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Why are there so many hurricanes this year?

Hot on the heels of Hurricane Harvey, which caused over 60 deaths and billions of dollars in damage as it barreled across Texas and Louisiana, came Hurricane Irma, which battered the western coast of Florida, having wrought devastation in the Caribbean. Now we are halfway through the hurricane season, we caught up with Jonathon Cavanagh, a catastrophe modelling analyst at Hiscox, to find out what’s going on.

First Harvey and now Irma, with possibly José on the way. Why are there so many hurricanes this year?

It isn’t that more powerful hurricanes are forming this year than in the past. It’s just that in the past ten or so years there have been a number of reasons why they haven’t made landfall in the US as major storms.

Most importantly, in the past there have often been strong wind shears – where winds at different heights above the Atlantic Ocean blow in different directions – which has weakened hurricanes, or prevented them from forming altogether.

For some time, El Niño has helped to weaken or stop hurricanes, because it produces warm temperatures in the Pacific Ocean, pushing high winds into the Atlantic and creating strong wind shear. But we’re now entering a transitional phase, when El Niño starts to wear off and La Niña may take hold, when the temperature of the Pacific cools, creating weaker wind shear in the Atlantic. So we’re likely to see more hurricanes.

Also, hurricanes have been forming further east in the Atlantic in recent years, meaning they’ve been more likely to re-curve further out at sea. But what we’ve seen so far in this season is that storms aren’t moving in the way they have done in the past. They aren’t re-curving as they move across the North Atlantic, so they are now heading towards the US coast.

Another reason for the low number of strong landfalling hurricanes in the past decade is that a high-pressure ridge has sat along the Eastern Seaboard, which has acted as a buffer against hurricanes. It was a big factor in why Hurricane Matthew [a Category 5 storm in 2016] veered away from Florida and followed the coast, gradually weakening before making landfall in Carolina. But this high-pressure ridge has moved away this year, meaning that the US coast is more vulnerable to hurricanes.

Have the hurricane forecasters got it right?

It’s too early to tell. Klotzbach and Bell at Colorado State University forecast two major hurricanes – so Category 3 or above – this season, while Tropical Storm Risk predicted three. So, although we’re still within the boundaries offered by the forecasts, we’re only at the beginning of September – the most active month of the hurricane season.

We’ve seen higher-than-average sea temperatures in the Gulf of Mexico, which is something that neither of the forecasters had predicted. In June and July, they were the warmest since the early 1980s, which could mean that more hurricanes form and that they develop into more powerful storms.

These hurricane forecasts take the current sea temperature and air pressure and project those forward to make estimates about the number of hurricanes there may be. But most of these projections are only good for about a month after they were made. The June forecasts are still two months away from the most active part of the hurricane season. Furthermore, localised, short time-frame conditions are important factors when predicting a hurricane’s path, something these forecasts are not yet able to capture. This means the forecasters do not yet have significant skill in accurately predicting the number of storms that make landfall in the US during a given hurricane season.

How accurate have the cat modelers been in predicting damage from Hurricane Harvey?

Again, it’s too early to say. But based on very early data we’ve received from some coverholders, losses are likely to be within the range of outcomes provided by the cat modelers, although the models cannot accurately predict localised “shock losses”.

The cat models still don’t include possible losses from hurricane-related rainfall. So we’ve had to find other sources of data on rainfall and the potential extent of flooding from hurricanes to help us to manage our catastrophe book of business.

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