Storm surges are the deadliest threat when a hurricane makes landfall. Flooding from the tidal-wave-like onslaught of ocean water pulled on shore by a hurricane's low-pressure center has killed more than a thousand people in several catastrophic hurricanes in U.S. history, and two years ago, surge flooding drowned dozens in New York City alone when the wave from Superstorm Sandy rolled onto the shore.
Storm surges also destroy property, of course. Water is much more massive than air, and it's nearly impossible to design and build a home to withstand the impact of battering waves riding atop a hurricane's surge. Elevating the house above the flood level on pilings is the only practical measure for making the building surge-proof.
That being the case, it's important for builders and homeowners to understand the risk of a storm surge at their location. Understanding can mean the difference between life and death: In Hurricane Sandy, as well as in previous storms, the National Hurricane Center has noticed that coastal residents tend to pay more attention to wind advisories than to flood warnings (that's one of the reasons so many people died when Hurricane Ike, bearing only a Category 2 wind rating, crashed on shore near Galveston, Texas, bringing with it an outsize and early storm surge that stranded hundreds of people on barrier islands with no hope of rescue).
So the NHC has been working on its methods for warning the public about surge risks. The agency's latest effort is a color-coded interactive online "inundation map" (see: "Storm Surge Inundation - SLOSH Maximum of Maximums") that lets the public view any location on the Atlantic and Gulf coast and see how deep the flood waters might get if a storm of a given strength were to make a direct hit at that location. (For examples of various map views, see Slideshow above)
The new maps are getting press attention in coastal areas. Jacksonville, Florida TV station News4JAX covers the story here (see: "New Surge Maps Show Possible Inundation During Hurricane," by Blake Mathews), and the Baton Rouge Advocate has a more detailed story here (see: "New storm surge map predicts worst-case scenarios for south La.," by Amy Wold).
Taken at face value, the map's aggregated "worst case" graphics exaggerate the risk. The bright red area indicating maximum storm risk extends along the entire U.S. coastline, and when a single city is viewed, the flood area seems to cover whole towns and suburbs. But that's a composite view of multiple possible storm tracks, compiled into a single map by a computer algorithm. In reality, only a small portion of the indicated flood zone would really be flooded during any given storm.
"Although alarming at first glance, the map doesn't reflect the effects of a single storm across the coast of Louisiana," the Advocate explains. "Instead, it shows the worst-case scenario for any particular area along the coast, said Barry Keim, state climatologist. 'That map can never happen (in total),' agreed Ken Graham, meteorologist in charge with the National Weather Service in Slidell."
Still, if your street is covered in red, it means that a storm could possibly flood your house — useful information if you're buying or building, or if an actual storm is headed your way. Unfortunately, the NHC map isn't precise enough to evaluate a particular house or building site: the storm surge overlay disappears when the user zooms down to street level. (Parcel-by-parcel flood modeling information is available — for a price — from real estate analytics firm CoreLogic: see "Millions of Homes at Risk from Storm Surge," Coastal Connection 7/22/14.)
"The NHC map is intended as an education and outreach tool to raise awareness on the storm surge hazard for the U.S. Gulf and East coasts," explained NOAA spokesman Dennis Feltgen in an email. "The tool is meant more for a community level view of the hazard, rather than a parcel level view zooming into a specific property. At this time, there are no plans to set the zoom level to go into a closer view. It is very important that the user seeks out evacuation zones or other protective action information from local sources, such as the county emergency management office."
Indeed, storm surge flooding, like the Wu Tang Clan, is nothing to fool around with. Hurricane expert Jeff Masters provides this sobering overview at the Weather Underground website (see: "Storm Surge Basics," by Jeff Masters): "The storm surge moves with the forward speed of the hurricane — typically 10 - 15 mph. This wind-driven water moving at 10 - 15 mph has tremendous power. A cubic yard of sea water weighs 1,728 pounds — almost a ton. A one-foot deep storm surge can sweep your car off the road, and it is difficult to stand in a six-inch surge. Compounding the destructive power of the rushing water is the large amount of floating debris that typically accompanies the surge. Trees, pieces of buildings, and other debris float on top of the storm surge and act as battering rams that can cave in any buildings unfortunate enough to stand in the way. If you receive an evacuation order for a hurricane storm surge, it is a very good idea to get out sooner rather than later. The storm surge can begin to rise a day before the storm hits, cutting off escape routes when low-lying highways are flooded. This is particularly true along the Gulf of Mexico shore."
The Hurricane Center's interactive map lets the viewer toggle between five intensities of hurricane, from Category 1 up to Category 5. At the Category 5 level, there's a confusing artifact when you look at the border between North Carolina and Virginia — the red flood zone ends abruptly at the state line. That's because Category 5 storms are thought to be unable to survive in the colder waters of the northern Atlantic. But the Category 4 layer makes it clear that Virginia faces a similar risk to the Carolinas, if perhaps not quite as severe a hazard. The potential for surge flooding from Category 1 through Category 4 storms continues all the way up into Maine.