The Historic Drought of EF5 Tornadoes

They are the most powerful category of tornadoes, capable of “incredible damage” with estimated winds over 200 mph. Before 2013, EF5-rated tornadoes in the United States were a rare occurrence, with less than 0.5% of all tornadoes receiving that rating. However, since 2013 that number has dropped to zero. The U.S. hasn’t recorded a single EF5 tornado since 2013—a gap unrivaled in the modern record, since 1950. A new study in the Bulletin of the American Meteorological Society from NOAA’s National Severe Storms Laboratory (NSSL) probes why.

Lead author of the study Anthony Lyza, a research meteorologist at NSSL, wondered whether the dearth of EF5 tornadoes was a real, natural phenomenon or if it was a consequence of changes to how tornadoes are rated. A statistical analysis points squarely to the latter. In February 2007, the U.S. moved from the original Fujita (F) scale to the Enhanced Fujita (EF) scale. Under the legacy F scale, a single-family home swept clean from its foundation qualified as F5. Under the EF scale, that same level of destruction for a single family-home, built to the current building codes, would be considered EF4.

By tweaking wind-speed thresholds tied to key damage indicators, like the aforementioned complete sweeping away of well-built homes off their foundation, the EF scale could restore consistency with the F-scale record back to 1880. By lowering the threshold for EF5 tornadoes from 201 mph to 190 mph,  that modest adjustment, the authors argue, would ensure truly catastrophic tornadoes earn the top rating, regardless of era. This would also reclassify 15 tornadoes since 2013 as EF5, with the most recent being the deadly Rolling Fork, Mississippi tornado of March 2023.

The team has shared these findings with committees of the American Society of Civil Engineers and the American Meteorological Society, which are now considering revisions to the EF scale, which may include not only these tweaks to the wind speed thresholds, but also take into consideration data from mobile Doppler radar units when rating tornadoes in the future.