The Significance of April 27, 2011 and the Enhanced Fujita Scale

Two tornadoes from last Wednesday have been rated EF-5, with more possible. The Fujita Scale was introduced in 1971 by Ted Fujita, and for years was used as a measurement of the severity of a tornado. Many think the F rating was based on the size, or wind speed of a tornado. In reality, it is the damage that can be verified that in turn rates a Fujita rating, and correlates to estimated wind speed.

In 2006, the Enhanced Fujita Scale was implemented. The enhancement was using additional information such as construction methods, radar data, and damage to vegetation. For example the construction of a house constructed in 1950 may be structurally different than the wall from a house built this year, so the collapse of that wall could mean two completely different wind speeds.

The first EF-5 tornado (the highest rating) was the Greensburg, KS tornado in 2007. The second was in 2008 in Parkersburg, IA. The significance of this is that since 2006, there have only been two EF-5 tornadoes. On April 27, 2011 a major tornado outbreak hit the Southeast. While watching this event unfold, I predicted something I normally would not, simply based on real time information. I estimated we would end up seeing three to five EF-5 rated tornadoes once the damage was assessed. The radar signatures on these storms was off the charts, the areas affected were highly populated (more damage to assess), and the damage reports coming in were very, very severe.

To date, two tornadoes in this event have been rated EF-5, which is already rare. These are the Smithville, MS tornado and the Hackleburg, AL tornado. I would be surprised if we don't at least see the Tuscaloosa tornado end up EF-5. Considering there have been only two in five years, and there are already two confirmed in one day, this is quite a historic event.

The Weather Channel has a great write up on F/EF-5 tornadoes.

Weather Channel information on the Enhanced Fujita Scale

I plan on posting soon about the death toll as well. As of this writing, there are 340 confirmed dead, which makes this the second worst tornado fatality event in our history. The only event that was worse was in 1925 where 695 people were killed. There are many reasons why these numbers are not apples to apples, and I would like to discuss that more later.