Storm surge explained
The storm surge from Hurricane Dorian is expected to cause problems all along North Carolina’s coast, but a predictive model developed by the University of North Carolina’s Institute of Marine Sciences offers a much more detailed forecast. It calls for an especially strong surge round Wrightsville Beach, parts of the Bogue Sound near Swansboro and Emerald Isle, the southern parts of the Pamlico Sound and the backsides of the Outer Banks.
Those specific areas are vulnerable because of the timing of low and high tides along the coast as well as the specific direction Dorian is traveling, said Richard Luettich, director of the Institute.
UNC’s model was accurate within a foot in most cases during Hurricane Florence, Luettich said. The Neuse River, as it did with Hurricane Florence last year, is also expected to bear the brunt of some of the worst storm surge.
“The sequencing between the surge and the high tide along the shore face is significant,” Luettich said because a high tide will add to the force of surge.
Most of the southeastern coastline of North Carolina will be in a low tide as Dorian approaches, he said, but it should be high tide when it nears Wrightsville Beach, causing a storm surge between Wrightsville and Topsail Island to reach between nine and nine and a half feet.
The Outer Banks should be at low tide, however, he added.
But for the lower Pamlico Sound — the shallow body of water between the Outer Banks and Pamlico and Cartaret counties — the wind direction will be more of a concern for flooding. On Thursday, Dorian’s winds were 110 mph around noon, making it a Category 2 storm, and it was moving north at 8 mph so close to the coast that forecasters say landfall could happen at any time.
“The current track of the wind is the worst case scenario” for the lower Pamlico, which is vulnerable to wind movements because it is so shallow, Luettich said. “Because the counter-clockwise wind is blowing up against the water toward eastern Cartaret County. We expect to see in the lower Neuse River a 10-foot flood surge.”
The model suggests that areas around Hobucken, Bayboro and the lower Pamlico River will see worse storm surge than it saw during Hurricane Florence last year, getting eight to 10 feet of storm surge consistently.
That surge will also cause the Neuse River to back up toward New Bern, Luettich said. The lower Neuse River, especially near Cherry Point, is expected to see 10 feet or more of storm surge, which is the worst the model is predicting in North Carolina.
New Bern saw some of the worst initial flooding when Hurricane Florence made landfall last year. The model suggests the parts of New Bern around the river could see anywhere from eight to 10 feet of storm surge.
As the storm leaves North Carolina, the backside of the Outer Banks could also face flooding. That is because the storm will initially push the Pamlico Sound away from the Outer Banks, and then bring it back towards the Atlantic Ocean on its way out.
“The storm will make areas (on the Outer Banks) around the sound look dry as the wind blows from northeast to southwest,” Luettich said. “But then the storm will bring that water back as it passes, so we are also predicting that there will be flooding on the backside of Portsmouth and Hatteras islands as the storm pulls away.”
Most of Portsmouth Island and the Buxton part of Hatteras Island will see the worst of it with around a nine-foot storm surge expected there.
There are some key differences between Dorian and the recent hurricanes that have devastated Eastern North Carolina, including Florence and Matthew.
While Dorian is stronger than Florence, it is expected to move along the coast of North Carolina quickly, making the inland flooding from rain that was seen during Florence less severe. Inland flooding is still a possibility, however, as nearly all coastal counties expect to get 10 to 15 inches of rain from Dorian, according to the National Hurricane Center.
Another difference is the track of Dorian is expected to go right over the Outer Banks, which dodged most of Hurricane Matthew in 2016, as that storm veered toward the Atlantic Ocean before hitting the banks. Dorian should send more powerful wind and storm surge to the Outer Banks this time.
Luettich has been fine tuning his storm surge model since the late 1980s. The data from it is live streamed on the government’s Coastal Emergency Risk Assessment website, and is often used by government agencies like the Federal Emergency Management Agency and the Coast Guard.
Historically, UNC’s flood modeling technology has been used as a forensic tool by the government — often to determine exactly how severe flooding was during a storm and the differences between wind and water damage. It was used in the wake of Hurricane Katrina to pinpoint exactly what went wrong with the levees that failed in that city and to determine how high to rebuild.
Since 2008, the team at the UNC Institute of Marine Sciences has been working to make it a more accurate forecasting tool as well. The model uses data points from the National Hurricance Center (updated every six hours), ocean depths and an enormous database of the topographical details of coastlines across the United States.
Compared to 2008, the system is much stronger and accurate these days. It is even backed up by super computers in Chapel Hill, Texas and Louisiana so that it can still crunch numbers if the Morehead City office loses power during a storm — which it is prone to do.
“We are probably producing 10 to 12 runs (of analysis) every six-hour cycle,” Luettich said. “That is 50 runs a day, and over a 10-day event, we are using 500 runs. Back in 2008, when we were first doing this, it was maybe a dozen runs.”
The forecast has also gained in popularity over that time.
In just the past week, he said, emergency response centers from Florida, Georgia and the Carolinas have all created accounts to view the storm surge models.
Even the White House has requested a registration since Hurricane Dorian began threatening the southeastern coast, Luettich added.
“We feel that we are pretty accurate given all the uncertainty of the event itself,” Luettich said.
This story was produced with financial support from a coalition of partners led by Innovate Raleigh as part of an independent journalism fellowship program. The N&O maintains full editorial control of the work. Learn more; go to bit.ly/newsinnovate