What Is Storm Surge? — In Plain Words

This page is general information only.

What this page covers

This page explains what storm surge is, in plain words. It is based on one page from the National Hurricane Center (NHC): the “Storm Surge Overview” page.

Cross-reference, in plain text only: Storm surge is not a rip current — a rip current is a different coastal danger, with its own page here. Storm surge is wind-driven water, not a tsunami. A tsunami is caused by a sudden displacement of the ocean.

Why storm surge matters

The National Hurricane Center says storm surge from tropical cyclones poses a significant threat to life and property along the coast, and is currently the leading cause of fatalities from hurricanes.

All locations along the U.S. East and Gulf coasts are vulnerable to storm surges.

Storm surge can travel up rivers and canals, reaching well inland from the coastline.

NHC's page points to Hurricane Ian (2022) as a reminder of the devastation storm surge can cause, attributing 41 lives lost to its impacts.

What the words mean: storm surge and storm tide

Storm surge is an abnormal water level rise generated by a storm, over and above the predicted astronomical tide.

Storm tide is the water level rise due to the combination of storm surge and the astronomical tide.

Storm tides can reach up to 20 feet or more in some cases, particularly when storm surge coincides with normal high tide.

That “up to 20 feet or more” figure is a storm tide figure. It is not a storm surge figure on its own.

Neither definition includes wave action occurring on top of the surge, which can add additional feet to the overall water level.

What causes storm surge

Storm surge is primarily caused by the strong onshore winds of a hurricane or tropical storm.

The wind circulation around the eye of a hurricane causes a vertical circulation in the ocean. In deep water, there is no indication of storm surge, because nothing interferes with this circulation.

Once the storm reaches the shallower waters near the coast, the vertical circulation is disrupted by the ocean bottom. The water can no longer move downward, so it begins to move upward and inland, resulting in storm surge.

Five things that change how high the surge gets

NHC says the maximum amount of storm surge at a particular location depends on several factors. These are the five factors NHC lists:

• Storm intensity
• Forward speed
• Size (radius of maximum winds)
• Angle of approach to the coast
• The shape and characteristics of the coastline

1. Intensity. Stronger winds will produce a higher storm surge.

2. Forward speed. A faster storm produces a higher surge at the immediate coastline. A slower storm produces a surge that penetrates farther inland.

3. Size. A larger storm produces a higher surge. Its winds push on a larger ocean area, and its strong winds affect an area longer than a smaller storm's winds do.

4. Angle of approach. A storm moving onshore perpendicular to the coast is more likely to produce a higher surge than one moving parallel to the coast or inland at an oblique angle.

5. Coastline shape and shelf. A wide, gently sloping continental shelf allows a higher storm surge. A narrow, steeply sloping shelf makes surge harder to create.

Areas along the Gulf Coast, especially Louisiana and Mississippi, are particularly vulnerable, because the ocean floor gradually deepens offshore. Florida's southeast coast has a shelf that drops off very quickly, making it less vulnerable.

A concave coastline — curved inward, such as Apalachee Bay in Florida — produces higher surge than a convex one — curved outward, such as the Outer Banks of North Carolina.

Local features and barriers also affect the flow of water. NHC's example is North Carolina, with its barrier islands, inlets, sounds, bays, and rivers.

Storm surge is only part of the total water level

Storm surge is only part of what raises water levels during a hurricane. Tides, wave setup, and freshwater flow also contribute to the total water level rise.

Why storm surge is dangerous

Battering waves on top of surge may increase damage to buildings directly along the coast.

Water weighs approximately 1,700 pounds per cubic yard. Extended pounding by frequent waves can demolish any structure not specifically designed to withstand such forces.

The surge allows waves to extend inland.

Currents created by tides combine with the waves to severely erode beaches and coastal highways. Buildings that survive hurricane winds can be damaged if erosion undermines and weakens their foundations.

In confined harbors, storm tides, waves, and currents can severely damage marinas and boats.

Saltwater intrusion in estuaries and bayous endangers public health, kills vegetation, and can send animals such as snakes and alligators fleeing from flooded areas.

This page explains what storm surge is. For warnings and instructions during a storm, follow the National Hurricane Center and local emergency officials.

This page is general information only.