People usually think of a hurricane's strength in terms of wind speed. But in any region where tropical cyclones can strike, and especially in a low-lying and confined area like the Florida Keys, storm surge is often the biggest threat.
Still, until now the National Hurricane Center has focused its forecast mapping exclusively on wind strength.
However, that will change this hurricane season, when the NHC rolls out storm surge prediction maps along with the advisories it issues every six hours while storms are a threat. Hurricane season began on June 1 and will last until Nov. 30.
"It's a game changer," said Jamie Rhome, Storm Surge Team Leader for the NHC.
The surge maps, the technology for which the hurricane center has developed over the past several years, will be color coded to show which areas are at risk. People in areas coded red should prepare for a surge of 9 feet or more. Orange, yellow and blue are used to show areas at risk of surges of up to 9 feet, 6 feet and 3 feet, respectively.
For now, the NHC is calling the maps experimental. Over the next two hurricane seasons modelers will be soliciting suggestions from the public on ways to improve the map interface. After that, the format will be set for long-term use.
The hurricane center also plans to augment the maps next year by implementing explicit storm surge warnings that are separate from the hurricane and tropical storm warnings it has long issued.
Rhome hopes the storm surge maps will provide people with a clearer understanding of the risks of a particular storm.
The Saffir-Simpson scale, which uses wind speed to label a hurricane's strength from categories 1 through 5, doesn't account for many of the factors that drive storm surges.
"Storm surges and wind don't necessarily go hand-in-hand," Rhome said.
Along with wind strength, storm surge is also impacted by the physical size of a storm, the pace of its forward momentum, tides, physical features of the coast line and even the angle with which a storm is moving.
Hurricane Wilma in 2005, for example, only brought sustained winds of tropical storm force to Key West and the rest of the Keys. But its 6 feet of storm surge inundated more than half of Key West. The surge flooded homes up and down the Keys and destroyed an estimated 35 percent of vehicles in Key West, according to the National Weather Service.
Rising waters were the cause of the vast majority of the millions of dollars in damage wrought upon the Keys by Wilma. In fact, storm surges, and not wind, are the leading cause of hurricane-related deaths in the United States, said Rhome. Storm surge accounted for most of the 1,200 people who died from Hurricane Katrina in 2005.
The Keys, where no point is higher than 17 feet, are especially vulnerable to the ocean's whimsy.
"There's no topography. There's no relief in the Keys," Rhome said. "That means if the storm surge happens the vast majority of it is underwater, so you've got to run."