What you need to know about hurricane storm surges
WATCH: Ginger Zee explains the science behind storm surges during hurricanes.

Storm surge poses the biggest threat to the southeast as Hurricane Florence barrels toward the coast.

It has the power to rip apart roads and buildings and is often the greatest threat to life and property along the coastline during a hurricane, according to the National Hurricane Center.

"People do not live and survive to tell the tale about what their experience is like with storm surge," Brock Long, administrator of the Federal Emergency Management Agency, told "Good Morning America" on Wednesday. "It's the most deadly part of the hurricane."

In this file photo, storm surge hits a small tree as winds from Hurricane Sandy reach Seaside Park in Bridgeport, Conn., Oct. 29, 2012.

During Hurricane Katrina in 2005, at least 1,500 people died "directly, or indirectly, as a result of storm surge," the National Hurricane Center said.

When Hurricane Florence hits the Southeast this week, storm surge is forecast to reach up to 13 feet in parts of North Carolina and up to 9 feet in parts of South Carolina.

But what is storm surge?

Storm surge could be as much as 13 feet in parts of the North Carolina coast.

Here's how it works:

As pressure falls in the hurricane's center, water levels rise. The water then piles up while the storm is still over the open ocean.

When the hurricane closes in on land, the strong winds push that water toward the coast and it has nowhere left to go but up on to land -- sometimes as high as 20 feet.

The sun rises over the ocean ahead of the arrival of Hurricane Florence, Sept.r 12, 2018, in Nags Head, N.C. Hurricane Florence is expected on Friday possibly as a category 4 storm along the Virginia, N.C. and S.C. coastline.

If you're inside a house on the coast, water can approach quickly and viciously, entering the home and climbing up the walls.

During Hurricane Sandy -- which hit New York and New Jersey in 2012 -- homes filled with water quickly, reaching 8 to 9 feet.

And when storm surge combines with high tide, the rapidly rising water can be devastating.

In this file photo, a 3-story condominium has collapsed after Hurricane Irma send a storm surge and eroded the building foundations, in Islamorada, Florida Keys, Sept. 12, 2017.
In this file photo, a car sits abandoned in storm surge along North Fort Lauderdale Beach Boulevard as Hurricane Irma hits the southern part of the state, Sept. 10, 2017, in Fort Lauderdale, Fla.