September Ushers in a Higher Chance of Activity in the Tropical Atlantic

Brace yourself - the busiest month of the Atlantic hurricane season is upon us. Recent history and statistics over the years demonstrate that September is the peak time for activity in this part of the world's ocean. Here is a look at what you can expect in the tropics in the month ahead.

Recent History of September Tropical Activity

You only have to go back to September of 2024 to understand why the month has a reputation for producing devastating storms. This month was marked by the arrival of Hurricane Helene, a monster Category 4 storm that came ashore in the U.S. in the Big Bend region of Florida.

Helene went on to kill at least 250 people as it moved to the northeast and into the Appalachians of North Carolina, unleashing a catastrophic flooding event. The storm was the deadliest hurricane to impact the mainland U.S. since 2005's Hurricane Katrina.

There have been at least six September storms to come to life in each of the past seven years. The past eight Septembers have produced three or four hurricanes. Nobody will soon forget the record-breaking 2020 season, a year that generated 10 named storms in the month of September alone.

While it is easy to get caught up in the overall numbers for the month, what is most important is how many of these tropical weather events impact land. By this metric, September is also responsible for the most hurricane landfalls, going back to the mid-19th century. In addition to Helene, recent September landfalls include the notorious Hurricanes Ian, Ike, Irma, Ivan, and Maria.

When looking at averages within the month, you can typically expect the greatest amount of activity during the second week of September. According to data from the National Hurricane Center (NHC), an average of four to five storms formed every September between the years 1991 and 2020. Of these storms, two to three of them went on to become hurricanes. At least one made it to the status of a Category 3 storm or higher.

How the ACE Index Stacks Up in September

The sheer volume of named storms is not the only way to measure the action over a particular month. Meteorologists also rely on the metric known as the Accumulated Cyclone Energy (ACE)index. This measurement also takes into account the duration and intensity of the storms, in addition to their volume. According to data from Colorado State University, September typically produces about 45% of the total ACE activity of a season.

Research from the Hurricane Research Division (HRD) out of the National Oceanic and Atmospheric Administration (NOAA) also backs up the fact that September is the most dangerous month for tropical weather. This agency details that just under 50% of all hurricanes measuring at least Category 3 or higher since the mid-19th century have all sprung up during the month of September. September has produced 166 of these storms since this time, compared to 94 in August and 68 in October.

Understanding Why September is So Busy

What explains why the tropics are so busy during September? The simple answer is that the Atlantic basin is more favorable to development over a large zone of the ocean during this time.

Exceptionally warm ocean waters combine with low amounts of wind shear to deliver a fertile breeding ground for tropical weather. This time of the season also features fewer instances of disruptive dry air. This means that tropical waves that move to the west off the coast of Africa are met with conditions that are more likely to support further intensification.

It should be noted that not all Septembers come with a flurry of tropical activity. For instance, only two storms formed in 2014. Neither of these events impacted the U.S.

Going back farther in time, September of 1997 only saw the formation of one named storm. The Leeward Islands dodged Erika after it came to life before the storm went on to become a Category 3 hurricane.

The lesson in the statistics is that residents of areas prone to tropical weather should be prepared for anything as the calendar flips from August to September.

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