September 2017 Was the Most Active Month on Record for Atlantic Hurricanes

At a Glance

• As measured by the ACE index, September 2017 has become the most active month on record for Atlantic hurricanes.
• About one-fifth of an average Atlantic hurricane season is left.

September 2017, featuring Category 5 hurricanes Irma and Maria and Category 4 Hurricane Jose, has been the most active month of any Atlantic hurricane season on record.

(MORE: Hurricane Central)

Though not necessarily indicative of impact, a good way to rank the most extreme months of a hurricane season is using a parameter called the ACE (Accumulated Cyclone Energy) index, calculated by adding each tropical storm or hurricane's wind speed through its life cycle.

Long-lived, intense hurricanes have a high ACE index, while short-lived, weak tropical storms have a low value. The ACE of a month is the sum of the ACE for each storm and takes into account the number, strength and duration of all the tropical storms and hurricanes in the month.

As of September 30, the month had generated more ACE than any other calendar month on record, surpassing September 2004, according to Colorado State tropical meteorologist Dr. Phil Klotzbach.

(MORE: Eight Straight Hurricanes Have Formed in the Atlantic Basin, a First Since the Late 1800s)

This was about three and a half times the average activity from 1981-2010 in the Atlantic Basin, according to the National Hurricane Center.

Included in September 2017's ACE calculation were Irma, Jose, Katia, Lee and Maria. All of those except Katia were long-lived hurricanes. Lee and Katia were the only ones to not reach Category 4 or Category 5 intensity at their peaks.

For comparison, the most active month during the notorious 2005 Atlantic hurricane season was also September, but the ACE value that month only registered at 68.205, not even half of this September's total.

(MORE: 2005's Record-Breaking Hurricane Season by the Numbers)

Stunningly, Klotzbach noted only one north Pacific - including the typically more active eastern and western Pacific Basins combined - September had more activity than September 2017 in the Atlantic Basin.

September 2017 also generated more ACE than the entire 2016 season (ACE: 141).

The Atlantic Basin also generated 40.25 hurricane days in September 2017, which is the most in the Atlantic of any September on record, Klotzbach added. The previous record was 34.5 hurricane days in September 1926.

A hurricane day is defined by Klotzbach as "a measure of hurricane activity, one unit of which occurs as four 6-hour periods during which a tropical cyclone is observed or is estimated to have hurricane-force winds."

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Klotzbach did caution there may have been some named storms, even hurricanes missed prior to the era of satellite coverage (since 1966) and aircraft reconnaissance (since 1944), since only ship reports and observations from land were available back then.

Named Storm, Hurricane Pace Similar to 2004, 2005

This season becomes even more compelling when comparing it to two of the most notorious recent hurricane seasons of the previous decade.

The eight-hurricane pace lags behind 2005, but is equal to 2004, when four hurricanes hammered various parts of Florida, among other areas.

(MORE: The 10 Most Extreme Atlantic Hurricane Seasons of the Satellite Era)

 

While 2017 is unlikely to touch 2005's record 15 hurricanes, it has chalked up the same number of major (Category 3 or stronger) hurricanes through Sep. 30 as that record-smashing 2005 season generated up to that point in the season. 

In 2005, Dennis, Emily, Katrina, Maria (yep, same name, but different hurricane) and Rita were at least Category 3 intensity through the end of September.

The 30-year average number of hurricanes for an entire Atlantic season is six. The 2016 season generated a total of seven hurricanes, needing Hurricane Otto over Thanksgiving to get to that season total. 

According to the National Hurricane Center, an average hurricane season typically sees another three named storms, two hurricanes and one major hurricane develop before season's end.

(MORE: Hurricanes Irma and Harvey Mark the First Time Two Atlantic Category 4 U.S. Landfalls Have Occurred in the Same Year)

Roughly 21 percent of an average Atlantic season's ACE index occurs after Sep. 30, according to Klotzbach's climatology.

Just an average amount of ACE the rest of this season would place 2017 knocking on the door of the top-five most active seasons in the satellite era. 

Given the entire, often active month of October is looming, 2017 may move into the Mount Rushmore of notorious Atlantic hurricane seasons.

(MORE: How a Borderline La Niña Could Impact the Rest of the Hurricane Season)