Days before Hurricane Ike reached U.S. coastlines, meteorologists and hurricane forecasters warned sea-side residents of possible widespread destruction and loss of life. The National Weather Service (NWS) initially predicted surge heights of 20-25 feet; but actual storm surge heights averaged closer to 13.5 feet. Still high enough, though, to cause the wide spread devastation that forecasters warned of.
Hurricane Storm Surges
Storm surges are caused by hurricanes when water is pushed towards the shore by the force of winds swirling around the storm. The height of a storm surge depends on a number of variables, most importantly, the intensity of the hurricane. According to the Saffir-Simpson Scale, a Category 2 hurricane, such as Ike, could produce winds up to 110 miles per hour and storm surges up to 8 feet above the normal tide-level. Hurricane Ike produced winds comparable to a Category 2 hurricane, but forecasters predicted a much higher storm surge than 8 feet.
Storm surges can be even more dangerous depending on the continental shelf – where the ocean meets land. A more gently sloping continental shelf will be more easily overcome by a surge and result in more flooding inland. However, a more steeply sloped shelf will cause the surge to slow down as it hits land and will result in less flooding inland. In the case of Hurricane Ike, the entire Gulf Coast is a gently sloping continental shelf and lies less than 10 feet above sea-level, so forecasters knew that a storm surge above 10 feet would be destructive.
Forecasting Storm Surge Heights
Storm surges are caused by many factors and current storm surge models use what is known about these factors to calculate the height of a storm surge. Currently, the NWS uses a model called the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model. This model takes into consideration the variables affecting the height of a storm surge such as:
- Atmospheric sea-level pressure
- Hurricane size
- Hurricane forward speed
- Hurricane track
- Winds
In order to make the model as accurate as possible, forecasters from the National Hurricane Center begin running surge height predictions from the SLOSH model 24 hours before it is expected to make landfall. Factors such as the hurricane’s forward speed and the hurricane’s track can cause uncertainties in the model. The NWS overestimation of Hurricane Ike’s storm surges were due in part to the slow speed of the hurricane, the large size of the hurricane and uncertainties in the hurricane’s projected path, according to National Hurricane Center Director Max Mayfield.
Even still, the storm surge heights forecasted for Texas coastlines were adequately used to warn residents and evacuate low-lying areas. According to Wilson Shaffer, chief of the NWS evaluation division, if lower estimates had been given, and the hurricane had changed - causing higher surges - the results would have been much more catastrophic.
Sources:
"Storm surges hard to predict", CNN
National Hurricane Center Storm Surge page
