Hurricane Matthew: What is storm surge?

As Hurricane Matthew heads toward a forecast landfall along the Florida coast Thursday, officials continue to warn residents there to evacuate in advance of the strengthening storm.

In its latest warning, the National Hurricane Center said Matthew will “produce devastating impacts ... along portions of the East Coast of Florida tonight.” The storm strengthened into a Category 4 hurricane Thursday morning.

The NHC warns of the dangers of storm surge, water pushed inland as a hurricane advances, because more people lose their lives because of flooding from the surge, then from the storm’s winds.

What is storm surge, how does it happen and why should you be wary of it? Here is a quick look at storm surge.

What is storm surge?

Storm surge happens when water is pushed onshore by a hurricane.

How does it form?

Imagine a bowl of water. Put your hand in the middle of the bowl and cup it. Now slowly push your hand toward the edge of the bowl. Those are the same dynamics as storm surge. The ocean water is pushed by winds and waves, and is also sucked into the air near the eye of the hurricane by low pressure.

Is it a “wall of water” that rushes in?

Rarely. It is usually a rise of water that can happen quickly, moving at the same rate as the forward speed of a hurricane.

How powerful is it?

Very powerful. Only 1 cubic yard of sea water weighs 1,728 pounds. A 6-inch surge can knock a person down.

How dangerous is it?

Storm surge kills more people in a hurricane than all other components of the storm. The overwhelming majority of deaths in the 10 deadliest U.S. landfalling hurricanes were the result of storm surge.

How can I stay safe?

Get away from it. A surge 1 foot deep can take a car off a road. Get out early, because the surge can begin up to 24 hours before landfall. During Hurricane Katrina, people stayed in their homes and died there when the surge filled their homes with water and they could not escape. Also, don’t leave pets at home. Many animals died when people left them in their homes during Hurricane Katrina.

What does a 9-foot storm surge mean?

It depends on the sea level at the point where the water begins to come ashore.

Weather Underground explains it this way:

“The storm surge is how high above current sea level the ocean water gets. The number we are most interested in regarding storm surge is how many feet above mean sea level (MSL) inundation will occur. This number is known as the storm tide ... The storm tide is the height of the storm surge above the MSL, corrected for the tide.

“For example, in a location where high tide is 2 feet higher than mean sea level, and low tide is 2 feet lower than mean sea level, a 15-foot storm surge would cause a 17-foot storm tide if the hurricane hit at high tide or a 13-foot storm tide at low tide."