Why flooding is a hurricane’s deadliest risk

When back-to-back hurricanes Helene and Milton hit Florida, much of the damage they caused came from storm surges, violent waves thrown ashore in the path of the hurricanes.

The National Hurricane Center has issued storm warnings since 2017 when Hurricane Irma threatened to make landfall on both coasts. However, emergency officials still struggle to convey the risk to the public. At least nine people drowned in Pinellas County during Hurricane Helene.

WLRN Environment Editor Jenny Staletovich spoke with Hurricane Center Storm Team Chief Cody Fritz about forecasting a hurricane’s deadliest threat. What follows is an edited, shortened version of a longer interview that aired on WLRN.

STALETOVICH: So if we can start with Helene and Milton. How did you forecast and model them differently? Kind of like, what were the challenges and what were the differences?

FRITZ: With Helene, yes, it was, it was a big storm. It started off great. It remained large. It certainly intensified as it got closer to the coast, compared to Milton, which was a very small storm to begin with, but a very, very strong storm overall. And then, as it apparently crossed the Gulf of Mexico and made its way up the coast of Florida, it weakened, but got even bigger. So it’s not just that intensity of the hurricane. It’s also the size of that storm. And that means how far hurricane force and those tropical storm force winds go? Because ultimately, this is the force that pushes the water towards the shore.

STALETOVICH: You know, I still tend to focus on where it’s going to end up. You know, that’s going to be the biggest threat. And clearly, with Helene, she was, I think, about 100 miles out to sea when she passed Tampa.

FRITZ: You are absolutely right. It was 100 miles out to sea. And I think for a lot of communities that live along the West Coast, they probably thought they were, in a way, okay because the storm was offshore. They were following that hurricane as it moved further north, northeast. But what you’re after is ultimately just the center of the traffic, right? It doesn’t even take into account any of the dangers associated with that storm. And so that’s kind of where I come in and try to communicate one of the specific dangers, which is storm surges. Because ultimately the storm is why we evacuate any coastal community. That has less to do with wind and more to do with storm surge. And the west coast of Florida is so vulnerable to storm surges. I cannot stress this enough. So even though Helene tracked west of the Tampa Bay area, her wind field was so large, in fact, that tropical storm force winds, I think, were in the 90th percentile, right? That means only 10% of storms exceed this size.

As it passed nearly 100 miles offshore, Hurricane Helene generated a powerful storm surge that washed away boats on Madiera Beach near Tampa Bay.

STALETOVICH: When you talk about that region and that coastal area as well, I’ve talked about how it’s capable of doing, I’m going to say you hit it harder. It’s probably not the technical term. But I mentioned that my brother lives in Saint Pete Beach. He said the water came into his house so fast, he kind of lost track of time. But he said it felt like under an hour. He went from just getting in to being so deep he couldn’t open the door. He had to crawl out of a window. And so you can talk about something like that? Because I find that surprising. It must have been surprising to him.

FRITZ: Yeah, so, you know, one of the reasons we’re warning about storm surges isn’t just how high the water can get. It’s really the speed at which the water can get in, right? So how fast can water move. Because the combination of high water and fast moving water is very harmful to life. It can cause death in many cases. And that’s why, finally, we’re telling you there’s a danger of a life-threatening storm surge. So yes, definitely in the case of, say, your brother. Right in St. Spots, when the storm comes, can rise several meters in minutes to hours. So it’s not just this slow rise of water. It can come very, very quickly and then give you very little time to respond to it. That’s why ultimately we’re trying to promote those evacuation decisions when a storm warning is issued so that people have time to do what they need to do, get out of harm’s way. Because when the water comes in, it’s going to come in very, very quickly, and you’ll have very little time to respond to it.

STALETOVICH: So sea level is rising. I spoke with a geologist last week who said that since 2000, sea level in southwest Florida has risen about six inches. Is that accurate? And how do you incorporate that into your designs?

