Whenever a hurricane threatens land, almost all of the focus is on preparing for the strong winds. It’s not always the wind that gets you—it’s the water. The storm surge in a hurricane is arguably the greatest threat with these landfalling terrors, and it’s one that too many people seem to ignore until it’s too late.

Storm Surge

A storm surge is an influx of sea or lake water pushed ashore by the strong winds in a storm. The vast majority of storm surges are tiny—usually a foot or two, if that—and only affect the immediate coastline with something locals would probably consider nuisance flooding. Stronger storms can create a storm surge great enough to destroy homes and businesses near the coast, and historic hurricanes like Katrina or Camille can generate a devastating storm surge that pushes the ocean miles inland and destroyed thousands of structures, potentially killing hundreds of people along the way. Not only that, but localized features like bays or concave/angled coastlines—not to mention high tide—can make the storm surge even worse.

The depth of a storm surge is relative to the elevation of the land affected by the flooding. If Hurricane Skittlebip pushes a 15-foot storm surge inland, the water will be 15 feet deep where the coastline is at sea level. If a parking lot a few thousand feet from the coast is 5 feet above sea level, that 15-foot storm surge would only (“only”) be about 10 feet deep at that point.

Coastal areas that have shallow elevation changes between inland areas and the shoreline can see the ocean push an incredible distance beyond the coast. Some parts of Mississippi saw the storm surge reach more than ten miles inland when Hurricane Katrina roared ashore back in August 2005. These surges can also move through bays, rivers, and any body of water connected to (or near) the coast.

How It Happens

The next time you drink a cup of water (or coffee, or whatever), gently blow across the top of the liquid. If you look closely, you’ll see the liquid starting to pile up on the opposite side of the cup—it won’t be much, but it’s a noticeable change in height.

Now, imagine that happening over hundreds of square miles of ocean, and instead of your gentle, minty breath, it’s a huge area of winds that are raging at more than 100 MPH. This is how a storm surge takes shape. Lower air pressure can account for a small amount of the rise in water levels, but the overwhelming majority of the surge is from the wind. Large, intense storms can push vast amounts of water out ahead of them, and as the ocean gets more shallow nearer to the shoreline, the water begins to pile up and grow in height.

A storm surge is like a slow-motion tsunami. If you saw those horrifying videos of the tsunami in Japan back in March 2011, storm surges don’t happen with the same speed and ferocity, but they can cause just about as much damage. Water is powerful. While it doesn’t smack the coast all at once like a tsunami, a storm surge can come up pretty quickly, and people in the path of such disasters don’t have much time to react once it begins

The video above shows the storm surge on Coney Island during Hurricane Sandy back in October 2012. The video starts around 6:00 PM and ends around 8:00 PM when the power went out and it was too dark to see anything. In that short period of time, the water on the roads rose from general, navigable ponding to a flood that engulfed cars on the street below. The surge can rise even faster in stronger storms, and with water pushed around by powerful winds, buildings can easily float away or shred apart like they’re made of paper.

Hurricane Katrina

This week marks ten years since Hurricane Katrina made landfall along the northern Gulf Coast, creating the largest natural disaster in American history. The storm was the worst to crash into the country since a category four hurricane destroyed Galveston, Texas, back in 1900, killing between 6,000 and 12,000 people. A death toll of that magnitude is unimaginable in this day and age, but in the grand scheme of storms, the fact that 1,833 people lost their lives in Hurricane Katrina is pretty unimaginable on its own.

The level of death and destruction seen in Katrina’s wake wouldn’t have been nearly as high if it weren’t for the flooding caused by the storm. Almost half of the people who died in Louisiana during Katrina drowned in the storm surge. According to a 2008 medical study, 971 people in Louisiana died as a result of Hurricane Katrina (that number, which fluctuates based on the source, has since been revised to around 1,500). Out of those 971 victims included in the study (800 of whom had listed causes of death), 387 (43%) were confirmed to have died by drowning, while only 246 (25%) of the victims died from injuries sustained during the storm (both wind/water-related as well as events like shootings, suicide, heat, and illnesses).

That’s in Louisiana alone, where the effects of the rising waters were exacerbated by New Orelans’ below-sea-level location and fragile system of levees. Louisiana actually escaped the worst of the hurricane’s storm surge. The storm’s eye passed over extreme eastern Louisiana—east of New Orleans-coming ashore on the southern coast of Mississippi. The worst storm surge almost always occurs in the right-front quadrant of a tropical cyclone’s eyewall, where the direction of the strongest winds match the storm’s forward motion. Pass Christian, Mississippi, recorded a 27.8-foot storm surge, which is the deepest storm surge ever measured in the United States. It was in this part of the state that the storm surge reached more than ten miles inland, demolishing just about everything in its path.

When it made landfall, Hurricane Katrina was only a category three storm with winds of about 125 MPH. That’s not as strong as storms like Charley the year before (category four) or Andrew thirteen years prior (category five), but a confluence of factors led to the storm turning into a nightmare even though it was relatively weaker than its historical counterparts.

Less than 24 hours before landfall, Hurricane Katrina strengthened into a monstrous category five storm with winds of 175 MPH. Meteorologically, it was the perfect storm—save for a few bumps and rainbands, Katrina was perfectly symmetrical on satellite imagery, with an incredible buzzsaw presentation and an enormous, dramatic eye smack in the middle.

