Friday, October 11, 2024

Hurricane Milton shows how a storm’s category doesn’t tell the full story


Photograph: National Oceanic and Atmospheric Administration; 
AP Newsroom

From Wired by Alec Luhn

Milton’s reclassification to a Category 3 storm suggests it is weakening, but the scale accounts only for wind speed and not hurricane size, storm surge heights, or rainfall—which are all catastrophically large.

As Hurricane Milton roared toward the west coast of Florida on Tuesday and Wednesday, its 180-mile-per-hour winds weakened to 145 miles per hour, rose again, and then fell.
What had been one of the quickest ever storms to reach Category 5 strength—which is when wind speeds top 156 mph—flicked back and forth between Categories 4 and 5.
It ultimately arrived at the coast on Wednesday as a Category 3 storm, with winds of 125 mph.

But while Milton’s wind speed was reduced, the inundation of water forecast for Florida remained just as massive as before.
Tampa, a city of 3 million that hasn’t taken such a direct hit in a century, faces a storm surge of 10 to 15 feet, along with nearby St.
Petersburg and Sarasota, according to the National Hurricane Center.
This comes less than two weeks after Hurricane Helene pushed an 8-foot surge into the area.
Central and northern Florida could also see 12 inches of rainfall, with up to 18 inches in isolated areas.

That Milton could decrease in category but still threaten such a high storm surge and volume of rainfall shows a major shortcoming of the Saffir–Simpson scale, by which we assign hurricanes categories 1 through 5: It’s based solely on wind speed, even though in an era of climate change, hurricanes have been unleashing more and more water on cities.
That has left hurricane forecasters trying to move beyond these categories and convey the risk of storm surge and flooding, so people will still evacuate even if wind speeds slacken.

“The public needs to not focus totally on the number in the category,” says Erik Salna, a meteorologist at the International Hurricane Research Center at Florida International University.
“The fact that it has already been a major hurricane, it will still have that momentum, power, and force on the water.
And the biggest killer is water, not the wind.”

The Saffir–Simpson scale is “a great way to show really the intensity of an open-water storm system,” says Brian Hurley of the US National Weather Service.
“But it should not be the end-all be-all for diagnosing the threats.”
Modern hurricane forecasting was born in 1943, when a US Army Air Corps pilot flew a two-person propeller plane into the eye of a hurricane on a barroom dare, and then repeated the stunt with a meteorologist on board.
This gave birth to subsequent innovations like aerial surveys, but while these gave weather watchers a lot more data—on how low the barometric pressure in the storm had fallen, how fast a hurricane was spinning, and how fast it was moving toward shore—they still struggled to express the level of danger to the public.

For instance, in 1969, many residents of Mississippi failed to evacuate before Hurricane Camille slammed into the coast with estimated winds of up to 200 miles per hour, despite detailed weather reports, and 256 people died.
National Hurricane Center director Robert Simpson subsequently decided to adopt a categorization of hurricane wind speed developed by his friend, Miami civil engineer Herbert Saffir, leading to the Saffir–Simpson scale.
Simple and evocative, it gave even the most uninformed people the sense that categories 3, 4, and 5 are major hurricanes, with major destructive potential.
“Its great advantage is everybody knows it, and everybody more or less knows what to be afraid of,” says Richard Olson, director of the International Hurricane Research Center.
This simplicity comes at a price, however.
Since it’s based on the maximum wind speed achieved, the scale doesn’t say anything about the size of a storm.

Hurricane Katrina, for instance, hit New Orleans in 2005 as a Category 3, weaker than Camille.
But it was much larger, with hurricane-force winds extending 105 miles from its center rather than 60 miles for Camille, and it did a lot more damage.
The scale also doesn’t address flooding—neither the storm surge of ocean water pushed onshore by a hurricane, nor the heavy rainfall it dumps as it passes over land.
Originally each category included an expected range of storm surge, but the National Hurricane Center removed this in 2010.
That’s because factors besides just wind speed influence the surge.

A hurricane that moves forward quickly or has a large radius will push more water onshore than a smaller, slower storm, especially if a shallow continental shelf forces that water mass upward.
The storm surge will be higher when squeezed into a bay like the one around Tampa or when a hurricane barrels head-on into the coast rather than at an angle.
Hurricanes like Katrina showed the potential for confusion: The gigantic storm surge of up to 28 feet far exceeded the 12 feet predicted based on its Category 3 wind speed, corresponding instead to what would be expected from a Category 5.

In response, StormGeo, an advisory firm that helps its clients decide when to shut down infrastructure like oil refineries and retail stores, developed the Hurricane Severity Index.
“We realized the Saffir–Simpson scale didn't accurately reflect the storm-surge capabilities of a storm,” says StormGeo meteorologist Bob Weinzapfel.
The index measures wind speed on a 25-point scale and a storm’s size—that is, how far these high winds extend—on another, to give a total rating out of 50, and compares that to historic hurricanes.
By that index, Milton has 11 size points and 12 intensity points.
For its part, the National Hurricane Center started issuing storm surge watches and warnings in addition to categorizing hurricanes based on windspeed.
Its bulletins now also include risks of rainfall, tornadoes, and high surf.
“We’ve worked to separate the impacts to best represent areas on the immediate coast and for those a few miles to hundreds of miles away,” says Maria Torres, a meteorologist at the center.

