How is Florence different from other hurricanes — it’s more than just size

Hurricane Florence is the first storm in known Atlantic hurricane history to reach the U.S. from where it started in the North Atlantic — all others had recurved harmlessly out to sea from there.

The big picture: It’s also unusually large and slow-moving, as the map above shows, and it’s also approaching the coast at a nearly 90-degree angle. These 2 factors are big reasons why the storm is such a concern, and why the cable news buildup to this event is actually (somewhat) warranted.

Hurricane Florence is a Category 2 storm on the Saffir-Simpson Hurricane Scale as of Thursday afternoon.

• But that scale only measures the winds inside a storm.
• Florence will bring a storm surge consistent with a stronger hurricane because of its large wind field.
• Its slow movement also guarantees astonishing rainfall totals in inland areas.
• Wilmington, North Carolina, is forecast to break its 7-day rainfall record by more than a foot, and the state of North Carolina may break its all-time record for the wettest tropical cyclone, given the forecast for up to 40 inches of rain.

Between the lines: This storm illustrates the limits of the official rankings of hurricanes. Once the storm was “downgraded” from a Category 4 to a Category 2 on Wednesday night, Twitter lit up with concerned meteorologists worried that people in harm’s way wouldn’t take it seriously.

Even the National Hurricane Center got into the mix, tweeting: “Do not focus on the wind speed category of #Hurricane #Florence! Life-threatening storm surge flooding, catastrophic flash flooding and prolonged significant river flooding are still expected.”

By only benchmarking a storm’s winds, the Saffir-Simpson Scale ignores the deadliest aspect of a landfalling storm: water.

• Both storm surge flooding and inland freshwater floodingare a hurricane’s deadliest threats by far.
• So why give the wind the most attention?

Perhaps it’s because high winds used to kill more people in such storms, and the scale was developed by civil engineer Herbert Saffir and meteorologist Robert Simpson in the early 1970s.

• There have been attempts to move to other scales or tack on storm surge to the current scale. There was an effort to develop a promising metric called the Integrated Kinetic Energy, or IKE, which took into account a storm’s total power. However, that failed to catch fire in the hurricane community.
• Perhaps Florence will jump-start discussions about a new scale, because talk of a storm weakening, when the surge dangers and inland flooding threats remain the same, complicates meteorologists’ efforts to communicate deadly threats to the public.

The bottom line: Some of the country’s most expensive hurricanes have been Category 1 or 2 storms, including Ike ($7.3 billion) and Sandy ($70 billion).