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Monday, May 5, 2014

Meteorologist defends EF4 rating on Vilonia tornado

Posted By on Mon, May 5, 2014 at 11:12 AM

click to enlarge BAD ENOUGH: Pictures tell a sufficient story of the Vilonia tornado, no matter where it falls on the Fujita scale. But, note: Thrown cars are not part of the measuring standard. - KARK
  • KARK
  • BAD ENOUGH: Pictures tell a sufficient story of the Vilonia tornado, no matter where it falls on the Fujita scale. But, note: Thrown cars are not part of the measuring standard.

Meteorologist John Robinson with the National Weather Service in North Little Rock issued an explainer today on why the Vilonia tornado earned an EF4 rating rather than the top rating of EF5. For one thing, thrown vehicles don't count in the computation.

As expected, we are getting all sorts of comments about why the tornado was not rated EF5 in Vilonia.

Here's something to keep in mind: First and foremost, if a house that has a foundation built with nuts, bolts, and appropriate-sized washers is swept away, leaving only the concrete foundation, the "expected value" of wind in the Enhanced Fujita Scale is 200 mph, which puts it at the top of the EF4 category, not an EF5. It would have to be an exceptionally well-built house to go over 200 mph and thus achieve an EF5.

It's my understanding that when the Enhanced Fujita Scale was put together this was done for a very specific reason — that some on the committee felt that a house should never be rated EF5. The final outcome was that the "expected value" was placed so that it would be very rare that a house could be rated EF5.

Note that, whether the rating was EF4 or EF5, it would be expected that the house would be destroyed.

So, for the house in Vilonia that came closest to exceeding EF4, we also followed a couple of other "rules" that we use in assessing tornado damage.

First, one structure should not be used to determine a rating. Second, there were still some tall, skinny trees standing along a drainage ditch/small creek about 100 yards away from the house. I have seen something like this in the past, and the expert rated lower because of vegetation that was still standing nearby. To put it another way, the people doing the rating are supposed to take into account the entire scene, not just one thing in the middle of it.

Another thing that has been debated considerably: When a structure has been hit by a large amount of debris, how much of the damage was truly from the tornado itself vs. the amount of damage that was caused by flying debris from other structures?

As I have noted previously, we saw a much greater than normal number of vehicles that had been thrown around. Vehicles are not one of the damage indicators in the Enhanced Fujita Scale, so whether they were thrown 50 feet, 50 yards, etc., does not tell us anything about what EF-Scale rating to assign.

While I'm talking about objects being thrown, the construction company that is working on the new school in Vilonia that was badly damaged was kind enough to calculate the approximate weight of the large storage container that was thrown onto the roof. It weighed approximately 25,000 pounds, counting the container and its contents.

Finally, I am on the committee that is working to update the Enhanced Fujita Scale. Very few people working in NWS field offices are on this committee, which is composed of meteorologists, engineers, tree experts and others. Thus, I get to hear plenty of discussions about all aspects of rating tornadoes.

Here's a more detailed discussion from the Weather Service on what it found about use of anchor bolts to hold houses to foundations (not often, or improperly installed). An engineering professor is quoted:

In the whole scheme of things, this type of tornado seldom occurs, and its path is fairly narrow (usually less than a mile). The odds are remote that any one location is impacted by such a tornado. While the low risk will settle some stomachs, the absence of anchor bolts does not sit well with Dr. Prevatt:

"Bottom-line - we either build them (our houses) stronger or communities must accept that certain number of homes randomly selected by nature's wrath, will be blown away and kill people in every moderately strong tornado for the foreseeable future."

UPDATE: The Weather Service also provided a discussion from the Department of Emergency Management about the death of one person whose family was otherwise protected in a "safe room" when a door caved in. The room had not undergone a government inspection, which ADEM recommends.
click to enlarge ADEMnewsrelase__1_.jpg

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