Recipe for a disaster: How this week's tornadoes formed
In Alabama, the hardest-hit state, the storms destroyed hundreds of businesses and homes, such as this one in Tuscaloosa.
April 29th, 2011
01:54 PM ET

Recipe for a disaster: How this week's tornadoes formed

A wave of storms swept through the South this week, laden with tornadoes that killed at least 300 people and left a multistate trail of destruction.

As authorities continue to assess the damage, recover bodies and restore power to thousands of homes and businesses in the storm zone, weather experts have many questions about the confluence of factors that formed such a violent weather system.

Did a perfect storm of sociological, meteorological and geographical events combine to create the second-deadliest tornado outbreak in the nation's history since 1950? The University Corporation for Atmospheric Research's Bob Henson, who recently wrote about the recipe for the storms, said it seems that way, but perfection is relative.

"You never know what’s perfect, because there may be another storm that’s even more perfect. Many say the 1974 super outbreak was the ultimate event," he said.

In 1974, a super outbreak of tornadoes churned through 13 states, killing hundreds of people.

One thing is certain: Way before Wednesday's storms came, forecasters saw the ingredients for trouble in the skies.

Weather forecasters had "very, very strong signals actually about five days out indicating a significant weather outbreak," Greg Carbin, warning coordination meteorologist at NOAA's Storm Prediction Center, told CNN on Friday.

But "saying a significant weather outbreak is coming is quite different from saying a massive tornado will move through Tuscaloosa at 5 p.m.," Carbin said. "So knowing the big picture is pretty good, but you don’t know the specifics; you only know really after the thunderstorm begins to form."

Around midday Wednesday in Mississippi, funnel cloud reports began.

“Tornadoes typically form from what are called super-celled storms,” Greg McFarquhar, a professor in the Atmospheric Sciences Department at the University of Illinois, told CNN on Friday. “You need a number of different ingredients. One is warm, humid air toward the surface. You need some sort of trigger that will start that air rising, that is associated with a cold front, and then a third ingredient is instability in the atmosphere,” he said.

“When a parcel of air starts to rise, if it’s warmer than the surrounding air, it’s going to be less dense than the surrounding air, and it will continue to rise,”  McFarquhar said.

Throughout the day Wednesday, the National Weather Service issued tornado watches - a "particularly dangerous situation" - for parts of Mississippi, Alabama, Tennessee, South Carolina and Georgia.

Other agencies were issuing advisories as well. NOAA’s Storm Prediction Center in Norman, Oklahoma, issued its highest threat warning - “high risk” - for parts of the South. Shortly after, the prediction center issued red-colored headlines on its advisories, noting the likelihood of “destructive tornadoes … large hail up to 4 inches in diameter … and dangerous lightning.”

Another crucial ingredient was wind shear: volatility in wind speed and direction.

"Storms happen all the time," Carbin said, "but for those storms to last, you need wind shear.  If not, it will actually self-destruct. Wind shear will allow the storm to form a very efficient chimney, so to speak," he said.

On top of it all was a fast-moving storm system that raked parts of six states.

"It first showed up in Mississippi," Carbin said, "and at the same time, it wasn’t just one storm, it was five, 10, 15 erupting one after another in a 30-minute span. We know that. What we don't know is how to provide a specific pinpoint forecast of a violent tornado. That's still out of reach."

Armed with data from the National Oceanic and Atmospheric Administration and the National Weather Service, meteorologists in the affected regions went live with coverage of the storms’ path. But there was a major problem in getting the word out.

High winds associated with the storm had already caused power outages in several areas of the tornado zone as tree limbs snapped and power lines fell. Henson said he heard anecdotally that some NOAA weather radio transmitters were down as well, adding to the confusion.

Among the series of storms was at least one, in Smithville, Mississippi, that was an EF5. The National Weather Service in Memphis, Tennessee, said Friday the twister was Mississippi's first EF5 in more than 40 years.

An EF twister generates winds of more than 200 mph, according to the National Oceanic and Atmospheric Administration.

Soon afterward, witnesses reported tornado touchdowns in parts of Alabama.

In Alabama, where most of the damage was done, media and residents are calling another twister, captured on video, simply a "monster."

But how could so many people perish in an event that was forecast?

"Sometimes, we assume that getting the warnings issued is the only task that needs to be done, " Henson said. "But we're seeing that even when the warnings are out, we can still have many people killed."

