What is a storm surge? How high could a surge get from Hurricane Helene in Florida?

Strong winds can flatten buildings and fling people like twigs, but storm surges are a hurricane's deadliest threat.

Storm surges shove seawater onto land far above what's normally seen at high tide, inundating streets and buildings. Victims caught by the surge get battered by rough seas and debris, often drowning in the rising flood.

Research of hurricanes hitting the U.S. mainland over the last half-century showed storm surges are responsible for about half of 2,500 "direct" hurricane deaths, a 2016 study showed.

What is a storm surge?

A storm surge occurs when rotating winds of a hurricane push down on the ocean, driving seawater in various directions depending on the coastline and other factors. When this surge hits coastland, it can result in sea levels high enough to flood a home's second story. Extra-high winds can make the problem worse, or if the surge coincides with the normal high tide.

People who live on or near the coast are most vulnerable to the danger, though the water of storm surges sometimes travels far inland, flowing up canals or rivers. The Gulf Coast and East Coast are most vulnerable to storm surges as most hurricanes occur along those shorelines, according to the National Weather Service .

How damaging are storm surges?

The surge from Hurricane Sandy in 2012 came at the same time as high tide and caused much of the storm's estimated $19 billion in damage. The combined "storm tide" caused a 14-foot rise in the sea level, swamping streets, subway tunnels and water treatment plants.

A 2021 report from the analytic firm CoreLogic found that 8 million homes on the East and Gulf coasts valued at more than $1.9 trillion are at risk from storm surges.

What kind of storm surge is expected from Hurricane Helene?

Hurricane Helene strengthened Thursday and may hit Florida's Gulf Coast as a powerful Category 4 storm. That means a potential for 10 to 20 feet of storm surge above ground level, with "destructive wave action on top of that," according to the most recent video briefing on Thursday afternoon by Mike Brennan of the National Hurricane Center.

Brennan said some areas will see "unsurvivable conditions."

Live updates: 'Extremely dangerous' Hurricane Helene now a Cat. 4 on way to Florida

This article originally appeared on Arizona Republic: What is a storm surge? How high could a surge get from Hurricane Helene in Florida?