How strong are tornadoes? The rating may change

The system by which tornado strength is measured is undergoing an overhaul — and Kansas tornadoes are a key reason why.

Tornadoes are currently rated based on the damage they do, using parameters outlined in the Enhanced Fujita scale. The ratings take into account what was hit and how well it was built.

But critics say that leads to flawed data because powerful tornadoes can end up with deceptively low ratings.

“There are a ton of tornadoes in Kansas like that — they don’t hit anything,” said James LaDue, a senior instructor at the Warning Decision Training Division of the National Weather Service in Norman, Okla.

“A lot of those fade from our collective conscience” because they’re given lower ratings on the Fujita scale.

Clearer data on where and how often strong tornadoes occur can be valuable to companies and governing bodies as they consider where and how to build operating plants and key infrastructure, LaDue said.

It could also lead to changes in residential building codes, such as what happened in Joplin, Mo., following the deadliest U.S. tornado in decades in 2011.

Tornadoes are always top of mind in Kansas, which observes Severe Weather Awareness Week this week. State emergency management officials are urging residents to check their home emergency kits and replace dead batteries and outdated items.

Gateway to better warnings?

Inaccurate tornado strength measurements are more than just a statistical inconvenience, weather researchers say.

They can deliver a skewed picture of what tornado-producing thunderstorms are capable of, LaDue said — especially in light of data being collected in recent years by portable radars.

Josh Wurman, founder of the Center for Severe Weather Research, said he has measured the wind speeds of more than 200 tornadoes up close. The results, he said, suggest the typical tornado is much stronger than ratings would suggest.

That has tremendous implications for a variety of reasons, LaDue said. Accurate measurements of tornado strength are a key part of detailed reviews of radar data for the storms that produced the tornadoes.

“That will improve our ability to discriminate weak versus strong tornadoes in the warning process,” he said.

According to the National Oceanic and Atmospheric Administration’s Storm Prediction Center, only 59 tornadoes since 1950 have been rated EF-5. That’s the top of the Enhanced Fujita scale, with winds of at least 200 miles an hour. There hasn’t been one since 2013.

But Wurman’s radar readings indicate 25 percent of all tornadoes have wind speeds topping 200 miles an hour. The average tornado has winds of about 130 miles an hour, which would place it just below EF-3, or severe tornadoes, on the Enhanced Fujita scale.

Yet the Storm Prediction Center database shows barely more than 5 percent of all tornadoes between 1950 and 2012 had winds of EF-3 strength or greater. That discrepancy helps explain why severe weather researchers say the climatology for Tornado Alley is incomplete at best.

EF-5s ‘much more common’

The portable radars are measuring wind speeds at just a few meters above the ground at times, Wurman said, and data suggests wind speeds at the surface may be even stronger than those about 60 meters above the surface.

Tornadoes with EF-5 strength “are much more common than the official ratings would lead you to believe,” Wurman said. “The reasons they’re not rated that high are mostly because those tornadoes aren’t hitting anything.”

Two of the tornadoes captured by portable radars had winds in excess of 300 miles per hour, Wurman said: Moore, Okla., on May 3, 1999, and El Reno, Okla., on May 31, 2013.

But only the Moore tornado has an EF-5 rating. El Reno was given an EF-3 rating because of the type of structures it destroyed. Winds speeds recorded by anemometers or estimated by radars aren’t taken into account, Wurman said.

“There really was not much to hit in the El Reno tornado, so it didn’t get rated EF-5,” Wurman said.

One of several tornadoes that touched down in southwest Kansas last May 24 is another example of the Enhanced Fujita scale’s limitations, LaDue said. A large tornado passed just west of Dodge City and hit a pump house, earning an EF-3 rating.

But if it had veered east a short distance and hit the city, LaDue said, it “quite likely” would have earned a higher rating.

Wurman, LaDue and others see the flaws in the Enhanced Fujita scale, and they’re working to address them. A 40-person committee has begun working on new criteria for determining tornado strengths.

“It’s much more rigorous” than the current criteria, Wurman said, and includes building code information from the American Society of Civil Engineers.

“We’re expanding it beyond just damage” and including wind speed measurements, data collected from mobile radars and other sources, Wurman said.

Even videos of tornadoes could be used to help establish a twister’s strength and speed, LaDue said.

The new rating system should be ready in about 18 months, Wurman said.

This story was originally published March 3, 2017 at 4:31 PM with the headline "How strong are tornadoes? The rating may change."