COMMENT: Ready for a Category 6 hurricane?

COMMENT: Ready for a Category 6 hurricane?

That’s just the sort of headline you want to read as the renewal season gets underway. But a fascinating article in Scientific American says that there is growing consensus that the five point Saffir-Simpson scale used to classify windstorms may no longer be appropriate.

It says there is actually talk of adding a sixth level to the current scale, on which category 5 intensity means sustained winds higher than 155 miles per hour (250 kilometers per hour) for at least one minute, with no speed cap.

Some scientists say the lack of an upper limit on the scale results in all of the most intense tropical cyclones getting lumped together, despite their wide range of power. “Category 5 becomes less descriptive when it includes 2005's Emily, which reached peak wind speeds of 257.5 kph (160 mph) and six hours in category 5; the same year's Katrina which held peak wind velocity of 280 kph (175 mph) for 18 hours in the category; and 1980's Allen, churning with peak winds at 305 kph (190 mph) maintained for 72 hours in the highest category,” the article says.

The prospect of more severe hurricanes adds to the problem – and the need for a Cat 6 classification, David Enfield, a senior scientist at the University of Miami and former physical oceanographer at the US National Oceanic and Atmospheric Administration (NOAA), told the journal.

He points to global satellite data from the past 40 years indicate that the net destructive potential of hurricanes has increased, and the strongest hurricanes are becoming more common. Data gathered earlier than the 1970s, although unreliable, show cycles of quiet decades followed by active ones. The quiet '60s, '70s and '80s ended in 1995, the year that brought Felix and Opal, among others, and resulted in $13bn in damages and more than 100 deaths in the U.S.

A category 6 label would most likely be applied to hurricanes with sustained winds of more than (280 kph) 175 mph.
Yet, as the piece points out, meteorologists and climate researchers say a category 6 storm would not necessarily be most powerful tropical cyclone in history, because the Saffir–Simpson scale fixates on maximum wind speed lasting for at least one minute and disregards the many other large-scale components that factor into a storm's level of devastation.

Some say the whole index should be thrown out the (hurricane-proof) window.

Bill Read, director of NOAA's National Hurricane Center (NHC) is one. “The whole indexing was done back in the '60s and '70s when we had no way to convey the variables of damage that the storm did. We didn't measure it that carefully; we didn't have the tools," he told the Scientific American.

Variables used by meteorologists and climatologists to assess damage can go beyond wind speeds to include duration over land and the extent of deadly storm surges. Read said: “Katrina, Rita, Ike – all of them made landfall at a 2 or 3 level, but look at the damage they caused. Obviously a category did not accurately describe the impact."

A transition to "impact forecasting" began last year when NOAA's National Hurricane Center simplified the Saffir–Simpson hurricane scale and renamed it the Saffir–Simpson hurricane wind scale, the article explains. This change involved stripping away the scale's former central pressure, flooding and storm surge estimates. These factors among others are now forecast separately. In 2009 the National Weather Service began using new probability models that provide storm surge estimates ranging from 0.6 to 7.6 meters (two to 25 feet).

The Atlantic hurricane season, which runs from June 1 to November 30 annually, is predicted to produce more and stronger storms than average this year, although active years have been the norm since 1995. That year the Atlantic entered a period of warm sea-surface temperatures of what is called the Atlantic Multi-Decadal Oscillation, and such cycles typically last two to three decades.

That means we should have another 10 to 15 years of this active period.

As for the addition of a new category 6, NOAA’s Bill Read insists it is not needed. "I'd be totally opposed to that, even if they did get stronger," he told Scientific American. "I'll fight 'em tooth and nail under my regime. We'll keep what we have now, but I'm going to focus more on the impacts.

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October 2018


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