22 April, 2017
Tropical Storm Arlene developed on Thursday from Tropical Depression One, which itself emerged from a subtropical system which developed Wednesday.
Arlene is only the second named tropical system to form in April since the advent of the satellite era (1960), the prior occurrence 2003's which formed on the same day, April 20th.
According to forecasters at the National Hurricane Center, Arlene is not expected to pose any threat to land and as of 5:00 p.m.
A subtropical depression in the Central Atlantic became the first tropical depression of 2017 Thursday, officials with the National Hurricane Center in Miami said.
The storm arrives weeks before the 2017 hurricane season officially starts on June 1.
April tropical systems are pretty rare, but not unheard of.More news: The 49ers Should Select Cornerback Sidney Jones in the NFL Draft
Meanwhile, an ocean-warming El Nino could emerge in the Pacific later this year, disrupting weather patterns across the Atlantic enough to cut down on the number of storms, Henson said. Arlene is a minimal tropical storm with a short lifespan. This happened in 2016, with Hurricane Alex in January and Tropical Storm Bonnie in May.
Just a year ago, a tropical cyclone formed on January 13 and was named Alex.
The formation of these tropical or sub-tropical lows usually requires sea surface temperatures (SSTs) of at least 26.5C. Those dates were chosen because they historically encompass 97 percent of Atlantic storm activity.
For comparison, an average season would have 12 named storms including 6 hurricanes, two of which would be major hurricanes. They remained below the tropical level threshold, below 39 miles per hour.
As a result, the strongest winds and rain become closer to the center and, with time, further intensification becomes possible.