Supercharged Hurricanes

San Juan National Weather Service radar loop as Category 5 Hurricane Maria bears down on the island packing 165 mph sustained winds.

Rapid Intensification

“Maria is developing the dreaded pinhole eye,” wrote National Hurricane Center forecaster Jack Beven on Monday evening, as the storm reached Category 4 intensity.

That inward contraction of a hurricane’s eye can be one telltale indicator of what hurricane gurus technically call “rapid intensification,” although a more evocative word might simply be “explosion.” Whatever you call it, it’s something we keep seeing this year. Harvey, Irma, Jose and now Maria have rapidly strengthened — and all too often, have done it just before striking land.

It’s a dangerous and scary phenomenon that scientists and forecasters are still trying to understand.

“It’s not a common event. Typically, that occurs in maybe 5 percent of our forecasts,” said Mark DeMaria, acting deputy director of the National Hurricane Center.

The dreaded "pinhole" eye

Hurricane Maria developed a tiny “pinhole” eye during its rapid intensification burst on Monday, September 18, 2107 with a diameter of 8 nautical miles (nm). The hurricane maintained a small eye through Tuesday, the 19th while striking Dominica, with the diameter fluctuating between 7 nm and 10 nm (10 nm = 11.5 miles).

Hurricanes that develop pinhole eyes often intensify into some of the strongest storms we observe, since they concentrate their wind energy around a narrow ring surrounding the tiny eye. These small eyes tend to be unstable, resulting in an eyewall replacement cycle (ERC) shortly after the pinhole eye is observed. Some other examples of tropical cyclones with pinhole eyes with a diameter less than 10 nm:

Hurricane Wilma - 2005 (175 MPH / 882 MB) - Western Caribbean - 1.5 nm
Hurricane Iris – 2001 (140 MPH / 950 MB) - Western Caribbean – 3 nm
Hurricane Beta - 2005 (115 MPH / 962 MB) - Nicaragua - 5 nm
Hurricane Dennis - 2005 (120 MPH / 930 MB) - Florida - 4 nm
Hurricane Charley - 2004 (150 MPH / 941 MB) - Florida - 2.5 nm
Hurricane Opal - 1995 (150 MPH / 919 MB) - Florida - 5 nm
Hurricane Andrew - 1992 (165 MPH / 921 MB) - Florida - 6 nm
Typhoon Forrest - 1983 (165 MPH / 883 MB) - Philippines - 4 nm
Cyclone Tracy - 1974 (125 MPH / 950 MB) - Australia - 7 nm

But DeMaria said that this season is seeing more rapid intensification events than usual and that Maria, in particular, appears to have set a key record for hurricane rapid intensification in the Atlantic.

“Looking back through the records, Maria went from a tropical depression to a Category 5 hurricane in just two and a half days,” he said. “I couldn’t find any other tropical cyclones in our historical record that went that quickly from a depression to a Category 5 hurricane.” 

That’s a big problem, because rapid intensification sets the stage for worst-case scenarios. Sadly, that’s what happened to the Caribbean island of Dominica on Monday night, hit by Maria at full Category 5 strength. 

There’s little chance to warn people or for them to prepare if rapid intensification occurs, so forecasters naturally want to be able to have a handle on it — but it’s a struggle.

“One of the key issues is that it remains quite difficult to predict on a day-to-day basis. And of course, it’s something we would very much like to be able to predict, especially when an intensifying storm is near land,” said Gabriel Vecchi, a hurricane expert at Princeton University.

The National Hurricane Center technically defines rapid intensification as a wind speed increase of at least 35 miles per hour in 24 hours. All four of the most intense Atlantic storms in 2017 beat that easily:

—On the evening of Aug. 24, a day before landfall, Harvey was a Category 1 hurricane with 85-mile-per hour winds. Twenty-four hours later, at landfall in Texas, the storm was a Category 4 with 130-mile-per-hour winds.

—At 11 a.m. on Monday, Sept. 4, Hurricane Irma was already a strong Category 3 storm with 120-mile-per-hour winds. But Irma then radically strengthened further, becoming a superpowered upper-end Category 5 storm with 180-mile-per-hour winds in just 24 hours. 

—Following behind Irma in the middle of the day on Sept. 7, Hurricane Jose was a Category 1 storm with 90-mile-per-hour winds. Twenty-four hours later, it was rated a high-end Category 4 with 150-mile-per-hour winds. 

—Beven’s “pinhole eye” language came as Hurricane Maria reached Category 4 intensity, despite having been a Category 1 just 12 hours earlier. But Maria wasn’t done. The storm would leap further to Category 5 strength, ultimately increasing in intensity by 65 miles per hour in 24 hours.

While scientists don’t fully understand rapid intensification, they do know that it has something to do with hurricanes being in a highly favorable environment for intensification in general.

Rapid strengthening tends to happen when waters are warm, when that warm water is deep, when the atmosphere is moist and when there’s little adverse wind flow that could disrupt the storm, according to research papers on the topic and interviews with experts.

And broadly speaking, what we appear to be seeing this year — similar to the catastrophic Atlantic hurricane season of 2005 — is that the environment is extremely hurricane friendly. Storms simply rev their engines and find that the fuel is of the highest grade, and there’s a deep well of it. Then they take off, and there’s nothing to disrupt them.

Potential Intensity

One key scientific concept that helps explain how such an environment creates the conditions for rapid hurricane intensification is the idea of “potential intensity” — defined as the maximum strength that a hurricane can theoretically achieve in a given environment. That doesn’t mean the storm will actually get there, but when potential intensities are high, you have to worry about what storms are capable of doing — including spinning up very fast.

