Hurricane Helen will bring strong winds, explosive flooding and storm surge

What makes Hurricane Helen such a dangerous storm?

By Andrea Thompson

Hurricane season doesn’t seem to be over on the Gulf Coast. Another storm, Hurricane Helen, is headed toward Florida just over two weeks after Hurricane Francine slammed Louisiana. Hurricane Helen is expected to be large and bring a storm surge of up to 15 feet to the state. It is also expected to bring heavy rains deep inland, with explosive flooding most likely in the southern Appalachians.

Helen formed just after the mid-September peak of the Atlantic hurricane season, but after months of summer sun, the ocean waters are still warm enough to produce storms, which is still a good time for a storm. The hurricane began in early September as Francine, which had been relatively inactive for several weeks. The downturn came after the season began with Hurricane Beryl, which became the fastest Category 5 storm on record in the Atlantic basin.

Now, Helen is hurtling into the Gulf of Mexico, where conditions are expected to be ideal for the storm to strengthen. Helen is expected to become a major hurricane before making landfall somewhere in Florida’s Big Bend region (the meandering area between the state’s panhandle and main peninsula).

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One of Helen’s major threats is storm surge, which is expected to reach 10 to 15 feet above ground level in the hardest hit areas of Big Bend. Lower but still potentially damaging surge levels are expected farther south in Florida, away from the center of the storm. One reason for the large surge footprint is the storm’s size, which is expected to be in the top 10% of hurricanes at similar latitudes.

Kim Wood, an atmospheric scientist at the University of Arizona, says part of the reason Helen is so big is because it “starts out so big.” Also, the center of the storm is currently moving like a needle between Cuba and Mexico. That keeps the storm above water, allowing the convection that drives the core of the tropical system to continue to gain fuel in the form of heat and moisture. Meanwhile, the winds swirling around the center are subject to friction as they pass over land. “That transfers energy and momentum,” Wood says, which can increase the storm’s size. Finally, if the storm moves into the Gulf Coast, there will be enough moisture and heat, and Wood notes that research suggests that wetter conditions breed bigger hurricanes. As an example, they cite Hurricane Katrina, which grew much larger after it moved into the Gulf Coast than it did when it was in the Atlantic east of Florida. “The Gulf Coast can handle bigger storms,” Wood says.

Warm, humid conditions and relatively little wind shear to disrupt the center of the storm are expected to steadily strengthen Helen. It is likely to strengthen to Category 3 or higher before making landfall. It is likely to experience what is called rapid intensification, which means the storm’s sustained winds will increase by more than 35 miles per hour over a 24-hour period.

Whether Helen will do so will depend in part on all of these external environmental factors and the internal structure of the storm. Hurricanes develop a closed “eye wall” of thunderstorms around their central “eye,” Wood explains, and can use the heat more efficiently to grow. That’s one reason experts and forecasters look to detailed satellite images to help them see what’s happening in the center of the storm. “It’s almost like an X-ray of what’s going on below the cloud tops,” they say.

Helen’s winds are most likely to cause problems farther inland. Tropical storm-force winds could reach northern Georgia, because the storm is moving fairly quickly. Wood says storms that move faster after landfall don’t lose energy as quickly.

Helen will also bring significant flooding threats from the heavy rains that fall far inland. The area most likely to be affected stretches from the Gulf Coast north to southern Appalachia, where the threat is greatest. That’s partly because the area is experiencing a drought, so the soil is dry and compacted. That means a lot of the rain will just run off, says Derek Eisentraut, a meteorologist with the National Weather Service office in Morristown, Tennessee. The mountainous terrain also means the rain could run off down slopes into lower valleys, he says.

Climate change is also playing a role in increasing the threat of hurricanes. Sea level rise is increasing the surge, storms are producing more rainfall than in the past, and there is evidence that a higher percentage of hurricanes are reaching peak intensity, so storms are also expected to intensify rapidly due to climate change.

Forecasters note that anyone in Helen’s path should heed local forecasts and local officials’ orders for evacuation and areas to avoid. Wood said the National Weather Service is also helpful. Hurricane Threat and Impact GraphicsThis allows people to toggle between different threats, such as floods or wind, and click to check conditions in a specific area.

Megan Bartels reported further.