Hurricane Florence and other Cape Verde Hurricanes

Florence becomes a Category 4 on September 10, 2018  (Source: NOAA)

Hurricane Florence is the first major Atlantic hurricane of 2018 and is poised to make landfall near the North and South Carolina border.  At peak intensity it was a Category 4 hurricane with sustained winds of 140 mph and a minimum central air pressure of 939 millibars – well below the standard sea level air pressure of 1013.25 millibars.  The National Hurricane Center expects Florence to weaken below Category 4 status before making landfall.

September is the busiest month of the Atlantic hurricane season because conditions are most favorable for tropical storm formation.  The longest-lasting hurricanes in the Atlantic are often “Cape Verde” hurricanes that develop in the eastern Atlantic Ocean near the Cape Verde Islands.  Florence is a “Cape-Verde” hurricane with a relatively long lifespan since it was first named a tropical storm on September 1. 

A “tropical storm” has sustained winds of 39-73 mph, while a “hurricane” has sustained winds of at least 74 mph.  According to NOAA, an average hurricane season in the Atlantic Ocean has a total of 12 tropical storms of which 6 becomes hurricanes, including 3 major hurricanes (Category 3 or higher with sustained winds of at least 111/+ mph).  That includes an average of up to 5 Cape Verde hurricanes in a given season. 

The track of Cape Verde hurricanes is largely dependent on a semi-permanent area of high pressure in the Atlantic Ocean known as the “Bermuda High.”  Air circulates in a clockwise direction around areas of high pressure in the Northern Hemisphere.  The Bermuda High is typically centered near the island of Bermuda, but its exact location often varies from one summer to the next. 

The Bermuda High has a large influence on where tropical storms and hurricanes track.  Cape Verde hurricanes can recurve northward over the open Atlantic and never threaten the East Coast of the United States, like Hurricane Erika (1997) and Hurricane Kate (2003) each did.  This tends to happen when the Bermuda High is situated farther east than normal.  There have been other seasons when the Bermuda High is centered farther west, and that guides Cape Verde hurricanes into the East Coast. 

There have been a significant number of destructive Cape Verde hurricanes that have made landfall along the East Coast, such as the 1933 “Chesapeake Bay” or “Chesapeake-Potomac” hurricane, Hurricane Gloria in 1985, Hurricane Hugo in 1989 and Hurricane Isabel in 2003.  Some people may also recall Hurricane Fran in 1996 and Floyd in 1999, which also fit this criteria.  Fran was the last Category 3 hurricane to make landfall in the Carolina’s, while Hugo was the last Category 4 hurricane to make landfall there.

Sometimes the Bermuda High doesn’t steer hurricanes northward until they’re much farther west.  For example, Hurricane Andrew in 1992, Hurricane Frances in 2004 and Hurricane Irma all made landfall in Florida and then had significant impacts along the U.S. Gulf Coast.  There have been other instances where Cape Verde hurricanes are kept south by the Bermuda High and impact Central America like Hurricane Dean in 2007.  Also, Hurricane Ike tracked over the island of Cuba and across the Gulf of Mexico before making landfall in Texas in 2008.

In conclusion, Hurricane Florence is the most recent in a long-line of Cape Verde hurricanes that is poised to become a destructive storm in the United States.