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

Brian Donegan
Published: September 26, 2017

(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 11 a.m. Monday morning, September had generated more ACE than any other calendar month on record, 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, Thanks to Hurricane Lee)

Included in this month’s ACE are Irma, Jose, Katia, Lee and Maria. All of those except Katia were long-lived hurricanes, and 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)

The Atlantic Basin has also generated 35 hurricane days this September, which is the most in the Atlantic of any calendar month 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.”

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 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 nearly chalked up the same number of major (Category 3 or stronger) hurricanes through Sept. 25 as that record-smashing 2005 season generated up to that point in the season.

In 2005, Dennis, Emily, Katrina, Maria (yep, same name) and Rita were at least Category 3 intensity through Sept. 25.

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 28 percent of an average Atlantic season’s ACE index occurs after Sept. 25, according to Klotzbach’s climatology.

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

Given Maria and Lee will still be out there for days, and the entire, often active, month of October is looming, 2017 may move into the Mount Rushmore of notorious Atlantic hurricane seasons.