Hurricane Central
Hurricane Season Update: Least Active in 30 Years
By Jon Erdman Published: Oct 11, 2013, 5:52 PM EDT weather.com
Graph of Atlantic Basin ACE index by year from 1970-2013. The 2013 total as of Oct. 11 is highlighted in red at the far right. (Credit: Weather Underground)
If the Atlantic hurricane season ended now, this would be the least active Atlantic season in 30 years.
Tracks of named tropical storms and hurricanes in the 2013 Atlantic hurricane season through Oct. 11. Red (orange) name labels and tracks denote hurricanes (tropical storms). Yellow (green) lines indicate portions of tracks during which each system was a tropical depression (post-tropical low).
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![Where Do Fall Hurricanes Form?]()
Where Do Fall Hurricanes Form?![An October Surprise?]()
An October Surprise?
Where Do Fall Hurricanes Form?
An October Surprise?
Yes, there have been 11 named storms through Oct. 11, only one storm shy of the 30-year average of 12 named storms. However, that doesn't tell the whole story.
A better method of summing up hurricane season activity than simply counting storms utilizes the Accumulated Cyclone Energy, or ACE, index. This is calculated by adding each tropical storm or hurricane's wind speed through its life cycle. Long-lived, intense hurricanes have a high ACE index. Short-lived, weak tropical storms, a low ACE index. Summing up the ACE indices of each storm or hurricane will give you an ACE index for the entire season.
The bar graph at the top from Weather Underground shows just that. Notice the paltry total of 28, so far, in 2013.
According to Erika Navarro, Ph.D. candidate at the University of Washington, there were only two other seasons with equal or less activity in the Atlantic Basin since 1981:
That stretch from 1982-83 was one of the strongest El Niño events on record. Increased wind shear during stronger El Niño events tends to suppress tropical cyclone development. Indeed, only 10 named storms formed during the 1982 and 1983 seasons, combined, though one was destructive Hurricane Alicia. In 2013, we are currently in a neutral state, neither El Niño, or its opposite, La Niña.
An average year ACE index is about 110. Navarro said Hurricane Ike in 2008 had an ACE index of 39, more than the entire seasons of 2013 (so far), 1983 and 1982.
(MORE: Weather Underground ACE Page)
Only two of this season's storms strengthened to hurricanes, Humberto and Ingrid. Humberto pushed the record for the latest first Atlantic hurricane of the season, waiting until Sept. 11. Ingrid became a hurricane in the far western Gulf of Mexico before sliding into eastern Mexico.
Furthermore, there has not been a single major hurricane, Category 3 or stronger on the Saffir-Simpson hurricane wind scale, this season. According to Digital Meteorologist Chris Dolce (Twitter), the last season lacking a single major Atlantic hurricane was 1994.
Seven of the season's 11 named storms eventually succumbed to either dry air, strong wind shear or both. This includes each storm forming in the central and eastern Atlantic Ocean, as well as Tropical Storm Karen.
Atlantic hurricane season energy (ACE index) average percentage by month.
Typical formation zones and tracks of October and November named storms in the Atlantic Basin.
Is the Season Really Over?
In an average season, we typically see two more named storms and one final hurricane from mid-October on.
As you can see in the animation below at right, late-season storms typically form in the Caribbean or southwest Atlantic Ocean.
Revisiting the ACE index, only 13 percent of the season's energy typically remains after mid-October.
So, climatology suggests there's at least one last hurrah left. In the recent past, these late-season hurricanes have been particularly destructive:
(MORE: Late-Season Hurricanes)
Interestingly, not a single named storm formed after late September or early October in both the 1982 and 1983 Atlantic hurricane seasons, according to Navarro. That said, it remains to be seen whether we can close the book on an unusually docile 2013 season.
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