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Active hurricane season comes to an end

Satellite image of Hurricane Irene (NASA)

December 2011 - After seven hurricanes and 12 other named storms causing several billion dollars of damage, the 2011 North Atlantic hurricane season has now officially come to an end.

It has been an unusual season in many respects. Firstly, the 19 named storms marks the joint second highest number based on records since 1944*. The average is about 11.

The season has also been characterised by a high number of relatively short-lived storms - with seven of the 12 tropical storms lasting just two days or less. None of the first eight storms of the season developed into hurricanes - something unprecedented in the records.

Joanne Camp, a long-range hurricane forecaster at the Met Office, explained that having so many short-lived and relatively weak tropical storms was a notable feature of the season: "If you look at the long-term record, this is unusual - but it has been an increasing trend over recent years.

"This is almost certainly due to the improvement of technology, such as satellites, which allows us to observe developments over the North Atlantic in ever greater detail. This means we are now identifying storms that could previously have gone undetected."

Many storms - but not much power

Because such a large proportion of this season's storms were short-lived and weak, the Accumulated Cyclone Energy (ACE) index (measuring the combined strength and duration of all the storms in the season) was only moderately above average at 124 - the average is about 101. Many seasons in the historical record have had a much lower total tropical storm count, but much higher ACE index.

The Met Office public forecast for the Tropical cyclones, which is issued in May, continued its run of providing good guidance on the ACE index - with this year's actual total well within the predicted range. On the number of storms, the total of 19 this year is outside of the forecast range for the first time in five years.Post tropical storm Katia Photo: EUMETSAT/Met Office

In May 2011, the Met Office ran its first experimental forecast of hurricane numbers. It predicted that the most likely number of hurricanes to occur during June to November 2011 would be eight, with a 70% chance that the number would be in the range 3-13. The average number of hurricanes is around six. In the event seven hurricanes occurred, which was close to the most likely number. Following further investigation of performance, forecasts of the number of hurricanes will be considered for release from March 2012.
On the Met Office seasonal tropical storm forecast for 2011 Chris Landsea, Science and Operations Officer at the National Hurricane Center in Miami, said: "It does look like 2011 was well predicted by the Met Office models. The number of tropical storms may be somewhat problematic to verify, as we do a much better job in identifying weak, short-lived systems as tropical storms now than just 15-20 years ago. Using ACE index or number of hurricanes would be a more stable measure, less prone to changes in technology during the last 40-50 years."

In terms of the storms which developed into hurricanes, two were notable this year. Irene was the first hurricane to make landfall in the US in three years. Interestingly, no major hurricane (those registering category 3-5 on the Saffir-Simpson hurricane wind scale) has made landfall in the US since Wilma in 2005 - a near-record amount of time.

Katia also made headlines this year - but this time in the UK. The Met Office's early forecasts of the hurricane's track after it became post-tropical highlighted its course across the Atlantic, leading to weather warnings of strong winds and heavy rain for the UK several days in advance.

Post-tropical storm Katia - September 2011, gusts of up to 98 mph were recorded as the northern half of the UK bore the brunt of the storm. Katia was the strongest ex-hurricane to make landfall in the UK since Lili in 1996.

Longer-term trends

Overall the relatively high amount of Atlantic hurricane activity continues a trend which started in 1995, with most years since then being above-average. To assess long-term cycles in North Atlantic hurricane activity the Met Office is trialling experimental forecasts for up to five years ahead. 

While this research continues, the Met Office's hurricane experts will be keeping a close eye on the drivers of tropical storm activity over the next few months as they prepare the first forecast for next year's season, which will be issued in March 2012. The main public forecast will be released in May 2012.

A verification report for this year's Met Office Seasonal Tropical Storm Forecast for the North Atlantic is now available online.

For regular updates on tropical cyclones worldwide follow @metofficestorms on Twitter.

* All references to records refer to data going back go 1944. Records going back to other dates are used in the verification report.

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