Off-Season Tropical Storms and Hurricanes

Tropical Storm Arlene, April 2017  (Source: NOAA)

Something very rare occurred in the Atlantic Ocean last week.  Tropical Storm Arlene became only the third tropical or subtropical storm to occur during the month of April.  Arlene was the first April storm since Ana in 2003 and an unnamed subtropical storm in April 1992.  No hurricanes have been observed in the Atlantic Ocean in April.

Arlene was also the strongest April storm on record, with peak sustained winds of 50 mph and a minimum air pressure of 993 millibars.  Air pressure is considered a more accurate measure of tropical storm intensity than wind speed.  There is an inverse relationship between lower air pressure and higher wind speeds in tropical storms and hurricanes.

Hurricane season in the Atlantic Ocean runs from June 1 through November 30.  The period from August 15 to October 15 is typically the most active portion of the hurricane season.  That’s when conditions tend to be most favorable with tropical ocean water at its warmest levels of the year and wind shear at its lowest level.  However, there have been 15 tropical storms in the Atlantic Ocean Basin (including the Gulf of Mexico and Caribbean Sea) outside of hurricane season.  Just last year, Hurricane Alex became the first January hurricane to develop in the Atlantic Ocean since Alice in 1955.

A lot of off-season tropical activity doesn’t necessarily correlate to a busier than average hurricane season overall.  For example, the 2012 Atlantic hurricane season saw two tropical storms develop in May as part of a busier than average season with 19 tropical storms, 10 hurricanes and 2 major hurricanes.  But, Tropical Storm Ana developed in May 2015 and the entire year had only 11 tropical storms, 4 hurricanes and 2 major hurricanes.  That’s slightly below the Atlantic’s 30-year average of 12 tropical storms, 6 hurricanes and 2 major hurricanes.  A “major” hurricane is defined as a Category 3 or higher on the Saffir-Simpson Scale, with sustained winds of 111 mph or greater. 

It’s important to remember that the overall number of storms is less important than whether or not any storms make landfall.  Scientists at Colorado State University are forecasting a 2017 season very similar to 2015, with 11 tropical storms, 4 hurricanes and 2 major hurricanes.  NOAA will issue its seasonal hurricane forecast for 2017 closer to June 1.  The next tropical storm that develops in the Atlantic Ocean will be named “Bret.”