Tropical Storm Arlene Is Moving Quickly in the Open Atlantic; No Threat to Land

April 21, 2017

Tropical Storm Arlene is moving quickly in the open Atlantic and is expected to weaken by late Friday.

Tropical Storm Arlene, the first named storm of the 2017 Atlantic hurricane season, developed in the north Atlantic Ocean Thursday evening. Arlene formed from Subtropical Depression One, which developed on Wednesday, and then became Tropical Depression One on Thursday morning.

Arlene is the first tropical storm in April to exist in the Atlantic Basin since Ana in 2003, which also formed on April 20, just a few hours earlier. It's also the farthest north a tropical storm has formed in the Atlantic so early in the season.

Prior to the beginning of the satellite era, which began in the 1970s, this type of storm was practically impossible to detect, according to the National Hurricane Center, so it is possible that others have occurred prior to 1970. 

This is the third consecutive year a system has formed in the Atlantic before the official June 1 start of the hurricane season.

(MORE: New Hurricane Season Forecast)

Tropical Storm Arlene is centered more than 1,100 miles west-northwest of the Azores in the central Atlantic Ocean, with maximum sustained winds estimated at 50 mph.

Current Informaion

Convection near the center of the system became more symmetric Thursday compared to Wednesday, when it was detected by the new GOES-16 satellite.

Additionally, some data indicated this system had developed a weak but warm core. Therefore, it was declared that it transitioned from a subtropical depression to a tropical depression Thursday morning.

The system continued to organize Thursday afternoon, and winds increased just enough to become a tropical storm. 

Tropical Storm Arlene will not last very long. Arlene is expected to become part of a large non-tropical low pressure system on Friday, and the storm will have no direct impact on any land areas.

(MORE: Hurricane Central)

Projected Path and Wind Forecast

This storm 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 had 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.

(MORE: 5 Changes Coming to Hurricane Season Forecasts)

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.

(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.

(MORE: When Hurricane Season Starts Early)

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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