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America Has the Worst Tornadoes on Earth

Putting yesterday's storm in context.

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A woman carries her child through a field near the collapsed Plaza Towers Elementary School in Moore, Okla., Monday, May 20, 2013.(AP Photo Sue Ogrock)

Action News 5 - Memphis, Tennessee

A massive and powerful tornado hit Moore, Okla., yesterday, causing widespread destruction, including at least 51 deaths. It's the deadliest tornado since 2011, and one of the worst in the last 20 years. The same evening, President Obama signed a disaster declaration for Oklahoma.

 



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Moore has a deep and tragic tornado legacy. The town could probably lay claim to being the very center of Tornado Alley, an area roughly defined from north Texas to South Dakota, and west of the Mississippi river. On May 3, 1999, Moore was hit by one of the worst tornadoes on record. That storm's winds were indirectly measured at 302 miles per hour, according to the National Weather Service, which called them "the highest winds ever found near Earth's surface by any means." That tornado killed 36 people, the most deadly in more than 20 years, although several storms have surpassed that number of deaths since then, including 2011's Joplin, Mo., storm, which killed 158.

Several sites are covering the tornado live:

 

Smithsonian's Colin Schultz put together a comprehensive explainer on understanding the power of the storm. Weather Underground's Jeff Masters is the go-to blogger for meteorological events and has a substantial post on the tornado.

The Red Cross is accepting donations via text message. Text REDCROSS to 90999 and you'll be billed $10. The municipal government of Moore is posting updates to a Facebook page.

We've assembled some common questions and research-based answers about the scientific and historical context of the disaster. If you have others you'd like answers to, e-mail me at alexis.madrigal[at]gmail.com.

  • When exactly did the storm hit?
  • Why do tornadoes strike Tornado Alley so frequently?
  • How are tornadoes measured?
  • Are tornadoes getting more frequent?
  • Are tornadoes killing more people than in the past?
  • How did Moore's emergency warning system work? Did people have time to take cover?
  • What was the housing stock like in Moore?
  • When do tornadoes strike most often?
  • How well can we predict tornadoes?

When exactly did the storm hit?

 

Meteorologist Mike Smith created a map showing exactly how and when the tornado approached Moore during the 3:00 p.m. hour. (It's worth noting the tornado warning went out at 2:40pm.)

 

Why do tornadoes strike Tornado Alley so frequently?

The simple answer is that warm, moist air from the Gulf of Mexico gets sandwiched between war, dry continental air and cold, dry air from drifting down from the Rockies. The combination creates the perfect conditions for thunderstorms to form. A more detailed explanation of the regional air movements is available here.

As these air masses collide, they can generate a type of particularly dangerous thunderstorm called a "supercell." They are characterized by their very strong, rotating updrafts accompanied by strong downdrafts. Tornadoes tend to occur at the interface between these two air movements.

While meteorologists are not quite sure why some supercells spawn tornadoes while others do not, it's clear that having strong thunderstorms makes supercells more likely and having more supercells makes tornadoes more likely. Hence, the areas with the most strong thunderstorms tend to have the most tornadoes.

75 percent of all tornadoes on Earth occur in North America. Per square mile, Oklahoma has the fifth-most tornadoes of any state, and the fourth-most tornadoes on an absolute basis after Texas, Kansas, and Florida.

It is also worth noting, however, that even in the very center of Tornado Alley, it is rare for any particular area to be hit by a tornado. As you can see in the graphic below, any location in the reddest part of the map could expect that a violent storm like the one today would touch down within 25 miles only four times per century.

How are tornadoes measured?

The "Enhanced Fujita" scale attempts to quantify tornado strength.

There is some controversy about the scale, particularly because wind speed is often inferred from damage, which is dependent on local construction practices, and not as precise as researchers would like.

Other factors affect the destructiveness of a tornado aside from its winds: how wide it is, where it touches down, and how long it remains on the ground.

The main thing to keep in mind is that anything EF4 or EF5 is likely to be very destructive if it hits near a human population center and stays near the ground for any period of time. Larger tornadoes do more damage for obvious reasons, too.

The preliminary reports are that the storm today was an EF4 and that it was large. Many think it will be upgraded to an EF5 after further damage assessment.

Only a couple percent of tornadoes are EF4 or larger.

Are tornadoes getting more frequent?

