Forecasters warn Hurricane Laura will bring 'unsurvivable' storm surge — a wall of water up to 20 feet high

• Hurricane Laura is forecast to make landfall on Thursday as a category 4 storm.
• The hurricane will bring "unsurvivable storm surge," the National Hurricane Center warned.
• Storm surge happens when a storm raises water levels above the normal tide line, which can be devastating to coastal communities.

Hurricane Laura, now a category 4 storm, is expected to make landfall near the Texas-Louisiana border on Thursday morning. Its wind speeds have already 140 mph.

On Wednesday, the National Hurricane Center issued a dire warning: "Unsurvivable storm surge with large and destructive waves will cause catastrophic damage from Sea Rim State Park, Texas, to Intracoastal City, Louisiana," the center said.

That life-threatening storm surge could penetrate up to 30 miles inland, according to the NHC. In some areas of Texas and Florida, the water could be as high as 20 feet.

A storm surge warning is in effect for the coastal area from Freeport, Texas, to the mouth of the Mississippi River.

A storm surge happens when a storm raises water levels above the normal tide line, and it can be highly destructive.

"Only a few hours remain to protect life and property and all actions should be rushed to completion," the NHC said.

Why storm surge is so dangerous

Storm surge is not factored into the Saffir-Simpson scale, which determines a hurricane's category number — that ranking is based on wind speed alone. But storm surge is highly dangerous and can be deadly.

"Imagine having a high tide that is 30 feet (9.1 m) higher than normal," scientists Orrin Pilkey and Rob Young wrote in their book, "The Rising Sea." "Now, imagine there are hurricane-driven waves on top of that extra-high tide. That is the impact of the storm surge."

Hurricane Katrina, which hit New Orleand and the Gulf Coast almost exactly 15 years ago, brought a storm surge of 30 feet in some areas.

The surge breached levees in New Orleans, causing 80% of the city to flood. That's what was responsible for most of Katrina's 1,800 deaths, according to a 2008 study.

Several factors increase the likelihood of a dangerous storm surge: Larger, faster-moving storms with strong winds tend to cause bigger surges, as do storms that move toward the coast at a perpendicular angle. Coastlines are curved inward, like much of the Gulf Coast, are also generally prone to more intense storm surges.

Hurricane Laura checks many of these boxes — it's large, has strong winds, and is moving towards land at a relatively perpendicular angle. Laura is also forecast to hit a curved part of the coastline.