Hurricane Preparedness Week: Why storm intensity ratings are flawed

Weather

SAVANNAH, Ga. (WSAV) — Nothing strikes fear during the summer and fall months more than a forecast for a landfalling hurricane as we conjure up images in our minds of roaring winds effortlessly ripping roofs off of homes, trees toppling over, and debris flying through the air. After all, it is the peak winds that determine a tropical system’s category. And yet, there is an impact even deadlier than wind: water.

It may be surprising to learn that flooding rainfall and storm surge are not part of the intensity rating system even though they often take far more lives and cause even more expansive damage than wind. Storm surge alone accounts for roughly half of all tropical cyclone related deaths in the United States.

Storm surge is defined as the abnormal rise of water over and above the tide level. Powerful winds blowing water toward the coast combined with extreme low pressure cause the water to swell and inundate not just the coast, but even inland areas near rivers. Homes and businesses can be completely destroyed, people drown, and the shape of a coastline can be forever changed in the most severe cases.

While it may seem to reason that the stronger the storm, the higher the surge will be, it’s actually not that simple. Given the right set of circumstances, it is entirely possible for even a tropical storm to produce more surge than some hurricanes. That is one reason why tropical storms are often underestimated. While on average hurricanes will produce more damage, that is not always the case.

To understand why that is, it is important to know exactly what is being conveyed when looking at a storm’s classification and intensity rating. The Saffir-Simpson scale is the standard tropical cyclone ranking system used by the National Oceanic and Atmospheric Administration.

The categories you are familiar with (1 through 5) really only represent one component of a hurricane: the maximum sustained wind speed. Those peak winds are located in a relatively narrow band in the eye wall of the storm, sometimes only 10 or 20 miles wide while the storm itself can be hundreds of miles wide. Although sometimes deadly, maximum winds do not give you nearly enough information about the potential danger of an approaching tropical cyclone.

Storm category does not consider the size of a storm, how fast the storm itself is moving from one location to the next, the amount of rainfall flooding it might produce, or whether it may threaten an area at high tide or low tide. Combined, these details along with many others have far greater implications for the severity of potential damage by a hurricane than just its maximum wind speeds.

It is entirely possible to get significantly more surge out of a large, slow-moving tropical storm that hugs the coast at hide tide than a compact, fast-moving hurricane that makes its closest pass at low tide or farther offshore.

Despite taking a similar track to Hurricane Matthew, Hurricane Dorian did not cause nearly as much flooding in the Coastal Empire and Lowcountry when it passed offshore in early September of 2019. The reason for that came down to the fact that winds were blowing offshore at high tide, preventing any serious flooding from occurring.

Sometimes it just comes down to timing. Had the storm moved a little faster or a little slower, we may have seen the strongest winds coincide with high tide and the damage would have been orders of magnitude greater. We got lucky, but it was an important reminder that there is far more to consider than category alone. Every storm is different and it is critical to keep up with forecast details that can significantly change leading up to their approach.

While it used to be a bit more complicated, these days when you are looking at a storm surge forecast, you are really looking at the predicted inundation levels, or water above ground. Unlike in the past, this much simpler system does not require any math to determine how much water to expect in your area. Inundation factors in elevation and does the math for you. If the inundation is forecast to be 4-6 feet at your location, that’s how much water to expect above ground.

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