The new maps predict likely flooding in advance of incoming storms, showing exactly where and how deep the waters will rise. They’re meant to take away some of the public confusion surrounding the storm surge phenomenon.
“Most people would never think that a storm surge can get 20 miles inland. In the past we could tell you that in a public advisory, but until people see it on a map they don’t necessarily realize it,” said NHC Storm Surge Specialist Brian Zachry. “Now we can show people clearly: This is the storm surge that you should plan for.”
The center spent two years developing the new maps, which could assist emergency managers in their efforts to enforce evacuation orders in times of crisis.
Real-Time Maps
Surge occurs when storm winds push the sea toward the shore, raising water levels above astronomical high tide, Zachry said. It is a life-and-death matter, responsible for roughly half of all hurricane fatalities.In their efforts to create an at-a-glance warning, planners devised a four-color map: Blue represents 1 to 3 feet of surge; yellow is 3 to 6 feet; orange is 6 to 9 feet and everything over 9 feet is in red.
Designers saw no point in making further delineations. “If you have 9 feet of storm surge, you might as well have 15 feet of storm surge,” said Zachry. “Either way, you need to get out of there at that point.
The hurricane center will generate the maps in real time when there is an active hurricane threatening the U.S. East and Gulf coasts. The maps will be available roughly 48 hours before the arrival of the storm surge hazard and will show up in the NHC website (hurricanes.gov) under the active storms for the “Atlantic - Caribbean Sea - Gulf of Mexico,” where a thumbnail or icon will take visitors to the interactive maps.
The NHC has been developing the new surge maps, using a supercomputer to churn through vast volumes of hurricane forecast data, including real-time information generated during two significant weather events. In July 2014, Hurricane Arthur made landfall in North Carolina, with a peak storm surge of 4.5 feet. In June 2015 Tropical Storm Bill produced widespread rainfall across east Texas, Oklahoma, the Midwest and the mid-Atlantic.
In each case, planners asked the computer to generate 500 to 1,000 alternative scenarios in the face of the approaching storm. Maps were then based on the reasonable worst-case scenario. For both Hurricane Arthur and Tropical Storm Bill, the mapped predictions “validated very well” against the actual impact of the storms, Zachry said.
‘Tools We Need’
Drew Pearson finds this encouraging news. As director of emergency management in Dare County, N.C., where Hurricane Arthur made landfall, he describes the new maps as a valuable tool for coastal emergency managers.“As an emergency manager, I know there might be 6 feet or 9 feet of water above the ground. But now our citizens will know that as well,” he said.
On North Carolina’s Outer Banks, most local residents already are aware of the dangers inherent in storm surge, but they may not have sufficient information to make sound decisions during an actual weather event. “If you talk to somebody who lives on Hatteras Island, they know what storm surge is; they have lived with it,” Pearson said. “What they don’t know is: When is it going to come? When is it going to get here or how deep is it going to be? That is what is really new with these products.”
While the new, detailed information may help individual citizens, it will be especially useful to emergency managers trying to motivate whole populations. “These are exactly the tools we need, something that enables me to go to a citizen or visitor and say: Here are the orders and this is why. This is exactly what is going to happen,” he said.
Even prior to the hurricane center’s latest push, various state emergency offices have mounted efforts to inform the public about the potential hazards of storm surge. Virginia offers storm surge maps and a tool for identifying surge risk, and Florida has detailed maps by county.
While these maps are based on historic surge data, the hurricane center’s product differs in that it’s generated in real time. During a storm event, maps will be refreshed within an hour of any new weather advisory, said Zachry.
In the absence of such clear guidance in the past, storm surge has led to disastrous outcomes, according to the NHC’s running tally. Over recent years, catastrophic storm surge events have included:
2011 — Tropical Storm Lee made landfall in early September along the coast of southern Louisiana, with tides reported at 4 to 6 feet along the coasts of Mississippi and southeast Louisiana. The storm surge at Lake Pontchartrain inundated more than 150 houses. The storm killed three people and may have caused $1 billion in damages, mostly due to storm surge or fresh water flooding.
