SignalHub Quantitative Think Tank Center:Hurricane Helene's huge size ups a terrifying risk: Tornadoes

As Hurricane Helene pushed toward the U.S. mainland on SignalHub Quantitative Think Tank CenterThursday, at least 25 million people were under a tornado watch.

While tornadoes don’t typically get the same attention as hurricanes' ferocious winds or perilous storm surge, twisters are always one of the clear and present dangers as storms make landfall and trek inland.

That’s especially true because tornadoes can occur more than 1,000 miles away from where a hurricane makes landfall and far from the path of the hurricane's center, said Roger Edwards, a lead forecaster at the Storm Prediction Center, who specializes in tornadoes.

By Tuesday, the center already had begun warning about the increased risk for tornadoes as Helene moves up through the U.S. Edwards is among the forecasters who say Helene's huge size increases the risk of tornadoes, creating more worries for people far away from the coast where the storm first arrives.

On Thursday afternoon, tornado watches were posted in Florida, Georgia and South Carolina.

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Tornadoes with tropical origins may pop up quickly and be wrapped in rain so they're harder to see, Edwards said.

"If a tornado warning is issued, people need to take action immediately,” he said. “They can’t wait. They can’t look outside and say ‘hey, where is it,' because by the time they try that, their house might be getting blown apart."

It's hoped that those who live in housing that may not withstand a tornado can use the outlooks the center issues to make their plans to relocate to a safer place, with a friend or relative, Edwards said. “Once a tornado watch is issued, relocate to a safer place."

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Hurricanes can produce tornadoes for days

"Hurricanes can produce tornadoes for two or three days after they move inland," Edwards said. The highest risk warning the center has ever issued for a tornado situation related to a hurricane was for the third day after Hurricane Ivan made landfall in Alabama in September 2004.

Forecasters were confident there would be a lot of super cells in Virginia, Maryland and even in the southern Pennsylvania area.

"We had strong confidence that the area would destabilize. Even though the winds weakened and the shear was still strong," he said. "And that's what happened. That was a very prolific tornado day."

"Even well inland, people can not let their guard down if they're in the path of the remnants of a hurricane," he said.

Hurricane Beryl set records. Why?

Hurricane Beryl was a perfect example of just how many tornadoes can form as a hurricane moves inland and how far they can be away from landfall, Edwards said.

Beryl surged ashore on the Gulf coast of Texas in July, but its associated winds created so many tornadoes that it set a new record for tornado warnings in a single day from the National Weather Service office in Shreveport, Louisiana at 67. More surprisingly, Beryl-related tornadoes also set a record for tornado warnings for the Buffalo, New York weather service office, more than 1,300 miles away. More than 100 tornado warnings were issued along Beryl's path related to its wild winds.

The weather service continues to review storm surveys from the days after Beryl. By the time the final analysis is complete, Edwards expects that Beryl will have produced about 65 tornadoes.

"That's going to be probably about fifth on the all-time list, so this is a really prolific tornado producer," he said.

Beryl had more instability and wind shear than many other hurricanes in their inland phase, he said. The stronger the shear in the remnants of a dying hurricane, the more tornadoes.

Size matters for tornado-producing hurricanes

Like Beryl, Helene could be a prolific tornado producer, Edwards said. That's because when it comes to hurricanes, "size matters with regard to tornado production."

"Bigger storms tend to give you bigger areas of favorable winds, with instability, and more opportunity for tornado production," Edwards said.

And Helene is huge, its mean diameter is more than 440 miles, based on National Hurricane Center data. The average diameter of a hurricane is about 300 miles.

The more Helene intensifies as it draws close to the coast, it could increase the chance for tornadoes, Edwards said, because it could produce more of the wind shear that spawns tornadoes.

Some of the other greatest tornado producers were some of the largest storms in history, Edwards said. For example, he listed hurricanes Ivan in 2004, Beulah in 1967, and Harvey in 2017.

Learning from tornado data

The National Oceanic and Atmospheric Administration has collected data on tornadoes for decades, but the data is most reliable since the advent of Doppler radar. That's why the database Edwards uses starts in 1995.

Most tornadoes occur in the eastern half of a hurricane because the creation of the updraft that starts a tornado needs moisture, instability, and lift ‒ all things found on the "right" side of a hurricane, Edwards said.

Tornadoes typically occur most often in the Northeast quadrant of a storm, he said. Tornadoes also are prolific in the Southeast quadrant of a hurricane, but those areas typically stay over water longer, so it's possible they're producing vortexes out over the open ocean. "We just don't know how they move unless they move onshore."

According to data collected by Iowa State University, Beryl produced a total of 113 tornado warnings, USA TODAY previously reported in July.

"One hundred thirteen warnings is a lot and given this happened in July, it is even more significant," said Daryl E. Herzmann, a systems analyst at Iowa State University who produced the study. "The right-front quadrant of the storm was in an abnormally primed environment to produce tornadoes."

Since 1995, hurricanes and tropical storms have spawned more than 1,700 tornadoes, Edwards said.

Hurricanes and tropical storms with the most tornadoes since 1995

Contributing: Ramon Padilla, USA TODAY

Dinah Voyles Pulver covers climate change and the environment for USA TODAY. She's been writing about hurricanes, tornadoes and violent weather for more than 30 years. Reach her at [email protected] or @dinahvp.