Peak of Hurricane Season

Humberto has become the third hurricane of the 2019 Atlantic hurricane season.  Fortunately, it won’t have any direct impacts on the United States as it tracks north and east away from the East Coast.  However, it could impact the island of Bermuda.  Aside from Humberto, there are several other tropical disturbances that the National Hurricane Center is monitoring for possible development.

Hurricane season in the Atlantic Ocean Basin is long: spanning half the year from June through November.  The middle of September is commonly the most active time, when atmospheric and environmental conditions are the most favorable.  That’s when sea surface temperatures are at the warmest levels across the largest geographic area in conjunction with the lowest levels of wind shear (winds of differing speeds at different levels of the atmosphere). 

Tropical storms and hurricanes rely on warm ocean water – typically at least 80° Fahrenheit to a depth of at least 200’ – as their primary energy source.  That’s why June and July aren’t as active as August and September.  The second half of October and November typically see diminished tropical activity as atmospheric conditions become less favorable with higher amounts of wind shear.  Wind shear disrupts the circulation of a tropical storm or hurricane and causes weakening.

Earlier in the year, meteorologists called for a near or below average Atlantic hurricane season due to a weakening episode of El Nino.  However, by early August El Nino had fully dissipated and NOAA updated its seasonal hurricane forecast to call for an average to slightly busier than average season.  There have already been eight tropical storms, three hurricanes (including Humberto) and one major hurricane (Dorian).  NOAA's current forecast is for a seasonal total of 10 – 17 tropical storms and 5 – 9 hurricanes, including 2 – 4 major hurricanes.  A hurricane becomes “major” when sustained winds around the center of the storm reach 111 mph or greater.  That makes it a Category 3 on the Saffir-Simpson Scale.

Despite being just past the season’s midway point, the current hurricane season has already broken records.  When Dorian became a Category 5 hurricane on September 1, this year became the fourth consecutive year that one developed in the Atlantic Ocean.  The previous record for most consecutive years with a Category 5 hurricane was three from 2003 – 2005.  Although more Category 5 hurricanes developed (6) during the three-year window through 2005 than during the past four years (5).  That earlier three-year period was skewed by the record-setting year of 2005, which saw four Category 5 hurricanes.  

Fortunately, except for Hurricane Michael last October, no Category 5 hurricane has made landfall in the United States since 1992.  That helps illustrate that as rare as Category 5 hurricanes are, landfalls by such storms are rarer events still.