Fox News Weather Center

Unusually early Subtropical Depression 1 forms in Atlantic

The first subtropical depression of the 2017 Atlantic hurricane season has formed on Wednesday. The system could soon become Subtropical Storm Arlene.

A subtropical depression or storm has both tropical and non-tropical characteristics.

Tropical storm formation over the Atlantic basin is very rare during the months of January, February, March and April. There has been only one tropical storm on record during April from 1851 to 2016, according to the Atlantic Oceanographic and Meteorological Laboratory.

"An area of showers and thunderstorms spinning about 700 miles southwest of the Azores was beginning to take on some tropical characteristics this week," according to AccuWeather Hurricane Expert Dan Kottlowski.

Static Invest 91L loop 9am

This is a live loop of the potential tropical storm over the central Atlantic. (NOAA/Satellite)

A depression has a cyclonic circular motion with surface winds under 39 mph.

Should the system advance beyond the depression stage, the storm would be given a name as a result. The first name on the list for tropical storms for this year is Arlene.

A tropical storm has winds ranging from 39 to 73 mph that rotate around a well-organized center.

The clock is ticking on development and strengthening of the system, however.

"A non-tropical storm will sweep in prior to the end of the week and should create a more hostile atmosphere for development," Kottlowski said.

The system will not be a threat to land but will continue to cause rough seas over the middle of the Atlantic.

The Atlantic hurricane season does not officially begin until June 1 and extends through Nov. 30.