Tornado ratings are typically assigned post severe weather events, however, there exists a classification that may not be familiar to all – the EFU tornado, as recently witnessed in South Dakota.
In essence, tornadoes are graded on the Enhanced Fujita Scale, ranging from EF0 (causing light damage) to the rare EF5 (inflicting incredible damage), based on the severity of damage along their path, rather than according to their appearance or any real-time metric like Doppler radar.
Estimates of tornado wind speeds for these ratings are determined through damage inspections conducted by the National Weather Service (NWS) subsequent to the storm. Factors taken into consideration include the quality of building construction and various types of damage, ranging from trees to small structures like barns to large commercial complexes.
Delving deeper into the topic, the EFU rating is an anomaly in the system. What occurs when a tornado is observed, yet there is no damage to assess? This is where the EFU rating, with ‘U’ denoting ‘unknown’, comes into play.
A recent instance of this scenario took place on February 24 in South Dakota, when a brief tornado recorded by a storm chaser formed over an open field. Following an evaluation by the NWS, it was determined that the tornado caused no damage, resulting in an EFU rating assigned to the event, marking it as the first February tornado in the state’s records.
It is probable that the wind speeds of this tornado were on the lower end, akin to an EF0 (65 to 85 mph winds). Nevertheless, in the absence of visual damage for inspection, this rating could not be conclusively applied.
Further exploration reveals that the EFU rating category had been in existence prior to 2013, however, it was not practically implemented until 2016, as noted in a recent paper by NOAA meteorologists.
Other circumstances that may lead to a tornado receiving an EFU rating include occurrences in extremely remote locations where surveying is impractical, or instances where the damage indicators do not align with the standard elements used in the scale.
Many EFU-rated tornadoes are observed in the Plains or Midwest regions, where the scarcity of structures and trees limits the impact of tornadoes and consequently diminishes the potential for damage to be assessed by an NWS survey team.
Chris Dolce, a seasoned digital meteorologist with weather.com for nearly 15 years, launched his career with The Weather Channel in the early 2000s.
