Almost a week has passed since last Wednesday’s historic tornado outbreak in the South. Tragically, the death toll has reached around 350. In Tuscaloosa, Al, more than 200 people remain missing. It is still difficult to fathom how this catastrophic weather event could have happened, but several excellent blog posts have provided insight into the extreme environmental conditions that gave rise to such a violent outbreak and the regional vulnerability that led to the large loss of life.

Extraordinary environmental conditions

Over at’s blog, meteorologist Stu Ostro provides a beautifully succinct synopsis of the background environment that supported the tornado ambush:

The ingredients were “textbook.” I mean, literally what I learned from a textbook more than 30 years ago. The atmosphere was explosively unstable with summerlike heat and humidity, interacting with a classic wind shear setup as a strong jet stream and upper-level trough crashed overhead. Also, dry air aloft put a lid on things and allowed the energy to build up until it blew sky high.

Not only were the ingredients perfect for a tornado outbreak, they were present to an extreme degree. The observed EHI (”Energy Helicity Index”), a measure which represents a combination of instability and wind shear, was extraordinary...

In referring to the EHI, Ostro links to a technical, but terrific blog post by meteorologist Jon Davies, who documents just how off the charts this metric was.

Davies concludes: “The environment conditions where the storms formed and propagated were probably as optimum as we will ever see!”

This all elucidates how and why the atmospheric set up produced (preliminary count) two EF-5, 11 EF-4, and 21 EF-3 tornadoes.

High regional vulnerability

Most people saw these dangerous storms coming. Greg Carbin of NOAA’s Storm Prediction Center talked about how its predictions were good: watches with alarming language were issued hours ahead of the outbreak. National Weather Service warnings were then provided with an average lead-time of 24 minutes. There was abundant media coverage of the storms as they developed. So why did so many people die?

Both Tim McGill, a meteorologist at WGN in Chicago, and Bob Henson at the National Center for Atmospheric Research, blogged about some of the reasons.

McGill cites a 2008 study by Walker Ashley at the University of Illinois which found the Mid-South is the most vulnerable region in the country for tornadoes due to a whole host factors that include:

Mobile home density - the southeast United States has the highest percentage of mobile-home stock east of the Continental Divide. Slate magazine reported: “Even though mobile homes make up less than 20 percent of all homes, even in rural areas, they have accounted for about half of all tornado fatalities in the past three years.”

Forested area - the mid-South is more forested than the Great Plains, so it can be hard to see the tornadoes coming. That notwithstanding, Alabama ABC33/40 meteorologist James Spann credited local videographers with saving many lives by providing the station with footage of incoming tornadoes. He told Media Bistro: “If you can show a live tornado with a camera, there’s no doubt that people will react in a more urgent way.”

Complacency: Ashley indicated some in the South think the Great Plains tornado alley is “where you get tornadoes.” Of course, we’ve blogged about research which finds “Dixie Alley” has just as much tornado activity as so-called “tornado alley” in the Plains.

In addition to these factors, Henson suggests “disrupted communications” may have limited access to warnings due to power outages from storms that impacted many of the same areas earlier in the day. He also mentions the fast pace of storms “barreling forward at 50 mph or more” left people with little time to seek shelter.

So it was the coincidence of the development of extreme environmental conditions over a relatively vulnerable area that led to this catastrophe. We’ve seen these circumstances arise before with Hurricane Katrina the most obvious recent example.

Meteorologists, emergency managers, local/regional planners, communication specialists, social scientists, and goverment officials need to continue working together to reduce the risk of future extreme weather events.

While it’s true that for some weather events, we can’t stop them but only hope to contain them - we need to continue improving forecasts, communication of forecast, pubic education, and targeting outreach at the most vulnerable populations.

Although we don’t yet know exactly how most of the people died from the recent outbreak, based on statistics and experience, mobile-home residents might be a good audience to prioritize in storm education.

UPDATE: Andy Revkin at the NY Times has an excellent blog post on related issues - In Tornado Zones, Seeking Shelter From the Storm. He mentions “dearth of basements” in the South as another possible contributor to the death toll.