henri wind forecast sunday morning
Henri nearly a hurricane, warnings posted for New England.
03:31 - Source: CNN
CNN  — 

Much of the threat is due to storm surge, which CNN meteorologists have estimated could force 4 feet of water into coastal neighborhoods, damaging homes and flooding streets with frigid ocean water.

While many people focus on the wind speed of storms, the danger often comes from the water flowing in from the ocean.

Privately, you may be wondering (and you wouldn’t be alone): “What is a storm surge?”

“A storm surge is a rise in water level caused by a strong storm’s wind pushing water on-shore,” said CNN meteorologist Brandon Miller. “The wind literally piles up the ocean water and pushes it on the land.”

All that water has nowhere to go

That water doesn’t just leave. Depending on how much water was pushed ashore and the area’s watershed, it may hang around, causing further damage to communities.

Due to climate change, storm surge has become an even greater threat in recent years.

“Sea levels have risen in most places by about 1 foot over the past century. The higher baseline ocean level allows storm surges to reach even higher, increasing their destructive capabilities,” Miller said.

The National Weather Service in a 2014 report said that most surge deaths occurred in Hurricane Katrina and several other big, powerful storms. In a majority of storms, excessive rainfall that leads to drownings is the leading cause of death. Florence is expected to also be a rainmaker, with totals of 35 inches possible in some areas.

‘Reverse’ storm surge

One interesting phenomenon that can take place during strong storms is known as “reverse storm surge.” This is where the water actually recedes away from the shoreline back into the ocean as winds push water out of shallow areas.

Just before landfall, the water recedes along the coast, exposing land that was usually under water.

Ultimately that water comes back onshore with much greater force and inundating coastal communities.

This can be dangerous because people will often go out to the bare or uncovered land to take photos, grab seashells, etc. But the water can come back in just as quickly as it went out and people are often caught off guard.

CNN’s Brandon Miller, Gene Norman, Judson Jones and Emily Smith contributed to this report.