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Atlantic hurricane season off to an early start

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The 2017 hurricane season has gotten off to an early start with the formation of Subtropical Depression One in the Atlantic Ocean Wednesday morning.

This is the first subtropical or tropical cyclone to roam the Atlantic basin in April since Ana in 2003. It's also the third straight year a system has formed in the Atlantic before the official June 1 start of the hurricane season.

Subtropical Depression One was centered between Bermuda and the Azores as of early Wednesday evening, with maximum sustained winds of 35 mph. Little change in strength is expected into early Thursday, and the depression will have no direct impact on any land areas.

Current Informaion

Current Informaion

    A small increase in winds to 40 mph would allow this system to be upgraded to Subtropical Storm Arlene, but that is currently not expected to happen, the National Hurricane Center (NHC) says.

    Subtropical Depression One will not have a long existence. Another non-tropical area of low pressure is expected to absorb the depression on Thursday.

    Projected Path and Wind Forecast

    Projected Path and Wind Forecast

      How Rare Is This?

      You'd be right to think it's very early for development in the Atlantic. The Atlantic hurricane season, which officially begins June 1 and lasts until Nov. 30, accounts for about 97 percent of tropical cyclone activity in the basin.

      However, a tropical or subtropical cyclone developing in April is a rare, but possible, event.

       Ana in 2003 was the last April named storm to roam the Atlantic Basin. Ana began as a subtropical storm on April 20, 2003, and soon gained full tropical characteristics. That made Ana the only tropical storm on record to form in the Atlantic Basin in April.

      Visible satellite image of Subtropical Storm Ana taken by the OrbView-2 satellite on April 20, 2003. Ana would later become a tropical storm. (SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE)

      While Ana remains the only April Atlantic tropical storm in records dating to 1851, an April 1992 subtropical storm was found in post-analysis by the NHC. Since it was not classified as such at the time, a later hurricane that year, Andrew, got the "A" name.

      There have been numerous seasons that started early. On a long-term average, a tropical system forms prior to June about once every 10 years, and these storms tend to be relatively weak, due in part to cooler sea-surface temperatures.

      There has been a recent trend in early starts to the Atlantic hurricane season, with 2012, 2015 and 2016 all reporting tropical cyclone formation before June 

      Just last year, two tropical cyclones formed before the official start date. Hurricane Alex developed Jan. 13, 2016 and made landfall in the Azores. Then, Tropical Storm Bonnie formed on May 28 and made landfall in South Carolina over the Memorial Day weekend.

      Areas off the Southeast coast, as well as the northwest Caribbean and the Gulf of Mexico, are common locations for early-season development, especially in May.

      Difference Between Tropical And Subtropical Depression/Storms

      When an area of low pressure forms over waters with sea-surface temperatures of at least 70 degrees, a subtropical low can form. This is due to the core of the storm becoming warm, deriving some of its energy from latent heat, or energy released when water vapor that evaporated from the warm water is condensed into liquid.

      A subtropical depression or storm exhibits features of both tropical and non-tropical systems. This includes no cold or warm fronts, a broad wind field and thunderstorms removed some distance from the center.

      Subtropical systems also tend to have a large, cloud-free center and a less symmetric wind field. Maximum sustained winds are also much farther from the center, while the strongest winds in a tropical storm are close to the center.

      Subtropical Low

      Subtropical cyclones typically are associated with upper-level lows and have colder temperatures aloft, whereas tropical cyclones are completely warm-core and upper-level high-pressure systems overhead help facilitate their intensification.

      The NHC still issues advisories and forecasts for subtropical depressions and storms. They are assigned a number or name, just like a tropical depression or storm.

      Tropical Low

      If the subtropical storm remains over warm water, thunderstorms can build close enough to the center of circulation, and latent heat given off aloft from the thunderstorms can warm the air enough to make the storm a fully tropical storm.

      As a result, the strongest winds and rain become closer to the center and, with time, further intensification becomes possible.

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