The deadliest of the May 24 tornadoes, a massive twister that tracked 75 miles through El Reno, Piedmont, Cashion and Guthrie, was upgraded Wednesday to an EF5, the highest tornado rating.
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.”
The rating validates the unusually stern language forecasters used on the morning of May 24, when they warned “large, strong, long-track tornadoes” were likely to hit in central Oklahoma sometime after 3 p.m.
The first tornado warning was issued minutes after 3 p.m., and seven twisters touched down that evening, including the EF5 and two EF4 tornadoes. Wind speeds in the two EF4 tornadoes were just under 200 miles per hour, forecasters said.
Smith said no one at the weather service is celebrating the accuracy of the warnings.
The strongly-worded warnings early in the day spread quickly both in traditional news media and through social networking tools like Facebook and Twitter. Many businesses dismissed workers early so they could prepare for severe weather.
Smith said Oklahomans' response to the warnings was unprecedented. “It's encouraging to know our capabilities are getting better to get the word out in advance,” Smith said. “You can't measure how many people didn't die.”