At a Glance
- Based on long-term statistics, September is the most active month of hurricane season.
- At least six storms have formed in each of the last six Septembers.
- That includes many notorious recent U.S. hurricanes, such as Ian.
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September is usually the most active month of the Atlantic hurricane season, and recent years have borne out that dangerous reputation.
Visualizing the peak: When examining a graph of storms and hurricanes, September sticks out as the peak of the mountain that is hurricane season, as you can see below.
When averaged over the historical record, the number of storms and hurricanes have usually been highest around the second week of September.
In some ways, it's analogous to the peak of Major League Baseball's pennant races, also in September.
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What's usual: From 1991 through 2020, an average of four to five storms formed each September, according to data from the National Hurricane Center.
Of those September storms, two to three became hurricanes, with one reaching at least Category 3 or stronger.
Using a metric called the ACE index, which doesn't simply count storms but also factors how long-lasting and intense they are, September usually accounts for 45% of a total season's activity, according to data compiled Colorado State University tropical scientist Phil Klotzbach.
The "major" hurricane generator: No other month churns out as many intense Atlantic hurricanes as September.
According to data from NOAA's Hurricane Research Division (HRD), just under half of all hurricanes of at least Category 3 intensity (166) since the mid-19th century have happened in September. That's more than August (94) and October (68) combined.
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Busy, lately: The graph below shows the number of storms and hurricanes that formed in September in this era of more active hurricane seasons since the mid-1990s.
At least six September storms have formed each of the past six years in a row. And the past seven Septembers have each delivered 3 or 4 hurricanes. That included a record 10 September storms in the frenetic 2020 hurricane season.
Most U.S. landfalls, too: What ultimately matters is how many of these storms strike land.
Once again, September tops all other months for U.S. hurricane landfalls since the mid-19th century, per NOAA-HRD.
The list of recent September landfalls is a virtual who's who of U.S. hurricanes this century, including Ian, Ike, Irma, Ivan and Maria.
Why it's so busy: Put simply, the favorable conditions for hurricane formation overlap best in September over the largest area of the Atlantic Ocean, as you can see in the map below.
September has the most favorable overlap of low wind shear and ocean temperatures reaching their warmest. It also has less widespread surges of dry, thunderstorm-suppressing air from the Sahara than earlier in the hurricane season. It's in this favorable environment that a parade of disturbances known as tropical waves march westward off west Africa, occasionally the seeds of September hurricane development.
Not every September: A few years have had much quieter Septembers.
Ten years ago, during a season that generated just eight total storms, September 2014 only saw two storms form, neither of which struck the U.S.
And September 1997's only storm, Erika, avoided the Leeward Islands, then became the season's only Category 3 hurricane well east of Bermuda.
Be ready every September, regardless: Statistics and past history illustrate the importance of having your hurricane plan ready to go, especially in September.
While some September storms develop in the eastern Atlantic Ocean and may offer many days warning, others can develop quickly and much closer to the U.S.
Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.
