New Orleans Mayor LaToya Cantrell on Thursday decided not to evacuate residents of her rain-swamped city unless Tropical Storm Barry, a cyclone threatening the Gulf Coast, strengthens to a Category 3 hurricane.
But in the eyes of some leading scientists and researchers, a storm's category doesn't tell you the whole story — and it shouldn't be the only metric used for making important public safety decisions during potentially life-threatening natural disasters like hurricanes.
"We focus so much on the one big number — Category 5, Category 1 — because it's easy and sexy to talk about," Michael Mann, a professor of atmospheric science at Penn State, said. "But it leaves out lots of information you need if you're going to accurately reflect the risk and threat of a storm."
That's because the categories are part of the Saffir-Simpson scale, a measure of a storm's maximum wind speed. The scale, most notably used by the National Hurricane Center, does not account for other potentially devastating criteria, such as storm surge and rain.
"Saffir-Simpson was always meant to be a measure of wind, so it works perfectly at what it was designed to do. But I always preach that we need to focus more on other impacts that can actually be far more deadly than winds," said Marshall Shepherd, the director of the atmospheric sciences program at the University of Georgia.
James Done, a project scientist at the National Center for Atmospheric Research, said the general public should be concerned about wind speeds only up to a point, and public agencies should highlight the major risks posed by water, such as flooding and drowning, that do not factor into a storm's category.
Done pointed to statistics showing that water-related incidents, not wind speeds, were responsible for roughly 90 percent of deaths during tropical cyclones between 1963 and 2012, most of them by drowning. (The statistics applied only to the 2,325 individuals for whom a cause of death has been identified.)
In particular, the inundations of rising water moving inland known as a storm surge were responsible for almost half — or 49 percent — of deaths during that period, according to research from Edward N. Rappaport of the National Hurricane Center.
Rappaport's research also found that the deadliest storms were not necessarily the strongest when they made landfall, with only three of the 10 deadliest ranking as "major hurricanes" — Category 3 or higher on the Saffir-Simpson scale — when they hit coastlines.
Hurricane Katrina, for example, one of the deadliest and costliest storms in modern history, made landfall over southeast Louisiana and Mississippi as a Category 3.
"If you look at some of the most devastating storms in history, you really could not have predicted their devastation based solely on wind speeds," Mann said.
The National Hurricane Center, for its part, said the Saffir-Simpson scale was always intended as a "measure of the damage potential from wind only," and that it should not be viewed as the be-all and end-all when the public assesses the severity of looming storms.
"We have worked hard to communicate other impacts, including the addition of the storm surge watch and warning, which highlights the risk of storm surge, historically the leading cause of fatalities in tropical systems," Hurricane Center spokesman Dennis Feltgen said.
In phone interviews, the weather experts did not directly criticize the Hurricane Center for continuing to use the Saffir-Simpson scale and its associated terminology. Mann and Done, however, said they have been hard at work tinkering with new storm assessment systems that might be more all-encompassing.
Mann was one of the co-authors of an academic article published last year that proposes an alternative called the K-Scale, a model that includes storm surge and other key factors.
Done said he had been collaborating with the private insurance industry on what he called the Cyclone Damage Potential Index, which weighs maximum wind speed against how fast a storm is moving ashore and the area of damaging winds. But even that scale, Done conceded, did not include storm surge and rain.
"There's no scale that's going to catch everything," Feltgen said. "That's why people should not ignore other impacts, especially water. There's no such thing as 'just' a Category 1 or 'just' a Category 3. Everyone needs to pay attention."