Millions of tornadoes watch as Hurricane Helene rages on land

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

While tornadoes typically don't receive the same attention as the violent winds or dangerous storm surges of hurricanes, tornadoes are always one of the clear and present dangers as storms make landfall and move inland.

That's especially true because tornadoes can occur more than 1,000 miles (1,600 kilometers) from a hurricane's landfall site and far from the path of the hurricane's center, said Roger Edwards, a senior forecaster at the Storm Prediction Center who specializes in tornadoes.

On Tuesday, the center had already begun warning of the increased threat of tornadoes as Helene moves across the United States. Edwards is among forecasters who say Helene's massive size increases the risk of tornadoes and adds even more worry to people far from the coast, where the storm comes first.

Tornado warnings were posted in Florida, Georgia and South Carolina Thursday afternoon.

Tips about storms: What is the difference between a tornado warning and a tornado warning?

Tornadoes of tropical origin can appear quickly and be enveloped by rain, making them harder to detect, Edwards said.

“When a tornado warning is issued, people need to act immediately,” he said. “They can't wait. They can't look outside and say, 'Hey, where is that?' because if they try that, your house could get blown up.

“The hope is that those living in homes that may not be able to withstand a tornado can take advantage of the center's prospects to make their plans to move to a safer location, to a friend or relative,” Edwards said . “As soon as a tornado warning is issued, move to a safer location.”

Hurricane Dangers: Hurricanes can kill people – and cause other terrible dangers – from 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 occurred on the third day after Hurricane Ivan made landfall in Alabama in September 2004.

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

“We firmly believed that the area would destabilize. Even though the winds died down and the shear was still strong,” he said. “And that’s exactly what happened. It was a very productive tornado day.”

“Even far inland, people cannot let their guard down when in the path of the remnants of a hurricane,” he said.

Hurricane Beryl set records. Why?

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

Beryl came ashore on the Texas Gulf Coast in July, but the associated winds spawned so many tornadoes that it set a new record for tornado warnings in a single day with 67 from the National Weather Service office in Shreveport, Louisiana. 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 away. More than 100 tornado warnings were issued in Beryl's path due to the strong winds.

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

“This will probably be about fifth on the all-time list, so this is a really prolific tornado producer,” he said.

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

When it comes to hurricanes that produce tornadoes, size matters

Like Beryl, Helene could be a prolific tornado producer, Edwards said. Because with hurricanes, “the size matters in terms of tornado production.”

“Larger storms tend to result in larger areas of favorable winds, with instability and more opportunities for tornadoes to form,” Edwards said.

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

The closer Helene gets to the coast, the greater the risk for tornadoes, Edwards said, because it could create more wind shear that spawns tornadoes.

Some of the other largest 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 been collecting data on tornadoes for decades, but the data has been most reliable since the advent of Doppler radar. For this reason, the database used by Edwards has been around since 1995.

Most tornadoes occur in the eastern half of a hurricane because creating the updraft that triggers a tornado requires 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 are also common in the southeast quadrant of a hurricane, but these areas typically stay over water longer, so it is possible that they create vortices over the open ocean. “We just don’t know how they move unless they move on land.”

According to Iowa State University, Beryl issued a total of 113 tornado warnings, as USA TODAY reported back in July.

“One hundred and thirteen alerts is a lot, and when you consider that this happened in July, it's even more significant,” 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 unusually 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

hurricane	Year	Tornadoes
Ivan	2004	118
Frances	2004	103
Rita	2005	97
Katrina	2005	59
Harvey	2017	52
Fay	2008	49
Gustav	2008	49
Cindy	2005	48
Georges	1998	48
Florence	2018	44

The National Weather Service began widespread use of Doppler radar in 1995.

Contributor: Ramon Padilla, USA TODAY

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

Hurricanes can produce tornadoes for days

Hurricane Beryl set records. Why?

When it comes to hurricanes that produce tornadoes, size matters

Learning from tornado data

Hurricanes and tropical storms with the most tornadoes since 1995

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