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day of the dalek [soylentnews.org] writes:

Tornado strength is rated from 0 (weakest) to 5 (strongest) on the Enhanced Fujita (EF) scale [weather.gov], with roughly 2% of tornadoes being rated EF4 or EF5. The EF scale replaced the older Fujita scale to provide much more fine-grained detail in determining a tornado's rating. The EF5 rating corresponds to estimated peak winds of 200+ mph. However, it is purely a damage scale, from which the peak winds in the tornado are later estimated. Although meteorologists often discuss the wind speeds in tornadoes, measured wind speeds are never a factor in rating tornadoes.

This distinction was made apparent on April 26, 1991 [weather.gov] when the Andover, Kansas tornado [youtube.com] was rated F5 while the Red Rock, Oklahoma tornado [youtube.com] was rated F4 despite likely being the stronger tornado. A mobile radar from the University of Oklahoma measured 270+ mph winds in the Red Rock tornado, well into the F5 range, and the strongest tornado winds that had ever been measured to date. However, because the Red Rock tornado remained over mostly rural areas unlike the Andover tornado, there was little opportunity for it to do severe enough damage to be rated F5. This distinction remains true with the EF scale, where the 2013 El Reno, Oklahoma tornado [youtube.com] was originally rated EF5 on the basis of mobile radar observations, then downgraded to EF3 based on the lack of EF4 or EF5 damage in damage surveys.

A new article in the Bulletin of the American Meteorological Society [ametsoc.org] discusses the current "drought" in EF5 tornadoes, with that rating being most recently assigned to the 2013 Moore, Oklahoma tornado [youtube.com] that happened just 11 days before the 2013 El Reno tornado. The lack of EF5 tornadoes for over 11 years has raised questions of why, and if the EF5 rating is essentially obsolete [youtube.com].

The journal paper argues that the lack of EF5 tornadoes for 11 years is roughly 0.3%, and it's very unlikely that there have been zero EF5 tornadoes during that period. Instead, it's probable that this is due to stricter application of the EF scale standards, and several tornadoes were estimated to have peak winds of 190+ mph during that period. If those tornadoes were reclassified to EF5, it would be statistically consistent with the previous climatology of EF5 tornadoes. The authors note that some of the previous EF5 ratings such as the 2011 Joplin Missouri tornado [youtube.com] were based on damage indicators that were not part of the EF scale specifications.

One of the biggest reasons for not assigning an EF5 rating is the presence of areas with limited damage very close to near-total devastation. However, the strongest tornadoes are generally multi-vortex tornadoes [youtube.com], where the strongest winds are found within small vortices embedded within a broader tornadic circulation. This could explain the proximity of extreme damage to areas with much less damage. The damage severity also depends on how long structures are exposed to extreme winds, an example of which is the 1997 Jarrell, Texas tornado [youtube.com], which was rated F5 but damage was more severe due to the tornado moving slowly and exposing buildings to the tornado winds for a longer than usual time [wunderground.com]. This raises the question of whether the EF5 rating is obsolete based on how the EF scale is currently applied, and if it's time to again revise how meteorologists rate tornado strength.

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