How likely is Bermuda to experience a category 5 hurricane? And how devastating could it be? Jonathan Kent asks catastrophe modeler and meteorologist Andrew Moore the hard questions.
Storms are a part of life in Bermuda and climate change is likely to make them more frequent and more intense. A looming question is the likelihood of a visit from a Category 5 hurricane—something never seen in the island’s recorded history.
The horrific devastation caused by Hurricane Dorian in the northern Bahamas in 2019 was a graphic illustration of the havoc that a storm at the top level of the Saffir-Simpson Hurricane Wind Scale can wreak. After making landfall in Great Abaco with winds of up to 185 miles per hour, the slow-moving cyclone battered the island for many hours before moving on to Grand Bahama. Abaco suffered winds of at least tropical storm strength for three days.
Andy Moore, a meteorologist who works with Bermuda-based reinsurer Arch Re as an atmospheric research analyst, believes it’s not impossible for a Category 5 storm to rage as far north as Bermuda. The fuel of a storm is the heat in the ocean. With seas warming as a result of climate change—the UN World Meteorological Organisation has reported that the past eight years were the eight warmest on record—the chances of more powerful storms are increasing. “We’ve had a couple of storms pass close by in the last couple of years that were Category 4,” Moore said. “So, fours are absolutely possible. It would be rare for fives to come up to this latitude, but certainly not impossible. Our waters can get warm enough and the atmosphere can be favourable enough for it to happen.” Only last year, a reconnaissance plane tracking Hurricane Fiona, which passed to the west of Bermuda, had recorded strong Category 4 winds, even when the storm was about 100 miles to our northwest.
Moore said that any such ferocious storm making a direct hit on Bermuda would be highly unlikely to have the slow-moving characteristic that made Dorian so destructive. He explained that as Bermuda is a subtropical island, the “mixed layers” of warm water close to the ocean’s surface are much shallower than further south in the tropics. “By hurricane season this layer can get to 10 or 20 metres of water that is about 85 degrees,” Moore said. “A hurricane can generate 50-foot waves, and this results in a lot of mixing and upwelling, which brings up a lot of cold water. That’s when the hurricane’s heat engine will start to sputter, especially with a slow-moving storm. So, it’s quite difficult to sustain a Category 4 or 5 storm at this latitude. If you get one, it’s likely to be fast-moving.
“Bermuda is fortunate in that way. If you have a slow-moving Category 4 or 5 in the deep tropics, that layer of warm water might be 150 metres deep. In that case the water will be very slow to cool, even when a big storm moves across it.”
The damage potential of storms, or wind hazard, has increased by 5 to 7 percent in storms that have hit the island over the past two decades, compared to the long-term baseline, according to Climate Change and Bermuda, a two-part report authored by Dr Mark Guishard, an experienced Bermudian meteorologist and former director of the Bermuda Weather Service. The same study found that loss uncertainty has increased by a magnitude of 40 to 50 percent. It also cited an engineering study which concludes that designs required by Bermuda’s building code produce structures that are stronger than they need to be—in the current climate, at least. However, given that the climate is changing, Moore warned that there is no room for complacency, citing the stark differences between top-of-the-scale storms and the less powerful hurricanes to which Bermuda has been accustomed. Even a small increase in sustained windspeed can cause a significant increase in damage potential, he added. “We do have to be careful. People should not think, ‘Oh, we’ve had Category 3s before, so we’ll be fine with a Category 4’—it could be a rude awakening if we were to get the worst part of the storm,” Moore said.
A good example of what the “worst part” of the storm means was provided by Hurricane Fabian, a Category 3 cyclone in 2003 that proved to be the most damaging we have experienced in modern times. The category assigned by the National Hurricane Center is an indication of a hurricane’s peak sustained winds. “Usually, it’s only a small portion of the storm, between 1 and 10 percent, that have winds at or near that peak,” Moore said. “So, for Bermuda to actually receive Category 3 winds, that small part of the storm has to pass over the island.
“When you look at Fabian’s track and the location of the peak wind field, it was in the worst possible place, so we got the full, sustained 115mph winds. On top of that, Bermuda has unique topographical features that will enhance winds in certain areas—so in some places that 115mph could have become 130mph, or even higher.”
Another variable is on which side of the storm you happen to be. The counterclockwise movement of Atlantic cyclones means that being on the right side of the storm means experiencing higher windspeeds than being on the left side. Even close to the eyewall, there can be a left-right differential of 10mph or more, Moore said. Such factors may provide at least part of the explanation why storms of a similar category have had markedly different impacts on the island in recent decades.
Moore closed with the thought that the National Hurricane Center has become much better at forecasting the track of storms over the years. “Intensity forecasts have traditionally been tougher and the internal dynamics of storms, strengthening or weakening, are much harder to forecast, because there are so many factors involved,” he added. “Generally speaking, it is advisable to prepare for a storm at least one category above what is forecast, if you can.”
For information regarding how to prepare for an impending storm, contact BF&M.
Read more from our Hurricane Season 2023 series, HERE!