Hurricane Idalia could push a 15-foot wall of water into Florida. Here's what storm surge is and why it's so dangerous.

• Hurricane Idalia is forecast to push up to 15 feet of ocean water into Florida's Big Bend region.
• Storm surge happens when hurricanes reach shallow coastal waters, and it's extremely dangerous.

Hurricane Idalia is churning itself into a major hurricane as it nears Florida.

The cyclone is forecast to make landfall in the Big Bend region of Florida's Gulf Coast as a Category 3 storm on Wednesday. The wind speeds will be destructive and life-threatening, but forecasters are desperately warning of another threat that could prove more deadly: storm surge.

Before the cyclone even makes landfall, it will start pushing a dangerous flood of ocean water inland. That's called storm surge. The water can rush in quickly, carrying away people and property, washing out streets and other infrastructure, and flooding buildings. According to the NHC, just 6 inches of fast-moving water can knock over an adult, and it only takes 2 feet to carry away an SUV.

If the surge coincides with high tide, Hurricane Idalia could raise waters 15 feet above ground from the Aucilla River to Yankeetown, Florida.

That level of storm surge washed away homes during Hurricane Ian last year.

"Residents should urgently follow evacuation orders from local officials," the National Hurricane Center said Tuesday evening.

Lower, but still life-threatening, levels of surge are forecast across Florida.

The NHC has issued a storm surge warning — where forecasters anticipate a life-threatening surge of water — for a vast swath of Florida's coast, from Englewood to Indian Pass.

"This is a life-threatening situation," the NHC wrote in an update Tuesday evening. "Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials."

Here's how a storm surge happens and why it's so dangerous.

Hurricanes stir up ocean water, then push it onto land

Hurricanes are large low-pressure systems that create a cyclonic wind effect. Those winds force ocean water to spin down into the water column, creating vertical circulation in the ocean below. Once the hurricane reaches shallow coastal waters, the ocean bottom interrupts this cycling. Unable to go down, the water pushes out onto land.

These water levels can rise rapidly — several feet in just a few minutes, according to the Coastal Emergency Risks Assessment.

This generally happens where the hurricane's winds blow toward the shore, pushing the surge of water in that direction. The highest storm surge tends to occur with the hurricane's strongest winds.

A storm surge can arrive before a hurricane's winds do, closing off roads and cutting evacuations short.

Storm surge can be deadlier than hurricane winds

While hurricane-force winds can rip roofs off of homes and take down trees and power lines, the surge of ocean water rushing inland often causes more damage.

Hurricane Katrina's 20-foot storm surge in New Orleans breached the city's levees 18 years ago. Elsewhere, Katrina's storm surge reached 30 feet. More than 1,800 people died in that storm and the ensuing floods, which also caused $108 billion in damage.

Storm surge often arrives before a hurricane's winds, closing off roads, cutting evacuation routes, and leaving people stranded in flood zones. That's part of what makes the phenomenon so dangerous. Surges can also continue after the storm's center passes, preventing emergency responders from reaching flooded areas.

The Saffir-Simpson scale, which ranks hurricanes in categories 1 through 5 based on wind speeds, does not account for storm surge, so even cyclones weaker than Katrina or Laura can produce huge walls of ocean water.

If a surge coincides with high tides, it gets an extra boost. That's what happened when Hurricane Sandy flooded New York City in 2012.

Heavy rainfall can also compound the effects of a storm surge, dumping more water on top of the ocean flood. That was especially true during Hurricane Florence, which drenched the Carolinas in 2018, and Hurricane Harvey, which dumped more than 60 inches of rain on parts of Texas in 2017 and killed at least 68 people.

Forecasters don't think Hurricane Idalia's rainfall will be as heavy as that of either of those cyclones — the NHC expects 12 inches of rain in the hardest-hit parts of Florida.

Warming, rising oceans can make storm surges more devastating

Larger, stronger, faster cyclones generally produce higher storm surge. And as Earth's oceans and air get warmer, tropical storms overall are getting stronger, wetter, and slower.

Rising sea levels can make storm surges even more devastating by giving them a higher starting point and allowing them to reach further inland.

"Our confidence continues to grow that storms have become stronger, and it is linked to climate change, and they will continue to get stronger as the world continues to warm," James Kossin, an atmospheric scientist who studies how climate change affects tropical cyclones, told the Washington Post in 2020.

This story has been updated. It was originally published on August 26, 2020.