Quirks of the 2021 Atlantic Hurricane Season

 

Hurricane Grace, August 2021 (Source: NASA)

The 2021 Atlantic hurricane season has already been a busier than average season with 19 tropical storms, of which seven developed into hurricanes.  Four of those seven became major hurricanes with Hurricane Sam being the only currently active system in the Atlantic.  A “major” hurricane is defined as a Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale, with sustained winds of at least 111 mph.

The first major Atlantic hurricane of the season was Grace that peaked as a Category 3 storm.  Hurricane Ida made landfall in Louisiana late last month as a powerful Category 4.  Its remnants brought heavy rain and severe weather to the Mid-Atlantic Region.  Fortunately, Hurricane Larry had no direct impacts on the United States despite being a named storm for more than a week.  Hurricane Sam isn’t expected to have any direct impacts in the United States.

With Hurricane Sam dominating recent tropical weather headlines, it was easy to miss Subtropical Storm Teresa.  Not a fully tropical system, “Teresa” was named a subtropical storm on September 24 near the island of Bermuda.  However, it was declared a “remnant low” the next day having been a named system for less than 24 hours.  Teresa was hardly the only weak, short-lived system this season.

According to Dr. Philip Klotzbach of Colorado State University, 2021 has tied the 2007 Atlantic hurricane season with nine tropical storms that have lasted less than two days.  Some of this season’s other short-lived systems that didn’t impact the United States include: Tropical Storms Ana, Julian and Odette.  Meanwhile some others did make landfall in the United States: Tropical Storms Claudette, Danny and Mindy.  Fortunately, none of these systems were particularly destructive being relatively weak and fast-moving systems.

Having weaker storms that remain over the open ocean isn’t much different than a stronger hurricane that remains out to sea.  Prior to the satellite era, neither type of storm was detected unless a ship happened to be in its vicinity.  Unless a ship were in the path of a tropical storm or hurricane over the open ocean, the only other way to tell if a storm existed was if it made landfall.

Since a hurricane season could have a high number of weak systems that produces a higher overall number of named storms, scientists also look at “Accumulated Cyclone Energy” or “ACE.”  NOAA more formally measures ACE on the “ACE Index” that is a measure of how active the hurricane season is based on the overall “wind speed and duration of each tropical cyclone.”  An average amount of ACE during an Atlantic hurricane season is 96.7 units (10⁴ kt²). Last season’s final ACE Index value was 180.37 10⁴ kt² while this season’s current ACE Index value is 97.80 10⁴ kt².  ACE is independent of whether or not a particular hurricane season is a destructive one.