Tropical Storms and Hurricanes Are Coming

Hurricane Ivan, 2004  (Source: NOAA)

Tropical Storm Arthur could be forming off of the southeast coast of the United States.  If it develops, then that would make 2020 the sixth consecutive season with at least one named storm before June 1, which is the official start of hurricane season.

An early start to hurricane season doesn’t necessarily mean the entire season will be busier than average.  According to a May 13 tweet from hurricane expert Dr. Philip Klotzbach of Colorado State University, there is “No significant relationship between day of first Atlantic named storm formation and seasonal Atlantic #hurricane activity (as measured by Accumulated Cyclone Energy) in the satellite era (1966-2019).”  However, an early start to the season could offer some clues.

It’s true that eight of the last 10 seasons that saw the first named storm before June 1 have gone on to be above average, but starting early wasn’t the primary cause.  In many cases, there was either a La Nina in progress or ENSO neutral conditions were in place – meaning neither El Nino or La Nina conditions existed.  Environmental conditions are less favorable for tropical storm formation in the Atlantic during moderate to strong episodes of El Nino.  Meanwhile, La Nina conditions or ENSO neutral conditions can help create more favorable conditions for tropical development in the Atlantic Ocean.

There have been several clear illustrations of this fact.  For example, a major episode of El Nino developed as the 2015 Atlantic hurricane season wore on.  Consequently, atmospheric and environmental conditions became less favorable.  Ultimately, the 2015 season was the Atlantic’s last below average hurricane season with 11 tropical storms, 4 hurricanes and 2 major hurricanes.  An “average” Atlantic hurricane season features 12 tropical storms, 6 hurricanes and 2 – 3 major hurricanes. 

The 2016 season saw two named storms before June first, including an exceptionally rare January hurricane that developed in the Atlantic Ocean.  The strong episode of El Nino during the 2015-2016 winter season quickly dissipated during the spring and early summer.  That allowed for more favorable conditions in the tropical Atlantic.  It ended up being a slightly busier than average season with 15 tropical storms, 7 hurricanes and 4 major hurricanes.  A “major” hurricane is defined as a Category 3 or higher with sustained winds of at least 111 mph around the center of circulation.

Colorado State University issued its seasonal forecast for the upcoming Atlantic hurricane season early last month.  It called for a total of 16 tropical storms, 8 hurricanes and 4 major hurricanes.  NOAA will issue its seasonal hurricane forecast next week.