The Weather Authority answers your questions about storm surge

What is storm surge?

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

It’s the change in the water level that is due to the presence of the storm.

Since storm surge is a difference between water levels, it does not have a reference level.

What causes storm surge?

Storm surge is primarily caused by the strong winds of a tropical system. The wind circulation blows on the surface and produces a vertical circulation in the ocean.

In deep water, there is nothing to disturb this circulation and there is very little indication of storm surge.

Once the storm reaches the shallower water near the coast, the vertical circulation in the ocean becomes disrupted by the ocean bottom. 

The water can no longer go down, so it has nowhere else to go but up and inland. This is called Storm Surge.

In general, storm surge occurs when the winds are blowing onshore. The highest surge tends to occur near maximum winds.

Total water level

In reality, storm surges only contribute a small part of the cause of rising water levels along the coast during tropical systems. Other factors also play a role.

• Tides: Water levels rise and fall along the coast every day due to the gravitational pull of the moon. This is the tide.
• Waves: Breaking waves contribute to the water level rise due to large crashing waves, as the water is propelled onto the beach. This increases the chances of surge, especially when the waves grow larger and larger in size.
• Freshwater Input (Rain) Adding to Storm Surge: Heavy rainfall ahead of a tropical system can cause water levels to rise well inland from the coast as well.        
• Total Water Level = Storm surge + Tides + Waves + Freshwater Input

Many factors that influence storm surge

There are many factors that contribute to the amount of surge a given storm produces at a given location.

• Central Pressure: Lower pressure will produce a higher storm surge.
• Storm Intensity: Stronger winds will produce a higher storm surge.
• Storm Forward Speed: On the open coast, a faster storm will produce a higher surge. Consequently, a higher surge is produced in bays, sounds and other enclosed bodies of water with a slower storm.
• Width and Slope of the Ocean Floor:  Higher storm surge occurs with wide, gently sloping continental shelves, while lower storm surge occurs with a narrow, steeply sloping shelf. For Southwest Florida, we are particularly vulnerable to surge because the ocean floor gradually deepens offshore.
• Size: A larger storm will produce higher storm surge.

If tropical weather approaches Florida, coastal residents will see new, separate warnings about storm surge in addition to those about winds.

Historically, storm surge is the primary killer in a hurricane. The largest loss of life in a hurricane comes from water and flooding, not from wind as much.

Storm surge can strike at different times and in different places than a storm’s winds.

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