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Posted at 12:50 PM ET, 04/28/2011

Tornado outbreak for the record books: how did deadly, destructive event happen and what does it mean?

A tornado moves through Tuscaloosa, Ala. Wednesday, April 27, 2011. (AP Photo/The Tuscaloosa News, Dusty Compton) (Dusty Compton - AP)

(originally posted at 5 a.m., updated at 12:50 p.m.)

The devastating tornadoes that chewed up swaths of the South and Southeast Wednesday were rare, mile-wide plus beasts, causing hideous damage, including denuded trees, flattened buildings, and pancaked cars. Death totals have climbed to at least 249 people, the third deadliest outbreak since the 1930s. Thanks to the 164 tornadoes reported (this number will change as the National Weather Service conducts their damage assessments), April 2011 has now almost certainly seen more tornadoes than any other April on record since 1954 when an estimated 407 tornadoes descended from the heavens.

Similar to the tornado outbreak on April 14-16, during which dozens of tornadoes affected states from Texas to Virginia, the latest outbreak was accompanied by unprecedented media coverage via social media as well as television coverage and livestreaming Web video. The most gripping and heart-wrenching coverage came from ABC 33/40 TV in Alabama, which aired live footage from a Skycam tower in Tuscaloosa as it captured a massive tornado moving through that city, leaving parts of it in absolute shambles and killing many.

Above: Live streaming video from ABC 33/40 as tornado bears down on Tuscaloosa.

In a surreal twist, wreckage from Tuscaloosa made it onto The Weather Channel, albeit about a half hour later and 50 miles away, as pieces of sheet metal and other debris rained down onto a bewildered Jeff Morrow in Birmingham, a veteran meteorologist for the network. Morrow then stared helplessly as the same tornado barreled across northern Birmingham.

All these tornadoes, and the jaw-dropping destruction they have wrought, raise a number of questions. Here’s a breakdown of the tornado numbers so far this month and this season, why the severe weather season has been so extreme, and how this may or may not relate to broader climate factors.

Q. What caused yesterday’s tornadoes from the South all the way into the mid-Atlantic and Washington, D.C. area?

Yesterday, the South, Southeast and mid-Alantic had the necessary ingredients for severe thunderstorms and tornadoes, but in varying degrees.

In the South and Southeast, all the ingredients came together to create a once-in-a-generation outbreak of severe weather and tornadoes. Unstable air had been in place for several days (leading to tornado outbreaks in TX, AR, MS, and LA in days prior), and an approaching cold front. Normally, this type of set up would create severe weather, but not to this effect. This time, a strong jet stream drove deep into the southeastern states with westerly winds of over 100 mph. At the same time, a deepening surface low created low-level winds from the south/southeast, creating a severe turning of the winds with height or wind shear. With this extreme wind shear and instability, storms had little trouble starting to rotate and producing tornadoes.

Diagram of a tornadic thunderstorm, showing the airflow within the storm, and typical location of a tornado. Credit: National Severe Storms Laboratory

The mid-Atlantic area, including Washington, D.C., had a similar but much less extreme setup. It certainly had the warm air, but the mid/upper levels of the atmosphere were warmer, making in not as unstable as in the South. The jet stream was nearby, but it wasn’t as strong, nor was the mid-Atlantic in as favorable a region of it to create the wind shear necessary. Also, it was further removed from the cold front so and thus didn’t have as strong of initial trigger to get storms going to begin with. Instead of the cold front, mid-Atlantic storms relied on a little bit of upper-level energy moving through in a short-wave trough.

So in general, while both areas had the conditions that were required for tornado formation, the extreme levels that were in place in the Southeast greatly dwarfed the marginal, yet favorable, levels that we in the mid-Atlantic experienced.

Q. How did this outbreak stack up against historic outbreaks in the past, such as the Super Outbreak of 1974?

This outbreak probably did not quite eclipse the Super Outbreak of 1974, during which 148 tornadoes struck 13 states. Yet tragically, both caused tremendous damage and loss of life. Approximately 330 people were killed in the outbreak of 1974 and almost 5,500 were injured. While sadly, there were many fatalities and injuries in yesterdays events, the numbers do not appear to add up to anything close to those amounts - a testament to the forecasting advances of the past three decades and early warnings.

On Wednesday, 164 tornadoes were reported in at least 13 states. During the past two days (including Tuesday), 232 tornadoes were reported in at least 16 states, though many reports appear to be multiple reports of the same storm. As far as storm intensity, the Super Outbreak featured 24 F4 and six F5 tornadoes, whereas it doesn’t appear that so many violent tornadoes occurred yesterday, though images from Tuscaloosa and Birmingham suggest there were several that will approach these levels.

