Analysis | Hurricanes Harvey, Irma, Jose, and Maria have one thing in common. They ‘exploded’ quickly before striking land
Hurricane Maria went from a tropical depression to a category 5 hurricane in just 2 and a half days
“Maria is developing the dreaded pinhole eye,” wrote US National Hurricane Centre forecaster Jack Beven 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 all 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 per cent of our forecasts,” said Mark DeMaria, acting deputy director of the US National Hurricane Centre.
But DeMaria said that this season was 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 2 and a half days,” he said.
Watch: Maria spares Antigua, heads toward Puerto Rico
“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. That’s what happened to the Caribbean island of Dominica 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 and NOAA.
The National Hurricane Centre technically defines rapid intensification as a wind speed increase of at least 55 kilometres per hour in 24 hours. All four of the most intense Atlantic storms seen in 2017 beat that easily:
● On the evening of August 24, a day before landfall, Harvey was a category 1 hurricane with 136 kilometres per hour winds. 24 hours later, at landfall in Texas, the storm was a category 4 with 209 kilometres per hour winds.
● At 11am on Monday, September 4, Hurricane Irma was already a strong category 3 storm with 193 kilometre per hour winds. But Irma then radically strengthened further, becoming a super-powered upper end category 5 storm with 290 kilometre per hour winds in just 24 hours.
● Following behind Irma in the middle of the day on September 7, Hurricane Jose was a category 1 storm with 145 kilometre per hour winds. 24 hours later, it was rated a high end category 4 with 240 kilometre 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 105 kilometres 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 favourable 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.
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 per cent 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.
“You know, not that long ago, rapid intensification was sort of an esoteric term that only really in the field did people talk about it,” said Vecchi.
“And now, it’s there. It’s part of the parlance. It’s part of everybody’s lingo.”
