Hurricane Delta, forecast to make landfall in Louisiana on Friday, is expected to bring a dangerous storm surge of up to 11 feet to the Gulf Coast.
A storm surge warning is in effect for Louisiana and Mississippi.
Here's how storm surge works:
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.
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 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.