BATON ROUGE, LA (WAFB) - Today, November 30th, marks the official end of the 2015 Hurricane Season. There were 11 'named' storms in the Atlantic Basin this year, one shy of the 30-year average; those 11 systems included four hurricanes, two short of the seasonal average. Two of 2015's hurricanes became 'major' hurricanes (Category 3 or stronger): Category 3 Danny during August and Category 4 Joaquin in October.
The U.S. fared well through the season, suffering only two landfalls: an early T.S. Ana along the Carolinas in May and a relatively modest T.S. Bill along the Texas Coast in June. Bill proved to be the only Gulf storm of the 2015 season - notable in that, on average, the Gulf experiences roughly one third of all the seasonal activity.
Eleven 'named' storms were more than most pre-season forecasting teams had anticipated and at the upper end of the National Hurricane Center's updated August prediction. Most forecasters noted the anticipated evolution and strengthening of El Niño during the season as the key reason for reduced activity. El Niños are associated with increased mid-level wind shear over the Atlantic Basin during the hurricane season: this shear inhibits storm intensification and can even lead to a rapid demise of a developing system. In effect, El Niño usually means reduced tropical activity across the Atlantic.
While the number of 'named' storms that formed was near the long-term average, it still looks as though El Niño had an impact on tropical storm energy across the Atlantic by shortening the amount of time the storms survived over the open waters. As a general rule of thumb, non-landfalling storms (tropical storm intensity or greater) typically "live" for 7 days or more. However, of the 11 storms during the 2015 season, only four – Category 3 Danny, Category 1 Fred, T.S. Ida, and Category 4 Joaquin -- survived for 7 or more days. Shearing winds played a significant role in the reduced lifespans of several 2015 storms.
The Accumulated Cyclone Energy (ACE) Index is often used to assess the energy of individual storms and the collective energy of all storms within a hurricane season. While the number of storms this past season was close to average, the relatively short lifespans for most of the 'named' storms meant that their individual ACE scores were low, resulting in a low contribution to the seasonal total ACE for the Atlantic Basin.
How low? The (preliminary) seasonal ACE score of 60 for the Atlantic Basin is only 58% of normal. That's not as low as the ACE of 36 in 2013, but it ranks as the 3rd lowest ACE score for the Atlantic Basin over the past 10 years and the 4th lowest ACE over the past 20 years.