The “category” system—officially known as the Saffir-Simpson Hurricane Wind Scale—is a very simple scheme: The faster a hurricane’s maximum sustained winds, the higher its category. If scientists record winds anywhere in the hurricane between 111 and 129 miles an hour, the storm is Category 3. As soon as they detect 130-mile-an-hour winds, it becomes Category 4.
The scale’s inventors—Herbert Saffir, an engineer, and Robert Simpson, a meteorologist—hoped their scale would measure a storm’s total destruction potential when they developed it in 1971. But their system omits a tremendous amount of information about a storm’s ferocity.
“They took only into account the maximum sustained wind speed,” which is concentrated in a small part of the storm, says Rosimar Ríos-Berríos, a research meteorologist at the National Center for Atmospheric Research. “This doesn’t tell us anything about the storm’s size, or how far from the center the strong winds are located. And it doesn’t tell us anything about rainfall and other hazards.”
She continued: “The problem with [the Saffir-Simpson scale] is that not all hurricanes are the same. We can have two hurricanes with the same category, but one may be smaller and one may be large. Even though they are the same intensity, the impacts may be greater for a large system because the strong winds extend over a large area.”
Hurricane Florence could be the worst storm to ever hit north of Florida.
The category also only loosely describes the most severe hazard of a hurricane: storm surge, which is responsible for half of all hurricane-related deaths in the United States. “The leading cause of fatalities due to tropical cyclones is not the wind, but the water,” Ríos-Berríos said.
Nevertheless, a hurricane’s sustained winds inform the entire way that meteorologists talk about a storm. A hurricane whose peak winds blow a little faster has “strengthened.” If winds lag a bit, then the hurricane has “weakened” or “been downgraded.” This is in fact exactly what happened to Hurricane Florence on Wednesday afternoon. The storm weakened slightly through the day: Its most ferocious winds blew at only 120 miles an hour, not the 130 miles an hour they had achieved earlier this week. This 10-mile-an-hour change was enough to downgrade it from Category 4 to Category 3.
But the size of the storm had actually expanded, wrote Stacy Stewart, a senior hurricane specialist at the National Hurricane Center:
While the hurricane hasn’t strengthened in terms of peak winds, the inner-core and outer wind fields have continued to expand, resulting in an increase the cyclone’s total energy, which will create a significant storm surge event.
In other words, the hurricane looks like it will have worse storm surge now, even though it is officially “weaker” than it was on Tuesday. Hurricane Florence was also predicted to take a meandering, dangerous track that will strafe against the coast of North and South Carolina on Wednesday—another new and scary aspect of its forecast.