Hurricane Florence: How meteorologists classify hurricanes

Hurricane Florence is seen from the International Space Station as it churns in the Atlantic Ocean towards the east coast of the United States, on Sept 10, 2018.
Hurricane Florence is seen from the International Space Station as it churns in the Atlantic Ocean towards the east coast of the United States, on Sept 10, 2018. PHOTO: REUTERS

NEW YORK (NYTIMES) - As Florence spins toward the East Coast, meteorologists say that the hurricane, now a Category 4 storm, could strengthen to become a Category 5.

There is no classification stronger than a Category 5 (or, more colloquially, Cat 5). That means storm winds could be fast enough to cause extreme, catastrophic damage.

If Florence does reach that level, it won't necessarily stay there. The storm's strength may fluctuate as it travels, and hurricanes ultimately tend to weaken as they pass over land, said Bob Henson, a meteorologist with Weather Underground.

Still, he added, that doesn't make this storm any less serious. Even if the Category 5 ranking is brief, it would have drastic implications for a storm surge.

"It is a strong hurricane, and it'll be pushing quite a bit of water toward the coast," Henson said.

Florence is expected to make landfall on Friday. Experts say the storm surges could be life-threatening, heavy rains could cause flooding inland, and some areas could lose power for days.

WHERE DID THE SCALE COME FROM?

The five hurricane categories are based on wind speeds and are largely meant to help predict structural damage.

"It's not just meteorological," said Joel Cline, a hurricane support meteorologist with the National Oceanic and Atmospheric Administration (NOAA). "It's more of an impact-based scale."

Herbert Saffir, a structural engineer, created the scale in 1969.

It was expanded with help from Robert Simpson, a former director of the National Hurricane Centre, and it became known as the Saffir-Simpson scale in the 1970s.

The ranking system once took several factors into consideration, including storm surges and atmospheric pressure.

But in 2010, a tighter focus on wind speeds went into effect.

Today, a Category 1 storm is one with sustained winds of 74 to 95mph (119kmh to 152kmh), while a Category 5 storm has sustained wind speeds of 157mph (252kmh) or more.

ON A SCALE OF 1 TO 5

In a Category 1 hurricane, "well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters" and "large branches of trees will snap and shallowly rooted trees may be toppled", according to the NOAA's National Hurricane Centre.

Hurricanes in Categories 3, 4 and 5 are considered "major".

Hurricane Maria was a Category 4 storm when it hit Puerto Rico last year (2017), ultimately killing thousands and causing months of power failures.

If a hurricane's winds reach Category 5 speeds, "a high percentage of framed homes will be destroyed", according to the National Hurricane Centre, and "power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months".

 
 

Only a few hurricanes have ever made landfall in the United States as a Category 5 storm. The last was Andrew, in 1992.

WHAT THE NUMBERS DON'T TELL US

Meteorologists caution that a hurricane can be very dangerous no matter what its category, especially since the Saffir-Simpson scale doesn't tell the whole story.

It does not, for example, account for rainfall levels or storm surges - not directly, anyway - and those factors have a lot to do with flood risk.

Take Hurricane Sandy. In 2012, it never made landfall as anything stronger than a Category 2 storm. But it led to flooding, mudslides, power failures and dozens of fatalities in the Caribbean and the United States.

Hurricane Harvey made landfall in Texas as a Category 4 storm last year and was soon downgraded to a Category 3. That rating did not account for the days of heavy rain that would follow, which led to fatal, catastrophic flooding.

Even tropical storms, with winds too weak to register on the Saffir-Simpson scale, can result in serious damage and loss of life.

No matter what the storm's category, Cline said, people in the path of a storm like Florence should do what they can to "avoid the water" and move away from any oceans, bays, rivers or inlets.

"Be in a safe structure for these category winds, because they are to be taken very seriously," he said.