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The Atlantic hurricane season draws to a close on Nov. 30, but this does not always mean that the threat of tropical systems is over until the following season.
The Atlantic hurricane season officially begins on June 1 and lasts until Nov. 30. This span of time is when weather conditions are most favorable for tropical development.
However, under the right conditions, tropical systems can take shape outside of the traditional hurricane season.
According to National Hurricane Center records, dating back to 1851, at least one named tropical storm or hurricane has occurred during every month of the year in the Atlantic. However, off-season systems are not common and do not develop every year.
“If there’s going to be early season development, it’s most likely going to take place across the south-central Atlantic well east of Bermuda,” AccuWeather Hurricane Expert Dan Kottlowski said.
“The reason for that is that’s where a lot of upper-level storms develop, and if they can drift far enough south into the warmer water, they may take on a sub-tropical, or maybe even a tropical, appearance,” Kottlowski said.
A subtropical storm is a type of hybrid storm, showing both tropical and non-tropical characteristics, and can eventually transition into a named tropical storm or hurricane.
This was the case for Arlene in the 2017 Atlantic hurricane season. The system first became a subtropical depression on April 19, 2017, eventually becoming Tropical Storm Arlene over the open waters of the Atlantic.
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Many of the tropical systems that develop between December and May are similar to Arlene, spending their lives spinning over the Atlantic Ocean rather than the Caribbean Sea or Gulf of Mexico, closer to land.
“It would be very, very rare for something to development in the Gulf of Mexico as we go into December,” Kottlowski said.
This is due to strong upper-level winds, called the westerlies, shifting over the Gulf of Mexico and the Caribbean Sea during winter. These disruptive winds are usually entrenched over the region during the winter months, preventing disturbances from becoming organized into tropical systems.
These winds begin to weaken and shift northward later in the spring, making May the most common month for off-season tropical development in the Atlantic basin.
When tropical systems do develop between December and May, they tend to remain weak with a majority of systems never reaching hurricane status.
Since records began, there have only ever been two hurricanes that have reached Category 2 status outside of the tropical season, and none that have become a major, Category 3 or higher, hurricane.
Hurricane Amanda was one of the strongest off-season storms recorded in the Atlantic Basin, becoming a Category 2 hurricane with winds of 105 mph. This was also one of the few early-season storms to impact the United States directly, making landfall in Florida on May 28, 1863.
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