Both outbreaks also look to have a signature tornado that is identified with it. For the Super Outbreak, it is the Xenia, Ohio F5 tornado. This tornado famously destroyed much of downtown Xenia and killed 34 people. Yesterday’s signature twister will undoubtedly be the tornado that tore through both downtown Tuscaloosa and Birmingham live on TV, creating tremendous damage and multiple casualties.

As for April so far, it’s virtually certain that April 2011 has now eclipsed both April 1974 and April 1957 as the most active such month on record. As Jason Samenow reported Tuesday, the official monthly tally in 1974 was 267 tornadoes, but because of advances in tornado detection (Doppler radar, a national network of weather spotters, etc.), we know that the actual number was likely higher than that. For that reason, NOAA’s Storm Prediction Center’s historical tornado record, dating back to1954, makes a mathematical adjustment. The adjusted tornado count for 1974 is 404. And for April 1957, another active tornado year, the adjusted total is even higher, at 407.

Q. What is making this tornado season so violent compared to past years?

A large part of the answer lies in the airflow in the upper portion of the atmosphere, at the jet stream level. There has been a parade of energetic storm systems, accompanied by very strong jet stream winds, diving down into the southern states this spring. These dips, or “troughs”, in the jet have helped set up major clashes between cold air to the north and warm, humid air from the Gulf of Mexico to the south.


Above: Violent tornado around Tuscaloosa, Alabama from Crimson Tide Productions.

In order for tornadoes to form, several factors have to combine in just the right way. These include: a warm and humid atmosphere, strong jet stream winds, and atmospheric wind shear (winds that vary with speed and/or direction with height), as well as a mechanism to ignite this volatile mixture of ingredients - such as a cold front. The unusually powerful jet stream has helped ensure that these elements have been brought together on multiple occasions.

Importantly, all of the other ingredients were abundantly available too, making for a veritable “Goldilocks” scenario of tornadic weather - all of the pieces were there, at the right place and the right time. That does not happen often. But when it does, watch out!

Q. Does this severe weather have anything to do with global warming or El Nino/La Nina?

The short answer to this one is - it’s complicated. There is no evidence that tornadoes have become more common or more intense due to global warming, or that they are now occurring in different parts of the country than they used to. In fact, meteorologists refer to parts of the South and Southeast as “Dixie Alley”, in addition to the more famous “Tornado Alley.”

A review of the scientific evidence shows that La Nina, which is a natural cycle of climate variability associated with cooler than average waters in the Pacific Ocean, tends to be associated with a greater number of tornadoes in the U.S. One study found tornadoes during La Niña years had longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. The Super Outbreak in 1974 occurred during a La Nina year.

We explored the global warming question in detail earlier this week.

Another contributor to the recent severe weather is an unusually warm Gulf of Mexico, where sea surface temperatures are running 1 to 2.5 degrees Celsius above average. The Gulf is the main moisture source for storm systems as they move east from the Rockies, and the additional moisture is helping to fuel thunderstorm development.

As for global warming, this is an active area of scientific research, with some conflicting projections so far about whether a warming atmosphere will make it more or less likely that tornadoes will form. Since more moisture gets added to the atmosphere as the climate warms, additional water vapor may help severe thunderstorms and tornadoes to form. On the other hand, wind shear is expected to decline due to climate change, which would argue against an increase in tornado numbers.

According to some studies, though, by the end of the present century, the added water vapor will be enough to overcome the lower wind shear, and create more opportunities for severe thunderstorms to form.

Tornadoes are a bigger wild card for climate scientists than other types of extreme weather and climate events, such as heat waves and flooding. (Studies have consistently found that both of these hazards will occur more frequently and severely as the world warms.)

Q. Should we be concerned about similar tornadoes happening in D.C. as happened in Tuscaloosa and Birmingham?

The DC metro area does have a history of tornadoes, some of which have proven particularly violent. Nine years ago (in 2002) on this very date (April 28), an F4 tornado in La Plata, Maryland in 2002, which killed five people, injured 120, and caused about $100 million in damages.

Luckily, advances in forecasting have made it possible for forecasters to highlight extremely dangerous conditions well in advance, and issue watches and warnings with enough notice to warn most people in harm’s way.

By and  |  12:50 PM ET, 04/28/2011

Categories:  U.S. Weather, Thunderstorms

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