30.04.2011 - Blogbeitrag von Dr. Greg Forbes zum Thema "Tornado Outbreaks" in den USA

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Samstag 30. April 2011, 13:45

Tornado Outbreaks Then and Now - Apples and Oranges?

Dr. Greg Forbes, Severe Weather Expert

Early in my graduate school experience I had the privilege of surveying the damage from many of the April 3-4, 1974 Superoutbreak tornadoes as part of Dr. Fujita's team. 148 tornadoes struck in 48 hours. Six of the tornadoes in that outbreak were rated F5 (on the original Fujita Scale) and 30 were rated F4 or stronger. There were more than 300 fatalities, over 5000 injuries, and the sum of tornado path lengths exceeded 2500 miles. The Superoutbreak has been the benchmark for all tornado outbreaks since.

I always thought that there was a chance that some future tornado outbreak might be worse. The most recent one could be, at least in some respects. Damage surveys are still in progress that will bring us the true count of the number of tornadoes, their EF-Scale ratings, path lengths and widths, and other measures of the outbreak's fury. The National Weather Service, using preliminary data, has already indicated that its tornado count might exceed the number of tornadoes in the Superoutbreak.

Once Doppler radars were deployed across the United States in the early 1990s, though, I never thought I'd see a tornado outbreak kill hundreds of people again. How sad that it has happened on Wednesday. More than 200 people were killed in Alabama alone and more than 300 in total, according to news reports! The Doppler radars allow us to see tornadoes and their parent thunderstorms' rotating updrafts like never before, and the National Weather Service issues tornado warnings with an average of 13 minutes of lead time (in advance of the tornado), and often much more than that. Combine that with so many more - and more efficient - ways of getting the warnings in this internet era and it's a "different world" relative to the 80-character-per-second teletypes that gave warnings to the media (and not directly to the public) back in 1974. People can get timely and effective warnings on NOAA Weather Radio, on The Weather Channel, from services like TWC's "Nofify!" that can personally send you a message that a tornado is coming, and in many other ways.

It's apples versus oranges in the relative ability for people to know a tornado was coming in the two eras. Yet so many died in 2011. That must be a function of the violence of the tornadoes, combined with the fact that so many took aim on communities rather than rural areas. And when tornadoes are violent, even being warned and taking proper safety measures is no guarantee of survival. Finding proper shelter improves your odds, but only being in an underground shelter or specially designed in-home shelter can ensure your survival.

But it's also a "different world" in the way that tornadoes get rated. The mainstay of the original Fujita Scale used to rate tornadoes was that a home crushed into small pieces and blown away would earn an F5 rating, with wind speeds estimated at 261-318 mph. Engineers surveying the tornado damage back in 1974 began to tell meteorologists that it didn't take 300 mph winds to turn homes into piles of rubble and cast the pieces to the wind. Even well constructed concrete block and brick school buildings could fail in 220 mph winds, they said. And homes with damage apparently fitting an F5 description often happened because the house was not properly secured to its foundation in winds less than 150 mph.

In the years since, these engineering analyses began to work their way into damage assessments, and in 2007 an Enhanced Fujita Scale (EF Scale) system was officially implemented. An EF5 tornado has winds estimated as low as 201 mph. And it's difficult to rate a tornado as EF5 based upon it just demolishing a house.

Statistically, the number of tornadoes being rated 2-5 have been decreasing since the 1970s, despite the total number of tornadoes being recorded showing a dramatic increase. This is at least partly a consequence of the introduction of engineering concepts into the rating process. I was part of the team that developed the EF Scale, and it's a system that more accurately estimates tornado wind speeds. But it troubled me then (and still does) that it might be hard to compare past tornado outbreaks with future ones and determine which was worst. It's apples and oranges, to some extent, in the rating systems then and now.

April outbreak versus Superoutbreak?It will be interesting to see just how many tornadoes were in this April's outbreak, and how many get rated EF5. But the death toll, the number of tornadoes in a 24-hour period, and the total path length of the tornadoes may be more appropriate measures by which to compare it to the Superoutbreak than the number rated EF5. Amazingly, the April 2011 death toll already appears to be worse!
Quelle: http://www.weather.com/blog/weather/8_2 ... _mainindex
The 1974 Super Outbreak

Say "the Super Outbreak" to people who lived through one of the most violent tornadic events on record in the United States, and whether one is a meteorologist or not, nothing more need be said.

The 1974 Super Outbreak spawned 148 tornadoes, the largest number of tornadoes ever produced by one storm system. Thirty of these tornadoes were classified as F4 or F5 on the Fujita-Pearson Scale. Before the fourteen state rampage was finished, over 300 people had lost their lives in 48 killer tornadoes.

On the morning of April 3rd, an area of low pressure was located in central Kansas. A warm front extended east-northeastward through the lower Ohio River Valley. South of this front, extremely unstable air had gathered during the overnight hours and was rapidly spreading north.

A cold front stretched from the area of low pressure south through Texas. At the upper levels of the atmosphere, a powerful trough was spreading strong winds aloft over much of the eastern half of the country.

With the warm air in place and a cold front approaching, along with favorable upper air dynamics, intense thunderstorms developed rapidly in the afternoon of April 3rd. Those thunderstorms spawned nearly 150 tornadoes across parts of the Midwest, Ohio and Tennessee River Valleys and Southern states from the afternoon of the 3rd into the early morning hours of the 4th.

An astonishing six F5 tornadoes were spawned. An F5 tornado hit Guin, Alabama, destroying the entire town and killing 20. Fortunately for Huntsville, Alabama, the tornado lifted back into clouds just before reaching the city limits.

Nearly 30 people perished in Brandenburg, Kentucky when another F5 tornado touched down, leaving the town in ruins. Over 300 homes were destroyed and over 2,100 were damaged by an F5 in Xenia, Ohio, which killed 34.
Quelle: http://www.weather.com/encyclopedia/tor ... break.html
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