In this NASA handout image taken by Astronaut Ricky Arnold, Hurricane Florence gains strength in the Atlantic Ocean as it moves west, seen from the International Space Station on September 10, 2018. Weather predictions say the storm will likely hit the U.S. East Coast as early as Thursday, September 13 bringing massive winds and rain.

Scientists are watching the Carolina coast for what could be a historic storm surge from Hurricane Florence that pushes water inland at heights of up to 20 feet.

The surge from Florence will be particularly dangerous due to the flat nature of the coastline off North Carolina and South Carolina, allowing the storm to pile a lot of water over a large area. It's exacerbated by two bays in the storm's path that collect water and increase the height of a surge.

"This has the potential to become the storm of record for the state of North Carolina," said Robert Young, a professor of coastal geology at Western Carolina University. "If its track holds, it could break storm surge records and do a significant amount of coastal damage."

The National Oceanic and Atmospheric Administration is calling Florence a major and deadly storm. It's expected to drop between 20 and 40 inches of rain in some parts that could produce "catastrophic flash flooding," according to NOAA. The governor of North Carolina ordered a mandatory evacuation for more than 1 million people along the coastline.

The storm's direction, moving straight into the coast at a perpendicular angle rather than along it, increases the severity of the storm surge, Young said. Florence is following a similar sort of path hurricanes as Hurricane Katrina in New Orleans in 2005 and Hurricane Hugo in the Caribbean and southeastern U.S. in 1989, he said.

Young estimates the storm surge to be between 15 and 20 feet.

More than 750,000 homes in North Carolina, South Carolina and Virginia are at potential risk of storm surge damage from Florence, data analytics firm CoreLogic said Monday. The firm estimates threats to real estate total about $170 billion.

"Yes, the winds will be bad, but really the water is what will be deadly," said Rob Galbraith, director of underwriting research for insurance firm USAA. "The type of inundation we are talking about here happens very quickly. Six inches of water can knock you off your feet, and two feet of water can actually lift your car off the ground."