He said the death toll may speak more to how people live today. "You can have perfect warnings, but if people don’t act on them … and when you have an event this big, you’re going to have some casualties. The open question is: Did there have to be 300?”

"I am concerned that there are a lot of places where safe shelter is just not a priority," Henson said.

Carbin said storms need to be studied more, particularly when many factors are involved.

"Once a storm becomes part of an environment, does it enhance wind conditions or deteriorate them?" he said. "We still don’t understand all of those interactions yet."

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Filed under: Alabama • U.S. • Weather
soundoff (85 Responses)
  1. maclean

    where are the Storm Chaser nerds when you need them??

    April 30, 2011 at 4:08 pm | Report abuse | Reply
  2. Dsn63

    Plan for the future NOW because I will be back when you least expect it! Signed THE TORNADO:-(

    April 30, 2011 at 4:08 pm | Report abuse | Reply
  3. cbchs5744

    first off i live in limestone county, ive seen the devistation firsthand. making a building code that requires storm shelters or basements at residinces is not feasabile. most ppl who live around north alabama (with the exception of huntsville) are dirt poor , not just poor but dirt poor . they live on land that there fathers father bought in either houses that most wouldnt use to stable their horses or their in single wide trailers . they cannot afford to buy food so they hunt for their meat and grow their produce . we will not ask nobody for anything we cant get on our own . and we do have a distrust for all goverment state and federal. these ppl cannot afford to have storm shelters. but every community has their own community storm shelter and they are always open. most ppl who died did so because they did not heed the many many warnings to take shelter.

    April 30, 2011 at 4:20 pm | Report abuse | Reply
    • Dsn63

      Well put.

      April 30, 2011 at 11:08 pm | Report abuse |
  4. Kristi

    I pray for all the victims who have been affected by these tornadoes as well as the ones posting things on here that have turned political and hatered. May God Bless you all!

    April 30, 2011 at 4:45 pm | Report abuse | Reply
    • Jacob Jones

      Thank you Kristi. :)

      April 30, 2011 at 8:11 pm | Report abuse |
  5. Jay

    Carla just needs a thick c..k rammed in her mouth to get her to shut the f..k up

    April 30, 2011 at 5:51 pm | Report abuse | Reply
  6. Travis

    As many people have posted, and the article stated, one of the biggest problems is that people in this area are used to tornado warnings without a tornado. People are used to the NWS forecasting a high probability of tornados and destructive storms and not being affected by the resulting weather.

    I was watching doppler radar all day Wednesday and saw a brutal storm with tons of rotation headed for Chattanooga. I called a few of my friends who have lived in the south most of their lives to tell them to find shelter NOW. They'd been driving around photographing the weather with no idea that a deadly storm was headed towards them. They found shelter as an F4 tornado, 1/2 a mile wide, went within a mile of where they'd been. What they were doing wasn't smart AT ALL but had they heeded every tornado warning issued for their county that day they would've been in their closet all day.

    In my opinion there should be some sort of GPS based, hand held system that will give residents some kind of warning that they're in the path of a potentially destructive storm. As it stands, warnings for entire counties are misleading and lead to people not paying attention to the potential danger when they've been through so many warnings that are issued for a storm 10-20 miles from where they're at.

    April 30, 2011 at 7:14 pm | Report abuse | Reply
    • Mike

      One of the best comments made. To say that we only had or have ten to twenty minutes is not true. We actually had close to forty eight hours. But all to often we are bombarded with County wide warnings that people blow off. But I myself took heed all that day. For those who lost power that morning when we had storms race through with fifty mph winds, you were warned that this was going to be a serious outbreak. For those who didn't lose their power, the local weathermen do an excellent job of letting us know what part of the county is being hit . Hats off to my favorite weathrman James Spann. When my life is on the line I could care less what Boob tube program you break into to warn me of impending danger.

      April 30, 2011 at 8:07 pm | Report abuse |
    • Dsn63

      That's the problem,people tend to blow off these warnings when they think that they don't apply to them but the reality is Tornados don't care where you live they can disapate and reform very quickly not to mention the fact that their movement across the ground can be very unpredictable,it's not always straight line movement.If you here or see a warning then break out the old battery operated radio and keep it close to your ear.

      April 30, 2011 at 8:35 pm | Report abuse |
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