2017-09-18 - By Martin Hedberg

Why Hurricane Irma was so powerful

The flooding from Hurricane Harvey was barely over when the world was alerted about Hurricane Irma. How could it be that the USA, after twelve years’ absence from major hurricanes, felt the impact of two record-breaking hurricanes over the course of one week?

Hurricane Irma at the time of landfall in Florida

Hurricane Irma at the time of landfall in Florida, September 10. To the right, Hurricane José is visible. Photo: NASA

Let us start by noticing that nobody ever said that the USA, or any other state previously hit by hurricanes (tropical cyclones), would no longer be at risk of being hit by future hurricanes.

However, it is part of human nature to ascribe more importance to recent events than to those occurring a longer while ago. That things remain as usual (i.e. persistence) and patterns are a foundation of our intuitive grasp of the world. This can be in sharp contrast to how mathematicians, physicists and other scientists view the world when it comes to assigning probabilities for different possibilities and the existence of, for example, natural catastrophes.

Often, the assumption of persistence works well. Forecasts such as “the weather today will be the same as yesterday” and “dew in the grass, it will probably be sunny today” are more often correct than wrong.

But the truly interesting, and crucial, situations arise when these simple assumptions fail. While it was twelve years since mainland USA saw the landfall of a major (category 3 to 5) hurricane (Wilma, 2005), but this does not entail that it cannot occur again, say next week.

On the contrary, there were plenty of indications that 2017 could bring an increased risk of hurricanes and landfalls. At least, this was the overall view of the forecasts by independent institutions. Neither La Niña nor El Niño, warmer than normal surface temperature and weak wind shears in the Caribbean were the most important factors.

But forecasts did not indicate a situation this serious. And they are not intended to do, as it is not possible to make that kind of forecasts. We can say that there is an above normal risk, but the details are hidden beyond the horizon of the future.

In the aftermath of Irma, you could almost feel a disappointment in media and social media that it was “only” a category 4, and soon enough category 3, cyclone making landfall in Florida and that the hurricane did not completely destroy the wealthy Eastern coast, with Miami as the crown jewel.

Property owners in Florida should be thankful to Cuba that things did not turn out worse. On its course to Florida, Irma made a short, but crucial, diversion over Cuba. The hilly terrain and the absence of moisture and warmth from the sea did not only drain Irma of energy, but also put it on another path than the one forecasted, had Irma stayed over sea outside Cuba. You could say that Cuba took a hit, which meant that Miami just got away.

Of course, not everyone got away from Irma’s rampage. Irma devastated large parts of the Caribbean. In particular Barbuda, Saint Bartholome, Saint Martin and the Virgin Islands, where approximately 95% of all buildings are, more or less, completely destroyed.

Irma also set a number of nasty records:

  • Irma had 37 hours with wind speeds above 185 mph (83 m/s). The previous record, 24 hours, was held by the “monster typhoon” Haiyan (which devastated large parts of the Philippines in November 2013).
  • Irma was the most powerful cyclone (with its wind speed above 83 m/s) and the cyclone with the lowest central pressure (914 hPa) ever observed in the Atlantic outside the Gulf of Mexico.
  • Irma has released more energy, ACE (Accumulated Cyclonic Energy) than the previous named storms in the Atlantic this season.
  • Irma’s landfall in the USA marked the first time ever two hurricanes category 4, or above, made landfall in the same season.
  • Irma had a record-breaking lifetime, with 11.25 days as a hurricane category 3 or above. Irma also was a category 5 hurricane for 8.5 days.

Animation of Hurricane Irma’s path over Florida and the conversion to a tropical storm. Photo: NASA

Often when we discuss hurricanes (or typhoons/cyclones as they are called, depending on where they occur), we talk about the number of hurricanes and their category, i.e. the maximum wind speed. But Irma, as well as Harvey, demonstrate that this is a too crude metric to describe the destructive ability of hurricanes.

Irma was not only powerful, measured by wind speed, but these winds also covered a very large area and the hurricane persisted for a long time. Hence, Irma had the potential to affect many more people than a geographically small and short-lived hurricane has.

All this occurred just after a number of records by Hurricane Harvey, not regarding wind speed, but precipitation. This, too, was an effect of a warm sea surface, but also of the fact that jet winds did not behave as they usually do. The absence of jet winds caused Harvey to remain more or less stationary in the same spot. Hence, Harvey could pick up energy and moisture from the Gulf of Mexico and converted this to rain, dumped over Houston and other cities in Texas, to devastating effects.

But it is not only about size. Equally important is if and where hurricanes make landfall. The hurricane season this year, due to last for another two months, demonstrated clearly that nature, unfortunately, can combine record-breaking hurricanes with the ability to make landfall.

The USA should be thankful that they have been relatively spared from hurricanes in the past years, but this should not be construed that they cannot be affected. On the contrary, it can possibly have contributed to a decrease in risk-awareness, neglected systems, preparations and procedures to cope with natural catastrophes that, inevitably, hide beyond the horizon.

Even if reliable forecasts can be made a few days or a week before a catastrophe hits, there is no time to act other than trying to escape bringing the most important possessions.

I would like to conclude by noting that Harvey and Irma were far from the only natural catastrophes affecting us in the past weeks, but they have received unparalleled media coverage here in the Western world. In many other places, for example India, Nepal, and Bangladesh, we have seen torrential rain resulting in flooding, with many victims and large consequences for food production and infrastructure.

Even though the hurricane’s roar has ceased and the water has stopped falling from the sky, months remain of grief for missing people and the reconstruction of societies that, in the turn between August and September, within hours and days saw fundamental change.

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Martin Hedberg

Martin Hedberg

Chief Meteorologist

Martin has 20 years’ experience from meteorology and previous officer in Swedish Armed Forces, civil career as meteorologist at Swedish Television and as CEO for a commercial weather company. He also works as a climate expert and advisor about extreme weather and how climate changes affects society, finance and energy markets. Martin holds a Bachelor of Science with a Major in Meteorology from Stockholm University.

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