“Rapid intensification likes to occur when the potential intensity is far from the actual intensity,” said Jim Kossin, a hurricane scientist with NOAA and the University of Wisconsin at Madison. “Rapid intensification likes a lot of head room. Those warm waters have been creating some very high potential intensity, which increases the head room.”

But none of this is, necessarily, enough. Scientists don’t understand every single spark necessary to create a rapid intensifier, which is what makes solving this atmospheric problem so difficult.

“What makes it so mysterious is that there’s a lot of what we call necessary conditions, but they’re not always sufficient,” said Kieran Bhatia, a postdoctoral research associate at Princeton University who is studying the subject.

“We know that certain thresholds need to be met for rapid intensification to occur, but it doesn’t mean that as soon as these switches are turned on, rapid intensification will initiate.”

One of the most striking things about rapid intensification is that according to recent research, it seems to effectively separate out the storms that reach high intensities from those that do not. In a 2015 study, Chia-Ying Lee of Columbia University and colleagues found that hurricanes around the world tend to come in two big bunches — the ones that reach a relatively low intensity, and the ones that get quite strong. And the study found that 79 percent of the latter storms, the strong ones, undergo rapid intensification.

The researchers therefore inferred that this process may be fundamental to determining how many strong storms form — and how that will change under global warming. “A complete understanding of the most intense storms in either the current climate or future (or past) climates may need to include some understanding of [rapid intensification],” Lee and colleagues wrote.

MIT’s hurricane expert Kerry Emanuel has similarly provided reasons for thinking rapid intensification could get worse in a warming climate.

Three Major Hurricane Strikes and Counting:  A Policy Disaster

There's something unnatural about these disasters

As the United States struggles to deal with three back-to-back major hurricanes, it would be wise to reflect on why we keep having such calamities and whether they are likely to get worse.

We must first recognize the phrase "natural disaster" for what it is: subterfuge we hide behind to avoid our own culpability. Hurricanes, floods, earthquakes and wildfires are part of nature, and the natural world has long ago adapted to them. Disasters occur when we move to risky places and build inadequate infrastructure and particularly when we continue to destabilize the atmosphere.

In the United States, we have in place a range of policies that all but guarantees a worsening string of Katrinas, Sandys, Harveys and Irmas as far as we can see into the future. Climate change acts as a threat-multiplier to these policy-generated disasters, making them progressively worse than they would have been in a stable climate.

The U.S. hurricane policy disaster has its roots in the hijacking of politics by special interests. In a free market, risk is largely communicated through pricing. Smokers pay greater health insurance premiums to cover the added risk of their voluntary activity. In a rational world, premiums in hurricane-prone places would be sufficiently high to reflect the actual risk to the property.

But agitation by coastal property owners has resulted in a rigged system in which states place caps on property insurance premiums, or on the maximum difference between premiums charged to risky and less risky customers, forcing the latter to subsidize the former. Hurricane storm surges and freshwater flooding are covered by the National Flood Insurance Program, and here too agitation has resulted in rates that do not adequately reflect the risk. Congress revamped this program in 2012, only to retract many of those changes in 2014 in response to a backlash from flood-prone homeowners.

On top of this, federal disaster relief, as necessary as it may be, inadvertently subsidizes risk. As a consequence of these subsidies, coastal populations are rising much faster than the general population. Globally, the population exposed to hurricane hazards has tripled since 1970, and the trend shows no signs of abating.

To make matters worse, human population explosion feeding rapid climate change is increasing the probabilities of hurricane disasters in many places. Rising sea levels worsen storm surges, often the most deadly and destructive aspects of hurricanes. Sandy would probably not have flooded Lower Manhattan had it occurred 100 years earlier, when sea levels were about a foot lower in New York.

The physical cap on hurricane wind speeds rises in a warming climate, permitting more intense storms like Irma to develop, and observations show that this cap is indeed rising. Basic physics tells us that hurricanes produce more rain in a warmer climate. Computer simulations confirm that the incidence of intense, destructive storms rises and that hurricane flooding from rain and storm surges gets worse in warmer climates, though the frequency of weaker storms may actually decline.

We are beginning to see trends in hurricane observations. Katrina's storm surge was the largest in U.S. history. Sandy achieved the largest diameter of any Atlantic hurricane on record. Western North Pacific typhoon Haiyan of 2013 achieved the highest wind speed of any tropical cyclone in global history, a record broken in 2015 by eastern North Pacific Hurricane Patricia. Harvey dumped more rain than any hurricane in the United States, and Irma maintained Category 5 status longer than any storm anywhere on the planet.

Deniers point out that most trends in the noisy hurricane database do not rise to the high bar of 95 percent certainty that we scientists place on signal detection, implying that no action should be taken until that level of certainty is achieved. That's rather like saying that you will let your 8-year-old cross a busy highway unless it can be proven with 95 percent certainty that she will be run over. Being conservative in risk assessment is the opposite of being conservative in signal detection.

The confluence of rising sea levels and stronger and wetter hurricanes with increasing coastal population and unwise government interference in insurance markets portends ever increasing hurricane disasters. The hurricane policy disaster is a result of too much regulation; the failure to reduce human population growth and greenhouse gas emissions arises from too little. Citizens of all political persuasions should demand that their representatives attack both these problems lest we condemn our children and their descendants to increasing incidence of the kind of misery now being experienced in Texas, Florida, the Virgin Islands, and Puerto Rico.