Largest and deadliest May 24 tornado in Oklahoma ranked an EF5

Rick Smith, warning coordination meteorologist with the National Weather Service in Norman, said weather service officials upgraded the tornado's rating after data from a University of Oklahoma mobile Doppler radar showed wind speeds in the twister exceeded 200 miles per hour as it crossed Interstate 40 near El Reno.

“The area where the wind speeds were measured was in the same area where we saw some pretty extreme damage,” he said.

The tornado killed five people within a mile of that spot and heavily damaged an oil rig in the area. It would kill more people before it was done.

Smith said rating the strongest tornado was difficult because of the method used to come up with a rating on the enhanced Fujita scale.

Engineers and forecasters survey damage and estimate wind speeds based on the construction of the structures the twister hits.

If a house is completely swept off its foundation, forecasters know wind speeds were more than 200 miles per hour and can rate the tornado an EF5.

But there were no houses destroyed near Interstate 40, and oil rigs aren't among the common structures used to determine a rating.

“We can't rate a tornado exclusively on the ground being scoured or even on automobiles being thrown,” Smith said. “We have to look at a combination of all the evidence that we see and piece it all together. In this case, I think we got it right.”