Tropical Storm Arlene Only the Second April Atlantic Tropical Storm in the Satellite Era

(MORE: Hurricane Central)

This first named storm of the 2017 Atlantic hurricane season was first designated Subtropical Depression One by the National Hurricane Center just before midday on April 19.

It then became Tropical Depression One 24 hours later, before popping enough convection near its center to be designated Tropical Storm Arlene late in the afternoon of April 20.

Arlene then began a strange track westward, thanks to being ingested within a larger low-pressure gyre building to the west and south of Arlene.

(MORE: New Hurricane Season Forecast)

Eventually, on April 21, that low-pressure gyre cut off warm, moist air from Arlene, leading to its demise as a tropical storm.

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.

How Weird Was Arlene?

Arlene was 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.

(RECAP: April 2003's Tropical Storm Ana)

Prior to the beginning of the satellite era, which began in the mid-1960s, 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 that time.

While Ana had been 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.

Arlene was also the farthest north a tropical storm has formed in the Atlantic so early in the season.

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

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.

(MORE: 5 Changes Coming to Hurricane Season Forecasts)

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)

In 2016, 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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