The short answer is no, according to the National Weather Service, especially when it comes to more violent storms. "There has been little trend in the frequency of the stronger tornadoes over the past 55 years," the service notes. The longer answer is that more tornadoes are now reported than in years past, but that's probably due to an increase in the number of eyes scanning for tornadoes than the number of tornadoes. "Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA's Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by NWS personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been," the Service explains. "This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence."

Are tornadoes killing more people than in the past?

The short answer, again, is no, the 2011 Joplin event notwithstanding. Although there are more people, and therefore a greater number of injured parties, in the United States in the regions where tornadoes strike, better forecasting and warning systems have greatly reduced the rate of fatalities per million people and the overall number of fatalities, as you can see in the following two charts. The fall was particularly dramatic before the tragedy in Missouri in 2011. (Note that the second chart shows the period from 1875-2000.)

 

How did Moore's emergency warning system work? Did people have time to take cover?

The short answer is: Early warning systems worked as expected.

The National Weather Service's Norman, Oklahoma office says a tornado warning went into effect 16 minutes before the storm hit. That's three minutes faster than the current average lead time for a warning, according to NOAA.

On the ground, sirens would have gone off. Here's how the city of Moore describes its emergency procedures:

In Moore, our Emergency Management staff works closely with the meteorologists at the National Weather Service Forecast Office located in Norman. NWS personnel will generate warnings based upon not only radar information, but also information from our Moore severe weather spotters that are in the field. Therefore, when a warning is issued for Cleveland County, our EOC staff normally have played a part in the decision process. If our EOC staff determine that tornadic conditions will directly affect Moore, we will activate our local warning system, consisting of an outdoor warning siren system and a cable television interrupt. The NWS office will be triggering the alarm on NOAA Weather Radio, and local media will be broadcasting the warning as well.

The general problem with the warnings is that people might not hear or see them immediately, reducing the amount of time they have to find shelter. The second problem is that the tornado hit during early rush hour, a little after 3 p.m., when many people were on the move.

What was the housing stock like in Moore?

Because there was a large tornado in 1999, we have detailed information on some of the problems with the construction methods in the area.

Engineer Timothy Marshall, in a report on the 1999 tornado, noted that much of the construction in the area does not follow best practices for resisting a tornado's winds. Marshall's damage report found that homes that affixed their walls to their foundations in certain ways had serious problems during the storm. Yet, when he returned to see the rebuilding process, he found many homes rebuilding with the same methods that had failed before. That's the housing stock that was hit by the storm today. Here's how Marshall put it in his 2002 report:

The author revisited the disaster area three months after the tornado to check the quality of new house construction. A total of 40 houses were examined in Moore and southern Oklahoma City on sites at which houses previously had been destroyed. The author found that the quality of new home construction generally was no better than homes built prior to the tornado. Most newly built homes were attached to their concrete foundations with tapered cut nails or shot pins as had been noted in homes destroyed by the tornado.

Marshall is back on the ground now investigating the current tornado damage, so we expect to find out more about what happened soon.

When do tornadoes strike most often?

Statistically, the worst months for tornado formation are May and June, although they can happen all year round. Historically, they've hit most often between 5 and 6 p.m. local time.

How well can we predict a tornado?

A better way of asking this question might be this: On what time scale, can we predict a tornado might form? The answer is that forecasters have a pretty good sense that a tornado is forming about 15 minutes before it touches down. On the day of a storm, forecasters have a pretty good idea of where severe storms might occur. Below, I've embedded the forecast update that the National Weather Service in Norman, Okla., released at about 11:30 a.m. Generally speaking, they called the region and the time in which the tornado touched down: east of I-44 and south of I-40. Moore is less than 15 miles from where those two highways meet. But the region of worry that you see highlighted in the video is quite large: Meteorologists can't provide anything like a pinpoint forecast.

How well can we predict a tornado?

A better way of asking this question might be this: on what time scale, can we predict a tornado might form? The answer is that forecasters have a pretty good sense that a tornado is forming about 15 minutes before it touches down. On the day of a storm, forecasters have a pretty good idea of where severe storms might occur. Below, I've embedded the forecast update that the National Weather Service in Norman, Oklahoma released at about 11:30am. Generally speaking, they called the region and the time in which the tornado touched down: east of I-44 and south of I-40. Moore is less than 15 miles from where those two highways meet. But the region of worry that you see highlighted in the video is quite large: meteorologists can't provide anything like a pinpoint forecast.

On the much longer timescale, historical data, as in the map below (sent over by climate research Roger Pielke Jr), can tell you that a particular region is very prone to a tornado at a particular time of year, but not much more.

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