FRITZ: Yeah, I mean, sea level rise is happening. I mean, you can’t deny it, because you can see that the water level has been rising over the last few years, and it’s continuing to rise as we move forward here. Can we explain these increases in sea level rise? Absolute. So whenever we’re trying to forecast or anticipate storm surges, relative to the aspects that are associated with that hurricane, we also need to fundamentally know how big the water is in its current state. So we can initialize our models that we use to forecast storm surges. We can initialize those models with the current state, the current initial water level and then use that as a base reference and then apply the various wind forcing characteristics of the hurricane to that initial base state. And from there, you understand exactly what the meaning of the storm surge would be, relatively speaking. Because in the end it matters, right? If you raise the water level, you have a greater potential to not only increase the height of the water that is experienced on the coast, but the water can also penetrate further inland and flood areas that may not have actually experienced storm surge before . And for those who live in vulnerable areas like the west coast of Florida, many of the areas along the Gulf Coast that will experience sea level rise or have already experienced sea level rise, any hurricane that passes can definitely make this storm worse. danger.

STALETOVICH: So we have to watch out for sea level rise because safe land can no longer be…

FRITZ: Yes, I mean, absolutely. I think with or without sea level rise, I mean, if you live on the coast and if I can tell you something about storm surge and these areas along the Gulf Coast, parts of the Southeast Coast and the East Coast, many of these areas are very vulnerable to storm surges. And one of the biggest things that I’ve noticed, and I’ve talked about it, even with people’s experiences with Hurricane Irma. Hurricane Irma did not hit Miami. It hit the west coast and that’s where some of the worst impacts were. If Hurricane Irma had hit Miami directly, it would be a completely different event. And you can look at it from many different aspects of hurricanes, where people’s experiences are what they live in that storm. But what they saw may not be the full devastation of what that hurricane can do. So I always caution people not to just use their experience in that hurricane, but to understand that all hurricanes are different. It depends a lot on where the respective hurricanes end up. It really depends on how big and intense those hurricanes are. And if you live in a community along the coast, especially those on the west coast of Florida or the Gulf coast or the southeast, and you’re very vulnerable to storm surge, you can’t just use your experiences from the past to guide you. decision in the future, you know. It is our job to predict what will happen. And hopefully it’s your job to answer that prediction we make.

Hurricane Helene's extended wind field ranked in the 90th percentile, meaning only 10 percent of recorded hurricanes were larger. This allowed it to push ashore a storm surge in Tampa Bay, about 100 miles east of the hurricane's eye. — Hurricane Helene’s extended wind field ranked in the 90th percentile, meaning only 10 percent of recorded hurricanes were larger. This allowed it to push ashore a storm surge in Tampa Bay, about 100 miles east of the hurricane’s eye.

STALETOVICH: Well, it turns out that Helene and Milton are so close, both in time and location, and behave so differently, that there are lessons to be learned there.

FRITZ: There are absolutely lessons to be learned there. You have two hurricanes, very intense in total. But there are other characteristics about them that were different. Obviously, one was very large, remained large, and continued to grow in intensity over time. One was small at first, very intense, but got weaker over time, but then got bigger. They hit various locations, but many of the locations along Florida’s west coast were still affected to some degree. The specific thing about Tampa is that Milton ended up south of the bay. And so a lot of the winds that you experienced were offshore. All the water went out of the bay rather than in. What’s more interesting than that, as you could see, what can happen in a case where you’ve had a storm still off the coast and it’s made landfall to the north of you, and then you have more of the onshore winds and , therefore you got more storm surge. If Milton had made it 15 or 20 miles north of where it did, all that wind would have been onshore and blown all the water into the bay and you would have had a significant storm surge, probably greater than about three meters. .

STALETOVICH: So going forward, are there things you’re looking at doing differently?

FRITZ: Our ultimate goal is obviously to continue to improve our modeling capabilities and try to improve our ability to predict these storms. I think especially for storm surges, though, our ability to better resolve the overall structure of a hurricane, because again, it’s all about the wind, where the wind is and where the wind is pushing the water. That’s it for storm surge. Where does the wind push the water? And if we can better resolve and understand the structure of the hurricane from a modeling standpoint, we can better predict the total amount of storm surge. And then, from there, we can communicate these predictions even better to local officials, the public and the media, and have much more confidence in that prediction.

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