While Katrina’s maximum sustained winds weakened dramatically in the hours leading up to landfall, it didn’t matter when it came to the storm surge. The storm’s large, intense wind field scooped up an incredible amount of water from the Gulf of Mexico and sent it on a collision course with the northern Gulf Coast. The winds weakened faster than the water could catch up. The damage was already done. Katrina made landfall with the winds of a category three hurricane and the storm surge of the record-breaking category five hurricane it was just hours before.

Hurricane Sandy

The worst storm to make landfall in the post-Katrina era was Hurricane Sandy, an unprecedented storm that roared ashore in New Jersey under extremely strange circumstances. The confluence of several weather features came together to make marginal category one hurricane one of the worst storms in American history, and one which was immediately assigned a fake name—“superstorm”—to account for its unusual ferocity.

Even though Sandy came ashore with winds that were barely at hurricane strength, its wind field was gigantic. Near landfall, tropical storm force winds (39-73 MPH) extended nearly 500 miles away from Sandy’s center of circulation, and hurricane force winds (74+ MPH) extended nearly 200 miles from Sandy’s center. That is an unbelievably large wind field that covered most of the Mid-Atlantic and Northeast at one point.

While we usually see devastating storm surges with intense storms like Katrina or Andrew, it’s pretty unusual to see such a widespread, destructive influx of seawater like we saw in a relatively weak storm like Hurricane Sandy. What Sandy lacked in strength it more than made up for in sheer size. The storm had a minimum air pressure equivalent to a category three hurricane, but instead of translating that energy into wind speeds, it used that energy to expand into this gigantic, sprawling storm. It was this enormous wind field that allowed Sandy to create such a devastating storm surge in New Jersey and New York which directly killed dozens (if not more than one hundred) people.

Advances in Forecasting

The public’s lack of knowledge about the threat of storm surges seems to be, in part, a failure of communication. We’re so focused on relaying wind speed information that storm surge forecasts fall by the wayside; it’s almost like they’re included as an afterthought in forecasts and news stories. “The storm has 140 MPH winds, it’s moving west, you’ll see 10 inches of rain, oh, and by the way, the sea will rise 21 feet and engulf every home within five miles of the coast.”

Last year, the National Hurricane Center (easy-to-remember link: hurricanes.gov) launched a new product to better relay storm surge forecasts and information to the public. This new Potential Storm Surge Flooding Map will show which areas could see a storm surge in an approaching storm, and how high above the ground that storm surge could reach if current forecasts hold true.

In the above map for a hypothetical hurricane threatening Fort Myers, Florida, areas shaded in red could see a storm surge up to nine feet above ground level, while the surge could potentially reach up to three feet above ground level in areas shaded in blue.

This year, the National Hurricane Center will begin experimenting with storm surge watches and warnings that will be issued alongside existing hurricane and tropical storm advisories. A storm surge warning would be issued up to 36 hours in advance of a potentially life-threatening inundation of water along the coast, and a storm surge watch would be issued up to 48 hours in advance of the anticipated surge of water.

This product, along with the experimental inundation maps introduced in 2014, will be incredibly helpful in relaying useful information to coastal residents in advance of any storms that threaten land in the future. Knowledge is power when it comes to severe weather safety, and the more people know about and are aware of the threat of storm surges, the more they’ll pay attention the next time a storm makes landfall.

Luckily, we haven’t had a chance to use these products yet, and hopefully we won’t for a long time to come.

Don’t Stick Around

It’s horrifying to hear people brush off the threat of a hurricane when one is churning their way. Even systems as “weak” as a tropical storm can cause lots of damage—ask Texas how well they fared during Tropical Storm Allison. It’s pretty common for coast-dwellers in the southern United States to shrug off even a formidable hurricane as “nothing much” or just a little bit of wind and rain. People right on the coast brag about having hurricane parties and go toe-to-toe comparing moxie by how strong the storm has to be before they’ll give up and leave.

A solid number of people who fail to evacuate before a storm stay because they can’t leave, a result of finances, lack of transportation, or the inability to get time off from work to do so. That’s a major problem that local officials and organizations attempt to solve (when the system works, anyway), but it isn’t always effective.

However, there are countless stories of people who don’t evacuate before major storms like Katrina simply because they thought they could ride it out, only to realize that the storm is much worse than they imagined. Structures surviving strong winds and heavy rain is one thing, but there’s usually one constant in the regretful stories told by people who stayed on the coast and survived: storm surge.

Storm surge is one of those overlooked hazards that’s usually too late to escape once it begins. If your house catches fire or you have a medical emergency, you can pick up the phone and help will be there within minutes in most cases. We’re lucky to have near-instant help from emergency personnel when something bad happens.

During a hurricane, you don’t have that luxury. Once the winds pick up and the water starts rising, you’re on your own until the storm is over.

If you ever find yourself on the wrong end of an approaching hurricane, don’t underestimate the power of the storm. Even if you’re a few miles away from the ocean, depending on the elevation profile of the coast, an intense storm could push a surge pretty far inland. A hurricane’s storm surge can cause more damage than the wind itself, and it’s harder to escape rising water than it is to find safety from flying debris.

[Images: AP, NHC, NOAA, NASA, NHC | Video: YouTube]

Email: dennis.mersereau@gawker.com | Twitter: @wxdam

If you enjoy The Vane, then you’ll love my upcoming book, The Extreme Weather Survival Manual, which comes out on October 6 and is now available for pre-order on Amazon.