What is a storm surge and what causes it?

Flooding from hurricanes and tropical cyclones accounts for nearly 90% of hurricane deaths  — and about half of those are caused by the storm surge. Storm surges are dangerous not only for the abnormally powerful high tides that they produce but also for the lack of public awareness about how they work. 

Take, for instance, Typhoon Haiyan in 2013. When the tropical storm hit the Philippines, it brought a 7.5 meter storm surge to shore, causing the deaths of more than 7,000 people. Sadly, there were plenty of coastal flood warnings that the storm surge was imminent; however, few people understood the threat storm surges posed, and didn’t evacuate in time. 

Lack of understanding about the risk of storm surge impacts communities all over the world. For this reason, let’s break down what exactly a storm surge is, what causes it, and how to avoid becoming the victim of a storm surge. 

What is a storm surge?

A storm surge is defined by NOAA as “an abnormal rise of water generated by a storm, over and above the predicted astronomical tides.” Storm surges are not the same as storm tides. Storm tides are the rise in water level that occurs due to a storm surge and the astronomical tide. 

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Essentially, a storm surge occurs when ocean water is forced toward the coast by a combination of powerful hurricane winds and low pressure. Here’s how a storm surge forms and creates coastal flooding. 

What causes a storm surge?

The primary cause of a storm surge is strong winds. A storm surge starts to form as winds swirl around, pushing seawater into a “mound” at the center of the storm. The faster the wind speed, the more water piles up. In addition, low air pressure at the center of the hurricane or tropical storm adds to the mound. Roughly 5% of the size of the mound is attributable to low air pressure.  

Storm surges are complex, and can be impacted by a variety of factors, including:

• Intensity: higher wind speeds lead to bigger storm surges
• Central pressure: low pressure can account for 5% of the size
• Forward speed: slower storms can lead to a higher, broader storm surge inland, while faster storms can create more storm surge along the open coast
• Size: typically, a storm with a large wind field leads to more storm surge; storms with smaller wind fields account for less storm surge. 
• Angle of approach: those storms that are perpendicular to the coastline cause more storm surge, whereas those that are parallel will cause less. 
• Width and slope of continental shelf: wide, gentle slopes lead to more storm surge while sharp slopes cause less storm surge. 
• Local features: geography such as the concavity of coastlines, bays, rivers, headlands, islands, etc. can cause greater storm surge impact. 
  

These factors all interplay to create different coastal flooding outcomes. For instance, a Category 4 storm that hits Louisiana could produce a 20-foot storm surge due to the coastline’s wide, shallow continental shelf. However, that same storm surge in Miami might only lead to an 8 - 9 foot surge, given Miami Beach’s steep continental shelf. 

To understand what a storm surge looks like, researchers at the National Center for Atmospheric Research (NCAR) created an animation to show how quickly 9 feet of storm surge can flood a coastal city. Using data from Hurricane Matthew, which made landfall in the southeast United States in October 2016, the team used 3D GIS modeling software to show how the storm surge progressed. 

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Luckily, researchers aren’t limited to retroactively modeling a storm surge. NOAA’s Sea, Lake and Overland Surges from Hurricanes (SLOSH) model is used to predict coastal inundation risk and forecast the potential range of a storm surge. 

How are storm surges forecasted?

SLOSH is a computerized numerical model developed by the National Weather Service (NWS). It is used to estimate storm surge heights by taking into account the atmospheric pressure, size, forward speed, and track data. These parameters allow SLOSH to create a model of the wind field that drives the storm surge. In addition, the model can use the coastline’s unique geography, including bay and river configurations, water depths, and other physical features like levees, bridges, and roads to determine “maximum potential impact due to storm surge”. 

There are three approaches that SLOSH can use to estimate surge:

1. Deterministic approach: Using physics equations, this approach creates a single simulation based on a “perfect” forecast. It is heavily dependent on accurate meteorological inputs. As we’ve covered, however, the complex factors that play a role in the formation and landfall of a storm surge make the single simulation likely to be inaccurate. 
2. Probabilistic approach: the P-surge approach incorporates statistics from past forecast performances to create a number of SLOSH runs. It models the astronomical tide to create a distribution of surge impact. 
3. Composite approach: This model predicts surge by running SLOSH several thousand times with different storm conditions. This creates the Maximum Envelopes of Water (MEOWs) and Maximum of MEOWs (MOMs) outcomes, considered to be the most accurate forecast of storm surge vulnerability. 
   

“The MEOWs and MOMs play an integral role in emergency management as they form the basis for the development of the nation's evacuation zones,” said NOAA.

SLOSH models are considered to be the highest standard, providing forecasts for hurricane surge elevations with an accuracy of +/- 20 percent. With data from sources like Sofar Ocean’s global grid of Spotter buoys, these models can only become more accurate over time — thereby allowing teams to evacuate coastal communities with maximum advance warning. 

If you’re worried about storm surges in your area, check out NOAA’s National Storm Surge Hazard Maps. Updated in 2019, these maps incorporate SLOSH modeling to evaluate ​​an area’s risk of coastal flooding due to storm surge. 

To learn more about mitigating the risks of storms, visit the Sofar Ocean blog.