Storm surge, the temporary rise in sea level beneath a storm, becomes dangerous when the storm – typically a hurricane or other tropical storm - reaches land. The water is unable to escape anywhere but onto land as the storm moves towards the shore, which causes flooding along the coast and other types of hurricane damage. For example, the force of waves and currents can cause land to erode and buildings to be destroyed. Saltwater that gets into freshwater lakes, streams, and aquifers is hazardous to aquatic life and drinking water.
When high tide occurs at the same time as storm surge, the combination of the two is called storm tide. During a storm tide, the water level may be 20 feet or more above normal. This is especially dangerous for low-lying islands or coastal areas where the land is not much higher than sea level. (Watch a storm surge animation to see what it looks like.)
The amount of storm surge that occurs as a hurricane or tropical storm hits a coast depends on many factors, described below. Computer models, such as the National Weather Service SLOSH model (Sea Lake and Overland Surges from Hurricanes) take multiple factors into account, allowing forecasters to customize storm surge predictions for particular storms and coasts.
Wind piles up the water.
As winds swirl around a hurricane or tropical storm, seawater is pushed into a mound at the storm’s center. Faster wind is able to pile up more water. Because wind speed determines a hurricane’s category according to the Saffir-Simpson Scale, Category 4 and 5 storms are able to produce a larger mound of water than Category 1 and 2 storms. The mound of water isn’t noticeable out at sea, but as it approaches a coast, the impact is seen as storm surge flooding.
Low air pressure also plays a role.
At the center of a hurricane, air pressure is low. Low air pressure causes a slight bulge in the ocean, which adds to the mound of water that causes storm surge. Most of the water is piled up by wind, but about 5% of the mound is due to low air pressure.
The shape of the coastline makes a difference.
Larger storm surge is more likely along coasts that bow inward than those that bow outward. Places with bays are especially vulnerable because the mound of water can get funneled into a small area.
The shape of the seafloor matters, too.
Coastal areas where the adjacent seafloor is shallow are more likely to be affected by large storm surge as compared with coastal areas where the nearby seafloor is deep.
Is the hurricane heading straight for the coast?
The angle of approach makes a difference. A hurricane that heads straight towards a coastline is more likely to cause a large storm surge than a hurricane that hits the coast at an oblique angle or travels parallel to the coast.
The hurricane’s speed matters, but it’s complicated.
Fast moving storms can cause more storm surge on straight coastlines. Slow moving storms tend to cause more storm surge in coastal areas with bays and estuaries.
Learn more about hurricanes:
- Hurricanes: Overview
- How Hurricanes Form
- Hurricane Damage
- Make a Hurricane Interactive
- What Does Storm Surge Look Like?
- US National Hurricane Center
- Classroom activities