Storm surge poses the biggest threat to people and property as Hurricane Dorian, a Category 1, charges up the coast.
Storm surge could reach 7 feet in North Carolina where Dorian made landfall on the Outer Banks Friday morning.
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.
But what is storm surge?
Here's how it works:
As pressure falls in the hurricane's center, water levels rise. The water accumulates while the storm is still over the open ocean.
When the hurricane closes in on land, its strong winds push that water toward the coast and up onto land, creating walls of water sometimes as high as 20 feet.
The danger to people inside houses on the coast is the deluge of water that can flood homes and overpower walls rapidly.
When Hurricane Sandy hit New York and New Jersey in 2012, many homes filled quickly with water that reached levels of 8 to 9 feet.
The risks can be even greater if storm surge combines with high tide, creating a devastating, rapid rise in water levels.