The quietest Atlantic hurricane season in more than a quarter of a century is now entering its final phase.
Hurricane activity normally tails off during October, and unless there a marked, and unexpected, upturn in activity, 2014 will have had the fewest storms since 1986.
So far this year there have been just four hurricanes – Arthur, Bertha, Cristobel and Edouard. Only Edouard in September qualified as a ‘major’ hurricane, reaching Category 3 on the five point Saffir-Simpson scale.
With only two named storms during September – Tropical Storm Dolly being the other one – only 1997 recorded fewer storms, when Hurricane Erika was the lone system, albeit a ‘major’ one.
A useful measure of storm potential is Accumulated Cyclone Energy (ACE). This measures the accumulated wind energy of all storms in a season. The average ACE over the last 30 years is around 93 but this year it is less than half this figure.
Most storms that develop in October do so over the warm waters of the Caribbean. But here, conditions are not favourable.
The vertical instability over the Caribbean is a measure of the average difference between temperatures near the surface and those in the upper atmosphere. A high index favours thunderstorm development which may, in turn, lead to a tropical storm system.
Throughout the 2014 season the index has remained well below average and there is no sign of the index reverting to normal during the coming weeks.
Looking further out into the Atlantic there are signs of a tropical wave forming off the Cape Verde islands, but a lot of dry, sinking air, is expected to inhibit significant storm development here.