New Study Reveals The Lack Of EF5 Tornadoes

T​ornado ratings are commonly assigned after severe weather strikes, but there's one type of classification you might not have heard of before: the so-called EFU tornado, as we saw recently in South Dakota.

Big Picture: Tornadoes Are Rated On The Enhanced Fujita Scale

They usually range from EF0 (light damage) to the rare EF5 (incredible damage), based on the most severe damage they cause along their path, not by their appearance or any other real-time metric such as Doppler radar.

The tornado wind speeds for those ratings are estimated through damage inspections performed by the National Weather Service (NWS) after the storm. Factors weighed include the quality of building construction and other types of damage, from trees to small barns to large shopping centers.

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(​MORE: How Surveys Reveal A Tornado's Strength)

Deeper Dive: E​FU Rating Is An Oddball Exception

But what happens when a tornado is spotted and there is no damage to survey? That's where the EFU rating comes into play, where U stands for "unknown."

A prime example happened Feb. 24 in South Dakota, when a brief tornado documented by a storm chaser spun up over an open field. An assessment by the NWS said it caused no damage and assigned the twister an EFU rating, making it the state's first February tornado on record.

I​t's likely this tornado's winds were on the weaker end of the spectrum along the lines of an EF0 (65 to 85 mph winds), but since there was no visual damage to inspect, that rating can't be used.

(​MORE: South Dakota's First February Tornado)

G​eek Out More: The Rating Has Been Used In Practice Nearly 10 Years

• T​he EFU rating category was around before 2013 but wasn't used in practice until 2016, according to a recent paper by NOAA meteorologists.
• O​ther reasons tornadoes can have an EFU rating include ones that occur in such a remote setting that they cannot be surveyed, or damage indicators are of a type that's not standard to what's used in the scale.
• Many of the EFU tornadoes are in the Plains or Midwest states where there are fewer structures and trees for tornadoes to hit and cause damage that can be rated by an NWS survey team.

Chris Dolce has been a senior digital meteorologist with weather.com for nearly 15 years after beginning his career with The Weather Channel in the early 2000s.