The Truth About Tornado Ratings

The Truth About Tornado Ratings (Image 1)

I’ll get right to the point: if you see me or anyone else talking about a tornado in the area that was determined to be an EF-whatever with wind speeds of whatever…it’s nothing more than an educated guess. That’s the best we can do right now.

The complexity of tornadic winds, the varying degrees of construction quality in any one area, and even the National Weather Service’s own policies work against accurately labeling all tornadoes.  Proving that meteorologists and engineers still have much to learn about what tornadoes are capable of.

Let’s go back to earlier this year as the state of Oklahoma was dealing with a run of noteworthy tornadoes.  The focus here is on the storm that hit El Reno on May 31…a notable storm for many reasons.  What I will focus on here, though, is the tornado’s strength.

Tornadoes are officially rated using the Enhanced Fujita (EF) scale, a modification of the old Fujita scale.  The scale ties surveyed damage to the strength of wind that would cause such damage.  This change officially occurred in 2007, with the updated scale based on damage studies conducted by meteorologists and engineers.

Rating tornadoes this way is a necessary evil, as recording actual wind speeds inside a tornado is almost always not an option.  The exceptions are in small tornadoes that just happen to pass over a weather station (with weak enough winds to not damage the station) or any tornado that has winds measured through radar…typically mobile, hi-res research radar that can park just miles from the storm. 

It was such a radar that came up with a reading of 296 mph inside the El Reno tornado.  After the National Weather Service’s initial survey saw the type of damage that typically points to an EF-3 storm (136-165 mph), the storm was upgraded to an EF-5 based on the radar data.

At least until this past Friday, when the NWS reset it back to an EF-3.

The reasoning?  In a statement released by National Oceanic and Atmospheric Administration, spokeswoman Keli Pirtle says “Despite the radar-measured wind speeds, the survey team did not find damage that would support a rating higher than EF-3.  While the wind measurements from the mobile radars are considered reliable, NWS policy for determining EF ratings is based on surveys of ground damage.”

So in the face of obvious evidence, bureaucracy takes over.

Why have tornado ratings if real-world evidence isn’t used?  Isn’t the point to know how strong the wind was?  Stumped on that one.

The National Weather Service does acknowledge that a policy change will be explored with regards to incorporating radar information into the rating scale.

Another example…earlier this year, a study released by the American Society of Civil Engineers determined that the Joplin, MO tornado from May 22, 2011 produced no EF-5 damage, despite the storm being rated as such.  The reason for this finding was that the town’s structures, as a whole, were determined to be poorly built to deal with high wind.  (Gee, as I used to live in that area, that’s a not very comforting thought!)  Flying debris from these structures made damage to better-built structures even worse.  This made it appear as if the winds were higher than they actually were.

Thankfully, such large tornadoes are rather rare in our area.  When dealing with smaller storms, it’s a little easier to get a handle on what types of wind might cause what types of damage.  But as of right now, even if some other measurements are available, the only way to guess the wind speed of a tornado is through a somewhat subjective damage survey.

This may seem trivial in the grand scope of things…how many people really care if it was an EF-5 or an EF-4 if it killed people and wiped out part of a town?

It is important to engineers whose job it is to create construction guidelines and codes.  The rules and practices that could ensure that your house or place of work are safer places to be.  And that matters to everyone.

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