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A turn toward the northwest is expected later Thursday, followed by a motion toward the west-northwest by tonight and on Friday.
ATLANTIC BASIN | Subtropical Depression One

Arlene downgraded, first named storm of 2017 Atlantic season

Staff Writers

Friday, April 21, 2017, 6:17 - While Arlene has been downgraded to a post-tropical cyclone, it is the first named storm of the 2017 Atlantic hurricane season.

It was officially named Tropical Storm Arlene on Thursday, boasting winds of 53 mph, according to the U.S. National Hurricane Center (NHC).

The last tropical storm to form in April was Ana back in 2003.

Arlene has since weakened to a post-tropical cyclone as it has become absorbed by a large extratropical low. The cyclone is moving towards the southwest at 23 mph Friday morning, and this general motion is expected to continue until dissipating later in the day, the NHC reports. 

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Tropical versus Subtropical

While a subtropical storm shares some features with the more familiar tropical systems, there are some key differences.

  • Tropical storms and hurricanes require a warmer ocean surface temperature to get going (about 82oF/28oC), whereas subtropical lows are possible with sea temperatures of about 70oF/21oC.
  • While subtropical lows do get some of their energy from warm ocean water, the cooler water isn't enough to sustain them, and they require upper-level wind flow support, similar to the low pressure systems that drive weather systems on land.
  • Subtropical systems tend to be less organized, and lack the tell-tale shape of a tropical storm or hurricane. Convection happens further away from the centre of the system than in a tropical storm.
  • The strongest winds with a subtropical low are much farther from the centre than in a tropical system, where the strongest winds are found in the eyewall.

Subtropical lows occasionally remain over warm water long enough to take on the characteristics of a full tropical storm. This happens when thunderstorms are able to form close enough to the centre of the storm to give it extra strength and energy. Ana, the subtropical low that formed in 2003, was such a storm, making it the only named Atlantic tropical storm to form in April in the record books.

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