This year saw another active season in the North Atlantic with 19 named storms, of which 10 became hurricanes.

Both the number of named storms and hurricanes were well above the 1980–2010 averages of 12 and six respectively. However, only one of these (Michael) became a major hurricane, which is below the average of three.

Unusual season

It has been an unusual season in many respects. This is the third year in a row with 19 named storms, which is unprecedented in the historical records. Only one other season – 2005, which saw the devastating Hurricane Katrina – has experienced more named storms (28) since reliable records began in 1944.

The season has also been notable for the high number of relatively short-lived storms, with seven of the nine tropical storms lasting just two days or less. These storms contribute towards a high storm count, but relatively little towards the Accumulated Cyclone Energy (ACE) index – a measure of the combined strength and duration of all named storms in the season.

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.

“It 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 that we are now identifying storms that could previously have gone undetected.”

Many storms – but not much power

Because such a high proportion of this season’s storms were short-lived and weak, the ACE index was only moderately above average at 127. The average is 104. Many seasons in the historical record have had a much lower total tropical storm count, but much higher ACE index,  for example the 2004 season recorded only 14 named storms but an ACE index of 225 – nearly twice  that seen in 2012.

The Met Office public forecast for the North Atlantic hurricane season, 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.

Chris Landsea, Science and Operations Officer at the National Hurricane Center in Miami, said: “Because we are now better able identify weak, short-lived tropical storms than we were just 15 to 20 years ago, a simple count of how many storms occur in a season is perhaps not the most representative measure of how active a season has been. 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.”

Experimental forecasts run by the Met Office during the 2012 season show that there is skill for forecasting the number of hurricanes. In May 2012 the Met Office predicted that the most likely number of hurricanes to occur during June to November 2012 would be six, with a 70% chance that the number would be in the range two to ten. In the event ten hurricanes occurred.

Significant storms

The most notable storm of the 2012 season was Hurricane Sandy (also referred to as Superstorm Sandy), which became one of the largest storms on record, measuring over 1000 miles across. The storm resulted in 253 deaths (at least 122 of those in the Caribbean) and is estimated to have caused over $65 billion in damage – making it the second most costly hurricane in US history, behind Hurricane Katrina.

GOES-13 natural-color image of Hurricane Sandy at 17:45 UTC on October 28, 2012. CREDIT: NOAA/NASA GOES Project.

The Caribbean also experienced a number of tropical storms during 2012 season. Hurricane Isaac caused severe damage in Haiti and eastern Cuba before making landfall along the northern Gulf Coast of the USA. Tropical Storm Rafael passed close to Puerto Rico and the British Virgin Islands. Hurricane Sandy made landfall over Jamaica and Cuba before heading to the northeast coast of the USA and Hurricane Ernesto made landfall over Central America.

No major hurricanes (those registering category 3-5 on the Saffir-Simpson hurricane wind scale) have made landfall in the US since Wilma in 2005 – a near-record length of time.

Longer-term trends

Overall the relatively high level 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 continue to monitor 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 2013. The main public forecast will be released in May 2013.

Further details on the 2012 season can be found in this year’s verification report (PDF, 1 MB).

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