2011 — Hurricane Irene pushed water over a large portion of the western Atlantic basin for several days, with storm surge estimated at 8 to 11 feet within portions of Pamlico Sound, N.C., and 4 to 6 feet along the coast from New Jersey northward. A storm surge of 3 to 6 feet caused hundreds of millions of dollars in property damage in New York City and Long Island.
2010 — Hurricane Earl didn’t make landfall in the U.S. but still drove storm surge over 4 feet in North Carolina, with surge of up to 3 feet along the U.S. coast from North Carolina to Maine.
2008 — Hurricane Ike caused storm surges of 15 to 20 feet above normal tide levels along the Bolivar Peninsula of Texas and in much of the Galveston Bay area, “with almost every structure on parts of the Bolivar Peninsula completely wiped from their foundations due to the surge and accompanying waves,” the NHC reported.
2005 — Hurricane Katrina’s surge measured in at 24 to 28 feet along the Mississippi coast across a swath about 20 miles wide. The surge drove 6 miles or more inland along much of coastal Mississippi and up to 12 miles inland along bays and rivers. As a result of the surge, levees and floodwalls overtopped or were breached, ultimately flooding about 80 percent of New Orleans with depths up to about 20 feet.
The NHC’s earliest noted surge event offers a telling example of the dangers of storm-driven flooding. In 1889 a hurricane off the mid-Atlantic coast was downgraded to a tropical storm, and even at this lower intensity it didn’t make landfall. Yet the impact of the surge was staggering. Stalled near Hatteras, N.C., the storm caused surge and flooding from New England to North Carolina. The meadows along the Atlantic City, N.J., waterfront were swamped. In Lewes, Del., the life-saving station, located 40 feet above the high water mark, was flooded. Some areas were completely submerged and hundreds of residents had to evacuate.
Faced with more than a century’s worth of evidence of the destructive power of storm surge, NHC planners say they wanted to ensure their early warning maps would be not only useful, but also legible to the average person. To make that happen, they brought in social scientists.
Easy to Read
A professor of sociology at Florida International University, Betty Morrow specializes in the effectiveness of warning messages in times of natural disaster. She collaborated with the NHC on the production of the new maps.“We would give the maps to ordinary citizens and ask them to try to interpret what they saw, to explain what they would do with the map,” she said. “The best forecast is only as good as how well it is communicated. Do people really understand their vulnerability? Do they understand the hazard itself?”
Getting the presentation right was especially important, considering the circumstances in which an individual might be likely to encounter these maps — that is, in a time of high anxiety. “When we are talking about leaving your home, this is not easy; it is a very onerous action,” said Morrow. “People don’t want to leave their homes unprotected; they may not know where to go. They don’t want to go unless everyone is going. They are thinking through all of that and weighing it against how afraid they are.”
To make the maps as accessible as possible, Morrow and the planners worked through a range of variables like type font, size and color. The biggest surprise: People couldn’t find the legend on the map, at least not when it was placed on the upper right. When designers dropped it to the lower left corner of the map, users knew just where to look. A seemingly small change, but substantial when trying to connect with a user under stress.
At the urging of a test group of emergency managers, planners made an additional design change. Rather than use blue to indicate zero-to-3 feet of water, the map defines that flood potential as beginning at 1 foot. Emergency managers found it distracting to be notified of a zero-foot storm surge.
Looking ahead, the NHC hopes to expand on the project in the near future by designating these maps as the foundation of a future surge-watch and surge-warning system like one that’s now used to indicate the approach of tropical storms and hurricanes. Such a system of watches and warnings could go live on the hurricane center’s website by 2017 in a collaborative effort with the National Weather Service and local weather forecasters.
Watches and warnings are “the most powerful thing we have to get that information out there,” Zachry said. “That’s what goes across those local media channels, and it’s where we ultimately would want to be with this.”
