Hurricane Katrina in the Gulf of Mexico in August 2005. Image credit: NOAA
When you think of the danger a hurricane poses to the unlucky people caught in its path, your first thought is probably the ferocious winds that crash ashore and tear up just about everything exposed to the elements. While the winds are destructive and the flying debris is a serious hazard to anyone caught in the way, the greatest and quietest killer in a tropical cyclone is its storm surge.
1. A SURGE IS A SUDDEN INUNDATION OF SEA WATER.
The strong winds of a landfalling tropical cyclone thrust it inland. The flooding that results from storm surges is only a few feet deep most of the time, but the worst surges—like those seen in Hurricane Katrina—can exceed 20 feet or higher. A storm surge comes up quickly and can push water miles inland in the most vulnerable spots during the strongest storms.
2. THEY'RE NOT CAUSED BY HURRICANES ALONE.
Hurricanes are most closely associated with storm surges, but they’re not the only storms that can push water inland. Tropical depressions and tropical storms can also inundate coastlines if their winds are strong enough. Powerful winter storms can also generate a life-threatening storm surge. A blizzard that hit the East Coast in January 2016 produced a storm surge in Cape May, New Jersey, that was slightly higher than the one recorded there during Hurricane Sandy a few years earlier.
3. TRACK AND TIMING MATTER …
A diagram showing how a storm’s winds are influenced by its forward motion. Image credit: Chris Landsea/NHC
We tell people not to focus on the exact track of a tropical cyclone since the impacts can extend hundreds of miles from the center of the storm. But when it comes to a storm surge, track really does matter. The worst winds in a storm occur in the right-front quadrant of its eyewall, or the part of the storm that’s in front of the eye and to the right of its forward movement. This spot sees the strongest winds influenced by the forward motion of the storm, and it’s where the wind is able to push the most water with it.
Timing also determines how much flooding people at the coast will experience. Coastal flooding will be worse if a storm hits land at high tide since water will be a few feet higher. That couple of feet at high tide doesn’t seem like much, but it can mean the difference between a few roads washed out and a few neighborhoods inundated by water.
4. … BUT WIND MATTERS MORE.
The fury behind the surge is wind. The National Hurricane Center says that 95 percent of storm surge is driven by the wind—the other 5 percent is water that rises above sea level due to low air pressure at the center of the storm. A general (and obvious) rule of thumb is that a stronger storm will produce a more destructive storm surge, but surge also depends on other factors like a storm’s forward speed and the size of its wind field.
5. WIND IS WHY SANDY WAS SO DEVASTATING.
Hurricane Sandy’s wind field at landfall on October 29, 2012. Image credit: NHC
Even though Hurricane Sandy only had 80 mph winds when it made landfall in New Jersey on October 29, 2012, it was one of the most destructive storms to hit the United States in recorded history. The devastating storm surge that Sandy drove into coastal communities was the result of the immense size of the storm’s wind field.
When Sandy made landfall, the area covered by its tropical storm force winds (39–74 mph) covered more than 1100 miles from South Carolina to Maine. The enormous area covered by these strong winds made up for the storm’s relative lack of concentrated intensity, allowing it to push tremendous amounts of water into the coast.
Hurricane Katrina’s historic storm surge along the northern Gulf Coast in August 2005 was also driven by the sheer size of the storm. Katrina was a massive hurricane with scale-topping category 5 winds to boot. Katrina weakened by the time it reached the coast, but the size of the storm and its former strength still pushed enormous amounts of water into Louisiana and Mississippi.
6. CURVY COASTS MAKE A BAD SITUATION WORSE.
As if getting hit with a bad storm weren't bad enough, the very shape of the coastline itself will determine how much of an impact a storm surge will have on coastal communities. Shallow waters offshore and concave bays and inlets will exacerbate a storm surge and make the inundation deeper than it would have been otherwise.
7. LINGERING STORMS DO MORE DAMAGE.
After it made landfall in Florida and moved into the Atlantic Ocean, meteorologists were worried about tropical storm Hermine’s impacts along the Mid-Atlantic and New England coastlines because of how long they expected the storm to linger near land. Forecasts called for Hermine to meander off the coast of New Jersey at or near hurricane strength for four full days before beginning to dissipate. Thankfully, the worst-case scenarios didn’t come to pass, but the threat was real.
Even though Hermine wasn’t forecast to make landfall, the exceptionally long duration of the storm—powerful winds blowing inland for days at a time—threatened to generate a large storm surge along the coast. A slow-moving storm will cause more damage than one that moves through in a matter of hours.