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Tropical Depression One No Threat to Land; First April System in Atlantic Since 2003
Tropical Depression One, the first system of the 2017 Atlantic hurricane season, transitioned from a subtropical depression to a tropical depression Thursday. This system formed on Wednesday, becoming 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.
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Tropical Depression One is centered over 700 miles west of The Azores in the central Atlantic Ocean, with maximum sustained winds estimated at 35 mph.
Convection near the center of the system has become more symmetric compared to Wednesday, when it was detected by the new GOES-16 satellite.
In addition, there is some data that indicates that this system has developed a weak, but warm core. Therefore, it was declared that it had transitioned from a subtropical depression to a tropical depression Thursday.
A small increase in winds to 40 mph would allow this system to be upgraded to Tropical Storm Arlene. Little change in strength is expected, and the depression will have no direct impact on any land areas.
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Tropical Depression One will not have a long existence. Another non-tropical area of low pressure is expected to absorb the depression by early Friday.
Projected Path and Wind Forecast
This depression formed from what was a non-tropical low, which has been spinning in the Atlantic for the last several days. Seas as high as 40 feet were analyzed by NOAA's Ocean Prediction Center in association with the system at that time.
The low gained sufficient organized shower and thunderstorm activity late Tuesday into Wednesday that the NHC has determined it now has characteristics of a tropical cyclone. Arlene began as a subtropical depression. For more on what a subtropical system is, scroll to the bottom of this article.
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.
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.
(RECAP: April 2003's Tropical Storm Ana)
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 1.
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 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.
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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