As Hurricane Irma barrels down on Florida, making landfall Sunday morning as a Category 4 storm, forecasters and local officials have repeatedly warned of the often-overlooked danger posed by storm surge, along with dangerous wind and rainfall.

The National Hurricane Center has raised the alarm for days about possible "life-threatening" storm surge "from rising water moving inland from the coastline."

Florida Gov. Rick Scott echoed that concern Sunday, adding that Irma is potentially even more dangerous than Hurricane Andrew, a Category 5 storm which made landfall and wrecked Florida in 1992, because "we didn't have this storm surge." Earlier in the weekend, he said that storm surge "will cover your house."

The threat of storm surge spans all across the southern Florida coastline, on the east and west sides. Up to 5 to 10 feet of storm surge was forecast in the low-lying Florida Keys, and 10-15 feet in other parts of southeastern Florida, according to the NHC.

Videos of storm surge hitting parts of Florida from Miami to the Keys, where Irma made landfall earlier, have been shared on Twitter, showing water rushing ashore.

But just what is storm surge?

According to the National Ocean Service, a branch of the National Oceanic and Atmospheric Administration, storm surge is "the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide."

Forecasters have also warned that the height and threat of storm surge can be amplified when combined with waves.

It is caused "primarily by a storm's winds pushing water onshore," the NOA adds. "The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the local bathymetry." "Bathymetry" is the depth of the ocean.

"Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide," the NOS explains. "Astronomical tides are caused by the gravitational pull of the sun and the moon and have their greatest effects on seawater level during new and full moons — when the sun, the moon, and the Earth are in alignment. As a result, the highest storm tides are often observed during storms that coincide with a new or full moon."

See NOA's diagram of storm surge here. A further description of the dangers of storm surge from the NHC can be found here.