

NARRATOR: Even light materials on the loose can be deadly.

TIM MARSHALL: Now, a piece of cardboard's pretty flimsy, but if a piece of cardboard is travelling at 200 miles an hour, then it can go right through the human body.

NARRATOR: Tim wants to help people survive the worst that nature can throw at us.

TIM MARSHALL: Within the mangled mess left behind by tornadoes, that's Mother Nature's fingerprint to me. And I try to make sense out of it, putting back together the pieces and seeing where the fatal flaws are in building construction.

NARRATOR: He is hoping this knowledge can be used to strengthen buildings to withstand tornadoes, a force of nature we can't avoid.

TIM MARSHALL: We can't stop tornadoes. We're going to have to live with tornadoes. 2011 has been absolutely a gangbuster of a year for me in terms of disasters. I've never looked at that many major disasters in one year.

NARRATOR: And it's not over yet. It's late May, and Joplin is not the end of the vicious 2011 tornado season. More storms are heading for towns all over Oklahoma.

HOWIE BLUESTEIN: All the indications were that this was going to be a major tornado outbreak.

NARRATOR: 12:30 p.m., May 24: Just two days after the Joplin tornado, Gary England goes live from Oklahoma City, forecasting trouble.

GARY ENGLAND: (On Air)…moving that way, right now, so take particular precautions.

NARRATOR: His forecast is based on National Weather Service alerts, his own Doppler radar and tornado chasers phoning in from across the state.

STORM CHASER: Gary, this is going to be a large tornado, about half a mile wide.

NARRATOR: Forty years of experience have prepared him for days like this.

GARY ENGLAND: (On Air) If you live on Chickasha on, northeastward, take your tornado precautions. This is another giant tornado…preferably a cellar, basement or a safe room.

NARRATOR: A string of tornadoes touches down in Oklahoma. Spinning at 200 miles an hour, an EF-5 tornado travels for 65 miles through El Reno, Piedmont and Guthrie, Oklahoma.

GARY ENGLAND: (On Air) If you're in Piedmont, you need to take your tornado precautions, right now. This is a life and death situation.

This was the first tornado. This tornado, if you take eight football fields and put them end to end, that's how wide it was. We had four this size. It was absolutely amazing.

NARRATOR: It's the sixth EF-5 of the year. Three EF-4 tornadoes also develop on this day. But in Oklahoma, only nine people are killed, compared to more than 160 deaths in Joplin.

So what accounts for the difference? Part of the reason is that these tornadoes missed major population centers, but a more important factor may be a greater tornado awareness. In the heart of Tornado Alley, the people of Oklahoma are used to living with tornadoes.

GARY ENGLAND: Oklahomans are very aware of what's going on in the weather. They stay weather-aware. I think most of the audience understands what we're talking about. There's a lot of times I'll say we have a big supercell, they know that's a rotating thunderstorm usually. And usually a, a supercell has that rotation in it, I think they understand them. They, they love the weather here. Well, you know, if they don't pay attention, they die.

JIMMIE ZIMMERMAN (Oklahoma Tornado Survivor): So…went to the TV, turned it on, and Gary England was on there.

GARY ENGLAND: (On Air) Right now, its along interstate 40, which. for you guys down in El Reno,…

JIMMIE ZIMMERMAN: They were tracking the tornado. So, I went to the kitchen window, and I looked out, and I saw it. And that's, like I said, is the first tornado I've ever seen in my life.

GARY ENGLAND: (On Air) In El Reno, I strongly suggest you take you tornado precautions, 'cause it has produced a huge tornado.

JIMMIE ZIMMERMAN: So, I went to the safe room, and I was in there maybe 30 seconds, when it hit.

NARRATOR: The low death toll in this state may also be due to the fact that many Oklahomans have a safe room, a reinforced shelter, built to withstand an EF-5. This one saved eight lives in Piedmont.

SAFE ROOM SURVIVOR: The way the safe room is designed is to tie the whole structure together…is the reason these walls are standing right now.

NARRATOR: But a safe room is too costly for everyone to have. So, by simulating the effects of tornadoes, scientists at Texas Tech are trying to find cheaper ways to make ordinary structures more robust.

DARRYL JAMES (Texas Tech University): It boils down to cost. We could design a building or structure that would withstand a tornado. The problem is that most of us couldn't afford to live in that structure.

The tornadoes that we simulate, in here, are based on the mid-EF-3 range, because about 92 to 94 percent of all tornadoes fall in that range. The reason why we're doing that is we would like to understand the, the wind loading on structures, such as this scale model of a mobile home. Our preliminary work shows you have parts of the structure that experience a positive force. In other words, they're trying to push the force in, and then, as a tornado gets closer and closer, then you have, suddenly, parts of the force wanting to pull it apart. The other bad part is you have stuff falling down from your roof.

NARRATOR: Structural engineers at Texas Tech are also looking at debris impact, trying to replicate the forces of the worst tornadoes, like the EF-5 that hit Joplin.

LARRY TANNER (Texas Tech University): What we do in this facility is research debris impacts from severe storms. Tornadoes can send all kinds of debris, impaling buildings, livestock, people, cars. It's, it's phenomenal.

DARRYL JAMES: Ultimately, we would like to develop codes that say, if you live in these tornado-prone areas, you ought to think about reinforcing structures in this way or building a structure in this certain way. That's the ultimate goal.

NARRATOR: The devastation of 2011 makes finding ways to prevent future deaths from tornadoes an urgent priority.

One important concern is the national radar system. There are 159 fixed Doppler radars across the country, not enough to get full coverage of the lower atmosphere.

