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

Saturday, February 03, 2007

Tornado History Repeating Itself

When the phone rang early Friday morning with news of a tornado outbreak in Central Florida, vivid memories of a February nine short years ago came flooding into my mind. The series of powerful storms that tore through Lake and Polk counties just a couple of days ago were eerily similar to the state’s deadliest severe weather incident on Feb. 22-23, 1998.

While summer is the most common period for tornadoes in South Florida, the twisters that do form from May through August tend to be weak and short-lived. Stronger and longer-lasting tornadoes are more common during the winter months.

Still, the death and destruction in the 1998 incident were unprecedented.

The damage produced by the seven tornadoes was incredible. The National Weather Service reported that 3,000 structures were damaged with 700 completely destroyed. Yet, it was the death toll that stood out: 42 people lost their lives with another 200 severely injured.

What made the outbreak the worst in state history was the unusual strength of the tornadoes. There were three F1 tornadoes (winds 73-112 mph), one F2 tornado (winds 113-157 mph), and three F3 tornadoes (winds 158-206 mph). Experts believe the three F3 twisters produced sustained winds of 200 mph.

Tornadoes that strong are rare in Florida. Even more uncommon was how long some of the tornadoes remained on the ground. The F3 tornado that touched down near Kissimmee had the longest track of just under 38 miles.

To make matters even more complicated, the tornadoes struck in the middle of the night. The Kissimmee tornado, for example, occurred around 1 A.M. This tornado killed 25 people and injured more than 150, mostly in a trailer park.

Yet the death toll could have been much higher if not for the staff of the National Weather Service Melbourne office. Just one year earlier, Bart Hagemeyer, the meteorologist in charge, had completed a detailed study to define the character and history of Florida tornado outbreaks.

Just a few weeks before the incident, Hagemeyer had finished another study examining El Nino’s role in tornado outbreaks in Florida. And, the staff’s annual training session on tornadoes had been moved up to December from February based on Hagemeyer’s studies.

The staff was prepared, and it showed. Weather warnings were issued with a remarkable 100 percent accuracy. The Melbourne meteorologists were honored for their work with the Commerce Department’s Gold Medal, the agency’s highest award (The National Weather Service staff did another remarkable job with the most recent tornado outbreak, providing residents with 10 to 15 minutes of advance warning time.).

Still, a weather warning doesn’t do any good if no one is listening. Researchers at Florida State University discovered that only 1 percent of the people in the damage path had a NOAA weather radio and received the tornado warnings by that means.

Almost 90 percent of the respondents interviewed by FSU researchers relied upon local television; however, 25 percent of the persons interviewed after the tornado outbreak did not watch television or listen to the radio that night. Many of the victims were sound asleep when the tornadoes hit.

The sale of weather radios increased dramatically following the tornado outbreak. In fact, for several weeks, it was nearly impossible to find a weather radio anywhere in Central Florida.

Chances are weather radio sales will spike again in the coming days because of this most recent outbreak of twisters. Still, while the death toll was high, there were success stories. One elderly couple, both in their 80s, survived the tornadoes because they had purchased a weather radio and knew the storms were coming.

They say we must learn from history or we will be doomed to repeat it. We need to learn that Mother Nature can always strike twice.

Posted at 6:56 AM