Tornadoes in America: The Oklahoma Disaster in Context

A backgrounder for understanding the storm that hit Moore, Oklahoma.

Action News 5 - Memphis, Tennessee

A massive and powerful tornado hit Moore, Oklahoma this afternoon, 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. This evening, President Obama signed a disaster declaration for Oklahoma.

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 tornado in over 20 years, although several storms have surpassed that number of deaths since then, including 2011's Joplin 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, email me at alexis.madrigal[at]gmail.com. 

When exactly did the storm hit?

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

Screen Shot 2013-05-20 at 9.17.04 PM.png 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 having supercell 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.

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National Weather Service

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National Weather Service

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.

dayspercentury.jpg

How do tornadoes form?

The science of tornado formation is complex. In the video below, NASA Goddard's Tim Samaras explains what we do and don't know about the mechanics. While scientists understand the conditions that make tornadoes more likely, the final steps that lead to tornadogenesis remain mysterious.

How are tornadoes measured?

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

fujitascale.jpg

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."

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