Friday, February 16, 2007

New Tornado Scale Is More Accurate

It was May 3, 1999 and an outbreak of severe thunderstorms was racing through Kansas and Oklahoma. Thanks to the wonder of satellite technology, I was able to tune in our Hearst sister station in Oklahoma City, KOCO, and watch their coverage of the storms. I was amazed at what I saw.

Chief meteorologist Rick Mitchell, doing his usual outstanding job in describing the severe weather, was reading off tornado warning after tornado warning. It seemed as if every couple of minutes another twister was touching down in central Oklahoma. It was clear that this was no ordinary severe weather outbreak.

As it turned out, 58 tornadoes ripped through Central Oklahoma on May 3rd including the last F-5 tornado recorded in the United States. The National Weather Service believes the F-5 twister that produced extensive damage in Moore, Oklahoma had sustained winds over 300 mph. Other tornadoes reportedly produced winds of more than 200 mph.

Shortly after that Oklahoma outbreak, meteorologists, emergency managers and engineers got together to study the weaknesses in the structures destroyed by the tornadoes and to determine if the wind estimates of the twisters were accurate.

Engineers claim, for example, that many homes are rated to withstand winds to 100 mph. Therefore, the question was raised that if a tornado has over 200 mph winds, how can the structure reveal this estimate when much of it is gone?

It was clear to all involved that the Fujita Tornado Intensity Scale, created by legendry tornado researcher Dr. Ted Fujita in 1971, needed to be updated. Many of the meteorologists and engineers felt that the original scale often overestimated the wind speeds of tornadoes and produced inconsistent ratings.

So, a blue ribbon panel of experts from the Texas Tech University Wind Science and Engineering Research Center, along with a forum of wind engineers, universities, private companies, government organizations, private sector meteorologists and NOAA meteorologists from across the country got together and produced the Enhanced Fujita Tornado Scale (EF).

“The EF scale provides more detailed guidelines that will allow the National Weather Service to more accurately rate tornadoes that strike the United States,” said Brig. Gen. David Johnson, U.S. Air Force (Ret.), director of the National Weather Service. “The EF scale still estimates wind speeds but more precisely takes into account the materials affected and the construction of the structures damaged by the tornado."

The EF scale incorporates more damage indicators and degrees of damage than the original Fujita scale, allowing more detailed analysis and better correlation between damage and wind speed.

By the way, the original Fujita scale historical database will not change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may have been somewhat less than previously estimated. A correlation between the original Fujita scale and the EF scale has been developed. This makes it possible to express ratings in terms of one scale to the other, preserving the historical database.

The new Enhanced Fujita Scale officially went into effect on February 1, just one day before the devastating tornado outbreak in Central Florida. National Weather Service meteorologists, using the new scale for the first time, determined the strong storms produced three tornadoes.

Two of the twisters were rated EF-3 with winds estimated at 150 mph, while the third tornado was an EF-1 with estimated winds of just over 100 mph.

Admittedly, it will take a little while to get used to the new scale and it’s somewhat awkward labeling, but the Enhanced Fujita Scale will lead to more accurate damage surveys of tornado damage and, hopefully, a better understanding of severe storms.

Posted at 10:34 AM