|Above: Visible satellite image of Tropical Storm Arlene at 10:00 am EDT Friday, April 21, 2017. Arlene's small circulation is being enveloped by a much larger midlatitude storm.|
The 2017 Atlantic hurricane season got off to a premature start on Thursday, April 20, with the formation of Tropical Storm Arlene. Originally designated a subtropical depression by the National Hurricane Center (NHC) on Wednesday, Arlene was reclassified as a tropical depression on Thursday, and its top sustained winds reached 40 knots (45 mph) at 5:00 pm EDT Thursday, enough to qualify it as Tropical Storm Arlene.
Top winds in Arlene were increased to 50 mph on Thursday night, based on partial data from Arlene’s periphery gathered by the ASCAT scatterometer. Subsequent ASCAT data on Friday morning from Arlene’s center suggested that the storm might no longer have a closed center of circulation, so Arlene is likely to be declassified as a tropical cyclone on Friday, although winds at some distance from the core may still be above tropical storm strength. Update: NHC issued its final advisory on dissipating Arlene at 11 am EDT Friday.
Arlene’s circulation was evident days ago as a large non-tropical gyre drifting across the remote Eastern Atlantic, including a small center with some eye-like characteristics. Sea surface temperatures beneath the circulation were only about 20°C (68°F), far below the standard minimum of 26°C (79°F) for tropical development. However, the upper atmosphere was cold enough to support the formation of showers and thunderstorms, and the broad upward motion gradually enabled the system to develop a warm core, thus triggering the shift from subtropical to tropical status. Arlene began to move northwest and west on Thursday as the circulation was gradually captured by an approaching midlatitude trough.
How unusual is Arlene?
Arlene is a rarity, as Jeff Masters noted in a post on Wednesday. Getting a tropical or subtropical depression in the Atlantic in April is about a once-per-decade event, and a tropical storm in April is even more unusual. The NOAA Historical Hurricane Tracks website shows that only three April tropical or subtropical depressions are known to have formed in the Atlantic prior to Arlene, although many such systems would have gone undetected prior to the advent of routine satellite monitoring in the 1970s. Only two of the four April systems on record became named tropical storms: Arlene (2017) and Tropical Storm Ana (2003). An unnamed April subtropical storm in 1992 also achieved tropical storm-force winds, and two other April tropical depressions formed in 1981 and 1973.
Arlene is also the northernmost of the few tropical storms that have developed this early in any season on record, as reported by weather.com. Tropical activity hits a climatological minimum in the North Atlantic during late winter and early spring, as sea surface temperatures reach their lowest points of the year and strong wind shear often prevails. The only tropical cyclones on record in the Atlantic that made it beyond depression strength during February and March are a system on February 2-3, 1952, that moved across South Florida as a tropical storm, and an unnamed hurricane that struck the U.S. Virgin Islands at Category 2 strength on March 6-9, 1908. Because Ana and Arlene both followed a gap of more than six weeks between tropical-storm-strength systems, one might consider them roughly tied as the earliest tropical storms on record, although most records are oriented toward calendar years rather than the climatology of the hurricane season itself.
I’ll be back later today with a post on the upcoming March for Science.