Florence Will Drop an Inconceivable Amount of Water

In the next few days, the hurricane will likely dump 18 trillion gallons of rain, as much water as fills the Chesapeake Bay.

A color-amplified satellite image of Hurricane Florence, captured by NOAA satellites on September 14
A color-amplified satellite image of Hurricane Florence, captured by NOAA satellites on September 14 (NOAA)

On Friday, Hurricane Florence made landfall in North Carolina. It was also downgraded to a Category 1 storm: Its high winds, while still “extremely dangerous,” are no longer the storm’s scariest trait.

But then again, they never were.

Florence’s main threat has always been the water. In the coming days, Florence will besiege the Carolinas through two different mechanisms: First, it will inundate the coastline with powerful and deadly storm surge. Second, it will drop trillions of gallons of water across the region, from the barrier islands to the Appalachians, overwhelming dozens of rivers and creeks and initiating 1,000-year floods.

Florence, in a way, is a double threat, combining hurricane hazards new and old. Storm surge has long been dangerous: Historically, it has caused about half of all hurricane-related deaths in the United States. But recently, immense and slow-moving storms—like Hurricane Harvey—have dumped so much rain that they trigger devastating floods far inland.

Both of these dangers are likely made worse in Florence by the unique geography of the Carolinas and by the effects of human-caused climate change. The warming planet seems to amplify hurricanes’ water-related dangers more directly than it intensifies their high winds.

Take storm surge, for instance.

Storm surge is always caused by a hurricane’s huge area of high winds, which pick up ocean water and push it in the direction of the storm. As the hurricane approaches the coast, this excess ocean travels with it, literally displacing the sea onto the land. Even a few feet of storm surge can make for an unsurvivable situation, as this remarkable visualization from the Weather Channel makes clear:

A few local geographical traits in the Carolinas make storm surge particularly dangerous. Hilary Stockdon knows them well: She wrote her doctoral dissertation on the dangers of storm surge in the exact section of the North Carolina coast where Florence made landfall. Now Stockdon is a research oceanographer at the U.S. Geological Survey, where she leads national research into the hazards of the changing coastline.

“The shape of the shoreline really matters,” she told me earlier this week. “The Carolinas have those large embayments between the capes. It’s those capes and embayments that really start to contain and hold the water.”

In particular, she fretted about a large bay “between Cape Fear and Cape Lookout,” the two beachheads that essentially encircle Wilmington, North Carolina. Home to more than 119,000 people, Wilmington is the state’s eighth-largest city.

The storm ultimately made landfall just miles from Wilmington. The National Hurricane Center forecasts that storm surge captured by these two capes will rise to somewhere between seven and 11 feet. Some local values could be even higher, it warns.

This is not the only geographical element working against this part of the Carolinas. The height of storm surge depends partly on the width and depth of the continental shelf. A wider shelf gives a storm more space to start piling up water, while a shallow shelf directs more of that water toward the land. The continental shelf off southern North Carolina is both shallow and wide, Stockdon said. And if that wasn’t enough, the beaches and land on the Carolina coast are also low-lying, meaning that storm surge can travel even farther inland than it would elsewhere on the Atlantic coast.

And then there are the climatic effects. Dragging along at just 5 miles an hour, Florence is an exceptionally slow-moving storm. Since it slowed down about a day before it made landfall, it had time to capture more water and build up a larger storm surge. As I wrote earlier this week, hurricanes also seem to be slowing down worldwide, which some scientists hypothesize may be a result of climate change.

More directly, sea levels have also risen in this part of the Eastern Seaboard. Thanks to climate change, Florence’s storm surge is more than half a foot higher today than it would have been in 1900. This boost to storm surge is sometimes enough to break records: As the meteorologist Bob Henson noted, the city of Beaufort, North Carolina, broke its water-level record today only because of sea-level rise.

Seas will only continue to devour the land in the decades to come.“If you compare storm surge 75 years from now with [what happens] today, you’ll see its impacts more inland,” Stockdon said. “The whole area, being so low-lying, is very vulnerable to increases in sea level and the combined impacts of storms.”

North Carolina has not necessarily planned for this effect. In 2012, Republican lawmakers in the state banned the government from consulting dire predictions of sea-level rise when managing the state’s coastal development.

All this talk of storm surge may sound ominous—but it’s only half of what makes Florence so ominous. No matter how much storm surge inundates the coast, it will likely be followed by record flooding. Even after Florence’s hurricane-force winds slacken, it will remain a large storm, pouring out a tropics’ worth of rain, oozing over the land at a snail’s pace.

All that moisture, dropped at such a low speed, will lead to biblical amounts of rain falling on relatively small areas. The National Weather Service predicts that multiple counties in North Carolina will receive more than 20 inches of rain. In the next few days, the city of Wilmington will confront somewhere between 30 and 50 inches of rain.

Radar-estimated rainfall totals suggest that some parts of the state have already seen 20 inches of rain thanks to Florence, even though the hurricane could linger over the Carolinas for at least 36 more hours.

Ryan Maue, a private meteorologist based in Atlanta, has calculated that Florence will dump about 18 trillion gallons of water in the next several days. That’s as much water as sits in the Chesapeake Bay, or enough to “cover the entire state of Texas with nearly 4 inches” of water, reports the Associated Press.

National Weather Service

This level of rainfall is almost without precedent. Only last year—when Hurricane Harvey struck Houston—did the National Weather Service even make it possible for the color scheme of its maps to display 20 inches of rain. Now it’s had to pull out those deep purples and whites again.

This water won’t just disappear. It will trickle to rivers and streams, sometimes intermingling with storm surge to form a unified coastal flood. As such, large swaths of the Carolinas—including 13 North Carolina counties and Charlotte, the state’s largest city—are now forecast to endure a 1,000-year flood.

In other words, Florence seems strangely reminiscent of Hurricane Harvey: an exceptionally powerful hurricane in its own right that nonetheless wrought most of its damage through storm surge and endless, flooding rains. Some of the best research into this type of storm has been conducted by Jim Kossin, an atmospheric scientist at the National Oceanic and Atmospheric Administration. Lately, I’ve been remembering a conversation we had earlier this year.

“We’re seeing a significant change in patterns of exposure and mortality risk,” he told me at the time. Storms are moving slower, he said, and they’re appearing “more poleward,” meaning they survive into more northern latitudes.

“We get people out of harm’s way on the coast pretty well because historically, most mortality was associated with storm surge. But we don’t typically evacuate people who are inland,” he said. “This slowing [trend in hurricanes] is going to impact inland flooding.”

Eventually, he said, inland flooding could become the No. 1 mortality risk. Florence could help make that unfortunate vision a reality.