But the Saffir–Simpson scale remains the key measurement, unless you have the time to click deeper into your regional weather service website.
And as climate change supercharges hurricanes, which are fueled by warm water and air, the sufficiency of the system's five categories is increasingly coming into question.
After Milton’s wind speed skyrocketed from a 60 mph tropical storm to a 180 mph Category 5 hurricane in only 36 hours, experts are again discussing whether a Category 6 needs to be added.
Milton’s enormous storm surge has also highlighted the growing danger from water.
More intense hurricanes are pushing higher storm surges due to sea-level rise.
These “hurricanes on steroids,” as Olson calls them, are also dumping larger amounts of rain inland, just as Hurricane Helene did in North Carolina late last month.
Between 2013 and 2022, flooding due to heavy rainfall accounted for a whopping 57 percent of hurricane deaths, with storm surges responsible for another 11 percent, according to the National Hurricane Center.
Wind caused only 12 percent.

The International Hurricane Research Center is known for its “wall of wind,” a hangar of 12 giant yellow fans that can generate 157 mph winds to test the resilience of building materials.
Now it has a $13 million federal grant to design and prototype a new facility with 200 mph fans and a 500-meter wave pool, to test the effects of windier, wetter hurricanes.
“That’s real-world. You don’t get just wind, just water, just wave. You get all three,” Olson says.

Some meteorologists say we need a different scale entirely.
Carl Schreck, a research scientist at North Carolina University, has proposed a Category 1–5 scale based on sea-level pressure to better incorporate water.
A low pressure boosts both wind speed and storm size, and larger storms tend to have bigger surges and more rain.
A Category 5 would be a hurricane with a pressure lower than 925 millibars.
By this measurement, Milton would have remained a Category 5 until mid-Wednesday rather than vacillating between 4 and 5.
“Pressure is easier to measure, easier to forecast, and matters more for damage, but NHC, through inertia, they’re tied to the current system, and they think changing it would confuse people, unless there’s a silver bullet,” Schreck says.
“And there is no silver bullet.”

No single number can capture all hurricane impacts.
That was demonstrated by Helene, which made landfall in Florida as a Category 4 but unleashed “biblical” rainfall hundreds of miles inland in Georgia, South Carolina, and North Carolina.
The storm killed more than 200 people, half of them in western North Carolina, where mountain valleys channeled the rainfall into devastating floods.
The impact was compounded by a tropical storm that showered the Carolinas with historic rainfall two days before Helene.
Before Helene hit, forecasts compared its rainfall to hurricanes Frances and Ivan, which brought up to 18 inches of rain to some parts of North Carolina in 2004, triggering 400 landslides and killing 11.
They also cited a record-setting flood in 1916, warning that the “impacts will be life-threatening.” 
The storm two days before Helene was described as a “once-in-a-thousand-year event.” But the fact that so many people died nonetheless shows a “communication disconnect” between our storm warning system and the public, says Schreck, who lives in Asheville and was without power and water for days.
He’s also helped develop an “enhanced rainfall” scale, where a Category 5 event pours five times as much rain as an area would get once every two years on average, a Category 4 dumps four times as much, and so on.
The predicted rainfall would have made Helene a Category 3 extreme rainfall event in the mountains of North Carolina rather than just a Category 4 hurricane on the coast of Florida.
“Nobody knows what 500 or 1,000 years means.
It’s basically inconceivable,” Schreck says of probability-based systems.
“So it’s saying, take the biggest event you can remember and multiply it by three.”
 

Not everyone will evacuate even for a major storm, however, especially in a hurricane-weary state like Florida.
More than a million people were under an evacuation order there for Milton, with Governor Ron DeSantis urging residents to “run from the water” and the mayor of Tampa warning those who don’t are “going to die.” But one mother named Amanda Moss went viral with TikTok videos saying she didn’t have the money for flights and hotels to evacuate her husband, mother-in-law, six children, and four French bulldogs from Fort Meyers, which faces up to 12 feet of storm surge.

In the comments, some other users said they were also staying put, arguing they couldn’t get off work or were worried about gas shortages.
It’s not just “a pride or an ego thing,” as Moss put it.
Thirteen percent of Americans wouldn’t be able to cover an emergency expense of $400, and 38 percent would have to pay with a credit card, sell a possession, or take out a loan to cover it, according to the Federal Reserve.
“There is not like one sentence that you can get on air and say that is going to get everybody to evacuate,” says Samantha Montano, an assistant professor of emergency management at Massachusetts Maritime Academy, who favors retiring the Saffir–Simpson scale altogether.
Rather than wind speed or sea-level pressure categories, hurricane forecasts should focus on local impacts in certain areas, she says.

For Tampa right now, that’s 15 feet of water in the streets, winds that could tear off your roof, and rainfall that can overwhelm drainage systems and wreck your car.
“Any scale that we’re using to communicate with the public that isn’t accounting for what impacts are isn’t going to capture what the public needs to capture in order to be able to understand the risk,” Montano says. 
 
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