Tropical Update for the Atlantic Ocean Basin

Beryl and Chris on July 9, 2018  (Source: NOAA)

The Atlantic hurricane season, which runs from June 1 through November 30, is usually more active during the month of August.  An average hurricane season in the tropical Atlantic (including the Gulf of Mexico and Caribbean Sea) has 12 tropical storms, of which 6 become hurricanes and 3 of those become major hurricanes.  A system becomes a tropical storm and is given a name when sustained winds around the center reach 39 mph.  A tropical storm becomes a hurricane when sustained winds reach 74 mph.  A hurricane becomes “major” when reaches Category 3 status on the Saffir-Simpson Hurricane Wind Scale with sustained winds of at least 111 mph.

When hurricane season began a little over two months ago, scientists with the National Oceanic and Atmospheric Administration (NOAA) and Colorado State University (CSU) expected it to be slightly busier than average.  However, both groups update their seasonal forecasts in early August.  

The team of CSU scientists currently expect an additional nine tropical storms, of which three will become hurricanes, including one major hurricane over the remainder of season. That’s in addition to the four named storms and two hurricanes that have already developed during 2018.  Meanwhile, NOAA released its updated forecast today that calls for a total of 9 to 13 tropical storms, 4 to 7 hurricanes with up to 2 of those becoming major hurricanes.  One key factor that scientists consider when issuing a seasonal hurricane forecast for the Atlantic is sea surface temperature.  Cooler than average sea surface temperatures currently exist over much of the tropical Atlantic that is not favorable for development.  

There has also been an increased amount of wind shear across the tropics that tends to inhibit tropical development.  Several factors can contribute to increases in wind shear in the tropical Atlantic, such as the presence of El Nino.  NOAA expects ENSO (El Nino Southern Oscillation) neutral conditions to continue for the remainder of the summer and that means no El Nino currently exists.  

NOAA says the chances for an episode of El Nino developing increases to 65% during the fall months.  “El Nino” refers to the unusual warming of the central and east-central equatorial Pacific Ocean that has far-reaching repercussions in global weather.  Earlier this season, meteorologists identified several dust plumes on weather satellite that blew west from central Africa across much of the tropical Atlantic.  The presence of such thick dust plumes also contributes to making environmental conditions less favorable for tropical development. 

Conditions were much different during the very active 2017 Atlantic hurricane season when sea-surface temperatures were warmer than average and wind shear was less than it has been this season.  It’s important to remember, though, that damaging and deadly hurricanes have developed during quieter than average hurricane seasons such as Alicia in 1983 and Andrew in 1992.