Three Tornadoes From Saturday Were Rated EFU; Here's What That Means

Three tornadoes from Saturday's severe weather outbreak in the Midwest were not assigned a rating by the National Weather Service (NWS) because they briefly touched down in rural locations and there was no evidence of damage.

Tornadoes are typically rated from EF0 (light damage) to EF5 (catastrophic damage) on the Enhanced Fujita Scale based on the damage that they cause, not by their appearance or any other real-time metric. But when a tornado strikes a mostly rural area with few structures or trees, the damage may be so sparse it can be difficult for the NWS to estimate the strength of the winds.

When there is no tornado damage but there is evidence a tornado existed, the NWS can give an EFU rating for "unknown" to be used in official records. Two tornadoes in Iowa and one in Illinois from Saturday matched the criteria to be rated EFU.

Iowa's two EFU tornadoes were located between Des Moines and Waterloo. They both lasted a minute and tracked for 0.8 miles and 1 mile, respectively.

There were nearly 200 tornadoes rated EFU in 2019, according to NOAA's Storm Events Database. Many of them were in 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.

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