What causes storm surge?

Mostly wind, with a small assist from low pressure, said Jeff Waters, a meteorologist and senior product manager for RMS North Atlantic Hurricane Models . Think of a leaf-blower blasting water through a puddle. That is basically how a hurricane displaces water, only on a massive scale. In a northern hemisphere storm, the largest surge occurs along the front right side, where the strongest winds are blowing onshore. That wind, combined with the storm’s forward direction, pushes the most water. Low air pressure at the center of a hurricane causes a slight rise in the water level, a little like taking a sip through a straw. But Waters said wind around the eye is by far the most important creator of storm surge. Every hurricane is unique, but traits of each storm and the land it’s headed toward provide clues to the amount of surge the storm may generate, according to the National Hurricane Center . The first half of the equation is the storm’s characteristics. Wind speed: Stronger winds create more storm surge. That doesn’t mean every Category 5 storm will generate more storm surge than every Category 3. But if the two storms and the areas they strike are identical in every other way, the storm with stronger wind will create a greater surge. Size: The larger a hurricane’s total wind field , the more water it will move. So a giant storm that has wind bands stretching hundreds of miles will often generate more surge than a more compact storm, even if the smaller storm has significantly stronger winds. Forward motion: Some storms travel quickly across the sea and onto the land, while others seem to crawl. The speedy ones tend to cause more intense storm surge along the open coast — a big wham! And then it’s over. The crawlers build a slower, more relentless surge over a larger area and often push water farther inland. Angle of approach: A storm that slams directly into a coast will create more storm surge than one that comes ashore at more of an angle. The other half of the surge-prediction equation is the location where the storm will hit.

What makes an area prone to storm surge?

Shoreline features and the contours of the continental shelf help determine how much storm surge may make it onto land in a particular area — and what effect it may have. Low-lying coastline is of course more susceptible than higher ground. But so are naturally concave areas, such as sounds, bays and inlets, because water is funneled in and trapped. The problem is compounded at river mouths and estuaries, as the surge rushes in along the paths of least resistance, sometimes causing inland flooding. The continental shelf — where the sea floor drops off dramatically — is an extremely important indicator of how storm surge will behave. For example, in 2005, Hurricane Katrina ’s storm surge was incredibly destructive even though its winds at landfall were “only” Category 3. That’s because the huge storm had churned for days in the gulf as a Category 5, building up water that it then pushed easily up the shallow shelf and onto the low-lying coast. Hurricane Andrew, however, barreled into South Florida as a Category 5 in 1992, and its wind dismantled entire neighborhoods. But its storm surge was much less dramatic. Andrew was a powerful but small storm that moved less water. And the steep continental shelf along the Atlantic prevented much of the water from reaching land. Various computer models estimate how much surge a particular storm could generate in the area it is most likely to strike. The National Oceanic and Atmospheric Administration created one aptly named SLOSH (Sea, Lake and Overland Surges from Hurricanes). RMS uses one from the Danish Hydrological Institute.

Is storm surge the same as flooding?

Not exactly, but is a type of flooding and one of two main causes of flooding during a hurricane. The other is rain. Whether storm surge or rain most effects an area depends once again on characteristics of the storm and the land. For instance, Hurricane Harvey stalled over the Houston area for four days in 2017 and dumped several feet of rain. One gauge recorded a total of nearly 61 inches, the largest rainfall ever measured during a U.S. storm. So it’s not surprising that rain caused most of Harvey’s catastrophic flooding. Katrina, on the other hand, dropped less than a foot of rain in all but a few areas. But a 19-foot storm surge — even higher in some places — overran levees and inundated New Orleans and swaths of the Gulf Coast. If a storm surge happens to coincide with a normal daily high tide, the resulting mega-gush is called a “storm tide.” In 1900, storm tides of 8 to 15 feet from an unnamed hurricane swept over Galveston Island, Tex., and onto parts of the mainland, killing an estimated 8,000 people.

Does climate change contribute to storm surge?

Yes. Not only does warmer air and water make for larger, stronger and more rapidly intensifying hurricanes, but it also causes a general rise in sea level. That means any storm surge occurs on top of a starting point that is already higher than it used to be. Sea level is two feet higher in some parts of Louisiana than it was in 1950. In addition, warm air holds more water, so hurricanes can unleash more rainfall, exacerbating flooding. Studies found that human-caused warming increased Harvey’s rainfall by 15 percent .

What is the highest storm surge ever recorded?

According to Weather Underground , the highest measured on the U.S. coast was 27.8 feet, recorded in Pass Christian, Miss., during Katrina. It was measured from a high-water mark found inside a building, so the mark couldn’t have been caused by waves. Measuring storm surge is not as easy as dipping a yardstick in the water. Waters said data comes from many sources, including high-water marks, sensors, tidal gauges, aerial and satellite images and people on the ground taking photos and surveying damage. But not every source is available in every area, so data reliability varies by storm. “It’s not going to be possible to have high-water marks on every single location,” Waters said. “To that end, you try to get as dense of a sample size as you can, and from there you will have to make some assumptions in between. That’s where some of our modeling science comes in to help fill in the gaps.” Because so much money and technology has been invested in modern observational data, Waters said, measurements are much more accurate for storms that have occurred within the past five to 10 years. The all-time global record may (or may not) be held by Cyclone Mahina , which struck Bathurst Bay, Australia, in 1899. Accounts from the time indicate that Mahina may have produced a storm surge of about 43 feet. Scientists disagree on that number, but one thing is known: Dead dolphins were found atop cliffs half a mile inland.

What is storm surge?

By Bonnie Berkowitz

and

Artur Galocha

September 26, 2022 at 1:30 p.m. EDT

(Video: The Washington Post)

As a hurricane travels over the open sea, its powerful winds act like a giant bulldozer collecting water and pushing it forward. When this buildup of water runs into land, the sudden rise in sea level above normal tides is called storm surge, and it is sometimes the most deadly and destructive part of a hurricane.

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Every hurricane will generate some storm surge, and so will other storms with strong wind, such as nor’easters.

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