JERRY BROTZGE (National Science Foundation, Collaborative Adaptive Sensing of the Atmosphere [CASA] Engineering Research Center): Tornadoes form near the ground and the lower several kilometers, but one problem that we have is what we call the earth curvature. So, as you're moving away from the radar, the earth curves down underneath the beam, and so the problem we have is we're not seeing a lot of the atmosphere near the ground. And that's a huge limitation of the current network.

NARRATOR: Jerry Brotzge and his team are working on a solution.

JERRY BROTZGE: These radars, because they're looking low, they're giving us, giving us information in critical areas.

NARRATOR: These radar stations are closer to the ground and able to look below the current radar system. They also scan the skies faster. A prototype was put to the test during the 2011 tornado season.

JERRY BROTZGE: One of the large EF-4 tornadoes that occurred, here in Newcastle, Oklahoma, within several miles of one of these radars, that information was sent to emergency managers in the field: rounded everyone up, roughly 1,000 people, and got them in the shelter.

NARRATOR: Oklahoma-based meteorologist Kim Klockow is also looking for ways to reduce deaths. She studies how people react to tornado warnings.

KIM KLOCKOW (University of Oklahoma): The slice of psychology I am looking at is risk decision science. So people, when they have risky choices, they tend to overweight low ranges of probability and underweight higher ranges that aren't certain.

NARRATOR: Kim is investigating how people affected by the Oklahoma outbreak responded. People like Randy Tucker, hit by an EF-4 tornado near Chickasha.

RANDY TUCKER (Oklahoma tornado survivor): We came out and stood around in disbelief. Kind of out…walked down the street, all the homes at the end of the road was gone.

NARRATOR: Randy survived by running to a neighbor's underground shelter, but with only seconds to spare, despite sirens and warnings on TV and radio.

KIM KLOCKOW: Some of the most striking things that I'm finding are people aren't necessarily sheltering right away.

NARRATOR: Warnings are often ignored, because people don't believe a tornado will strike them, especially if they've experienced a false alarm. They wait until they actually see for themselves that a tornado is heading their way. And sometimes that's too late.

KIM KLOCKOW: After I've collected this data, I'll brief the National Weather Service. Now we should start thinking about new and creative ways to implement warnings to alert people.

NARRATOR: Kim's findings will be used to revise how tornado warnings are issued in the future. But the Holy Grail is still to extend lead time, to give people time to get out of the way.

So, on the same day that Gary England makes his alarming forecast, Howie Bluestein and his team are back on the road, hoping to catch a tornado forming. They're chasing the big question: why do some thunderstorms spawn a tornado and others don't?

Howie believes the size of raindrops could be a clue, but only now can his new mobile radar measure them. The challenge is to put it in the right place at the right time, and as tornado season draws to a close, this may be Howie's last chance for the year.

HOWIE BLUESTEIN: Yeah. Oh, wow. Holy mackerel, we've got three hooks. We have three hook echoes, right now, three potentially tornadic storms.

NARRATOR: The team has managed to place the radar in the path of a storm, picking up hook echoes, which signal the storm is beginning to rotate.

HOWIE BLUESTEIN: And the southern one is coming fairly close towards us, but it's a good thing we stopped.

NARRATOR: It looks like it could be evolving into a tornado.

HOWIE BLUESTEIN: Hope we don't have to move. That's remarkable.

NARRATOR: Half an hour later, the tornado is forming, and Howie's radar is recording it all.

HOWIE BLUESTEIN: This has never been done before, so we see how the storm evolved on a two-second time scale. We have a relatively complete look at the evolution of the tornado as it was beginning, as it was intensifying, as it became very strong.

NARRATOR: This is the closest anyone has ever come to capturing, on high-resolution radar, the complete formation of a tornado emerging from a thunderstorm.

SCIENTIST: I think that's some of the most spectacular tornado genesis I've ever seen.

NARRATOR: This new data on raindrops: their size, speed and direction, might help Howie prove his hypothesis. Could the size of raindrops influence why some storms form a tornado and others don't?

His new radar reveals something else: a narrow dry band, with no rain or hail. Howie and his team are still trying to determine its impact, but it could be another important clue in tornado genesis.

These findings will be integrated into a computer model designed to help forecasters predict deadly tornadoes earlier, increasing lead time and saving lives.

As the 2011 tornado season comes to an end, with its extreme devastation, scientists and communities at risk are grappling with further questions: Will destruction at this scale become more frequent?

TIM MARSHALL: And the reason why tornado disasters are on the upswing is that more people are getting in the way. As our cities expand, the target gets bigger. So, tornados are more apt to hit major cities now than they were a hundred years ago, and that can only increase in the future.

NARRATOR: And does global climate change mean it is getting worse?

HOWIE BLUESTEIN: As the climate warms up and the amount of moisture near the ground increases, that certainly is something which is favorable for producing more thunderstorms.

GREG CARBIN: Probably, there's a relationship, on the large scale, to a warmer climate. We know that there's a potential for more moisture in a warmer atmosphere, and we are seeing extreme precipitation events occur more frequently due to that.

NARRATOR: So could the danger zone expand beyond Tornado Alley?

GREG CARBIN: What we may see…we could see a redistribution of those areas that experience a greater risk from tornadic thunderstorms. We could see that the conditions that favor these outbreaks shift to perhaps the upper Midwest or Great Lakes or Northern Plains over a period of time.

NARRATOR: If this really happens, the impact would be enormous, placing more lives at risk from tornadoes, making the quest to understand these terrifying forces of nature ever more urgent.

STORM CHASER: …debris falling. Oh, it just hit a house! It just hit a house!

Stop, stop, stop! Power's down, power's down!

Oh my god!