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    • Millions of people beneath the tornado watch as Hurricane Helene hits land
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    Millions of people beneath the tornado watch as Hurricane Helene hits land

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    As Hurricane Helene headed toward the U.S. mainland on Thursday, at least 25 million people were under tornado watch.

    Although tornadoes do not typically receive the same attention as the ferocious winds or dangerous storm surges of hurricanes, tornadoes are always one of the clear and present hazards when storms make landfall and move inland.

    That’s especially true because tornadoes can occur more than 1,000 miles (1,600 kilometers) 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.

    On Tuesday, the center had already started warning about the increased risk of tornadoes as Helene moves across the US. Edwards is among forecasters who say Helene’s enormous size increases the risk of tornadoes, raising more concerns for people far away from the coast where the storm will arrive first.

    Tornado watches were placed in Florida, Georgia and South Carolina on Thursday afternoon.

    Tips for heavy weather: What is the difference between a tornado watch and a warning?

    Tornadoes with a tropical origin can emerge quickly and be shrouded in rain, making them harder to see, Edwards said.

    “When a tornado warning is issued, people should take immediate action,” he said. “They can’t wait. They can’t look outside and say, ‘Hey, where is it,’ because by the time they try, their house might be blown apart.”

    The hope is that those living in homes that may not be able to withstand a tornado can use the center’s outlook to make their plans to move to a safer place, with a friend or family member, Edwards said. “Once a tornado watch is issued, move to a safer location.”

    Hurricane Dangers: Hurricanes can cause fatalities – and other terrifying hazards – up to 1,000 miles away

    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 hurricane-related tornado situation was for the third day after Hurricane Ivan made landfall in Alabama in September 2004.

    Forecasters were confident there would be many supercells in Virginia, Maryland and even southern Pennsylvania.

    “We had high 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 productive tornado day.”

    “Even far inland, people can’t let their guard down if they are in the path of the remnants of a hurricane,” he said.

    Hurricane Beryl set records. Why?

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

    Beryl made landfall on the Gulf Coast of Texas in July, but the associated winds spawned 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. Even more surprising, Beryl-related tornadoes also set a record for tornado warnings for the weather service office in Buffalo, New York, more than 1,300 miles (2,100 kilometers) away. More than 100 tornado warnings were issued along Beryl’s path due to the wild winds.

    The Weather Service continues to review storm surveys from the days following Beryl. By the time the final analysis is complete, Edwards expects Beryl will have produced about 65 tornadoes.

    “That’s probably going to be about fifth on the all-time list, so this is a very 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 hurricanes that produce tornadoes

    Like Beryl, Helene could be a prolific tornado producer, Edwards said. That’s because when it comes to hurricanes, “size matters with respect to tornado production.”

    “Larger storms usually give you larger areas of favorable winds, with instability and more opportunities for tornado production,” Edwards said.

    And Helene is huge, its average diameter is more than 450 miles, based on data from the National Hurricane Center. The average diameter of a hurricane is about 300 miles.

    The more Helene becomes more intense as it gets closer to the coast, it could increase the chance of tornadoes, Edwards said, because it could produce more wind shear that spawns tornadoes.

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

    Learning from tornado data

    The National Oceanic and Atmospheric Administration has been collecting data on tornadoes for decades, but the data is at its most reliable since the advent of Doppler radar. Therefore, the database Edwards uses starts in 1995.

    Most tornadoes occur in the eastern half of a hurricane, because the creation of the updraft that produces a tornado requires moisture, instability and lift — everything you find on the “right” side of a hurricane, Edwards said.

    Tornadoes tend to occur most often in the northeast quadrant of a storm, he said. Tornadoes are also common in the southeast quadrant of a hurricane, but those areas tend to stay above water longer, so it’s possible they create eddies over the open ocean. “We just don’t know how they move unless they move on land.”

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

    “One hundred and thirteen alerts is a lot and since this happened in July it is even more important,” said Daryl E. Herzmann, a systems analyst at Iowa State University who conducted the study. “The right front quadrant of the storm was in an anomalously suitable environment to produce tornadoes.”

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

    Hurricanes and tropical storms with the most tornadoes since 1995

    Hurricane Year Tornadoes
    Ivan 2004 118
    Frances 2004 103
    Rita 2005 97
    Katrina 2005 59
    Harvey 2017 52
    Fay 2008 49
    Gustavus 2008 49
    Cindy 2005 48
    Georges 1998 48
    Florence 2018 44
    The National Weather Service began widespread use of Doppler radar in 1995.

    Contributing: Ramon Padilla, USA TODAY

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

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