Quick-Forming Cape Verde Hurricanes May Menace, But There's a More Frequent Homegrown Threat

Some of the Atlantic's most powerful and unsettling hurricanes develop quickly off the coast of Africa and produce days of anxiety and dread as they trek across 2,000 miles of open ocean in the general direction of the U.S.

So Weather.com analyzed 26 years of hurricane data to find out just how much of a threat to the U.S. these so-called Cape Verde hurricanes are. That analysis reveals that these hurricanes don't often reach the mainland U.S., but when they do, tend to be destructive. It also reveals that there's a more frequent threat much closer to home.

Every hurricane season, meteorologists zero in on a parade of weather disturbances known as African Easterly Waves, or tropical waves, that march westward off the coast of western Africa and into the Atlantic Ocean. According to the National Hurricane Center, about 85% of intense hurricanes and about 60% of other tropical storms and hurricanes develop from these disturbances.

Sometimes one of these tropical waves wastes little time strengthening into a tropical storm and then a hurricane after moving off Africa and into the open waters of the Atlantic.

These early developers are called "Cape Verde hurricanes", after the group of islands former by which they often track, now known as the Republic of Cabo Verde.

One recent example, shown in the satellite image at the top of the article, was 2019's Hurricane Lorenzo. Lorenzo not only developed south of Cabo Verde, but also exploded into the strongest hurricane on record in the eastern Atlantic Ocean.

How Often a U.S. Threat?

Just because a hurricane develops quickly in the eastern Atlantic Ocean doesn't mean it will ultimately threaten the mainland U.S.

To shed some light on this, Weather.com examined all 201 hurricane tracks from 1995 through 2020. The period was selected because hurricane seasons have been more active since 1995.

Fifty of the 201 hurricanes either made landfall or brushed very close to the mainland U.S. to produce some direct impacts. That's an average of about two U.S. hurricanes per season since the mid-1990s.

But the analysis found that only eight out of 50 hurricanes that reached the U.S. were of the Cape Verde variety.

What Keeps Cape Verde Hurricanes at Bay

There are several reasons why Cape Verde hurricanes don't make it to the U.S. that often.

The simplest is that there aren't many of them.

According to NOAA's Hurricane Research Division, most seasons produce zero to five Cape Verde hurricanes.

Of the 14 hurricanes in the frenetic 2020 hurricane season, only Paulette and Teddy were Cape Verde hurricanes.

Cape Verde hurricanes also have thousands of miles of ocean to cover to make it to the U.S. A lot of things often go awry along the way.

They can be carried farther north into the central Atlantic Ocean if the Bermuda high is weaker or less expansive than normal. These recurving storms then curl around the weaker high well off the U.S. East Coast.

Both Paulette and Teddy curved north over the central Atlantic Ocean and ended up well east of the U.S. coast.

Two other factors limit the number of Cape Verde hurricanes.

Surges of hot air from the Sahara Desert known as Saharan Air Layers move off the African coast every 3 to 5 days or so from late spring through early fall.

Wind shear is another factor that doesn't allow development in the eastern Atlantic Ocean early in the season. These differing winds with height can rip apart a system trying to become a tropical storm.

All of this means that Cape Verde hurricanes are usually confined to a narrower window of the hurricane season, typically either August or September.

Those That Do Make It Are Impactful

It's worth discussing the eight Cape Verde hurricanes that did make it to the U.S. as hurricanes over the last 26 years.

While Cape Verde hurricanes are few and make up a small fraction of U.S. hurricane landfalls, those that do make the entire voyage are almost always very destructive.

September 2018's Hurricane Florence was the last Cape Verde hurricane to hit the U.S. Despite downshifting its winds before landfall, its slow crawl over the Carolinas triggered catastrophic flooding.

Others included Irma (2017), Ike (2008), Ivan (2004), Frances (2004), Isabel (2003), Georges (1998) and Bertha (1996).

All but one of these - Bertha in 1996 - were so damaging their names were retired from future use.

In these cases, there was not enough dry air or wind shear to put a lid on their intensification, and expansive, strong high pressure to their north ensured they would be steered to the U.S.

Homegrown Hurricanes a More Frequent Threat

The bigger hurricane threat for the U.S. comes from storms that develop much closer to the U.S.

As the map below shows, of the 50 U.S. impacting hurricanes from 1995 through 2020, very few of them first became hurricanes between the Lesser Antilles and Africa.

The large majority of them first became hurricanes in the Caribbean Sea, Gulf of Mexico or near the Southeast coast.

Some of the reasons for this more homegrown hurricane threat are simply geography. The closer to the U.S. it becomes a hurricane, the better chance of a U.S. hurricane landfall.

For instance, a Gulf of Mexico hurricane has nowhere else to go except Mexico or the U.S. if it doesn't fizzle first.

But sometimes the tropical wave encounters a hostile environment of dry air and wind shear elsewhere in the Atlantic, only to find a much more hospitable environment closer to the U.S.

Deep, warm water is typically most plentiful in the Caribbean Sea and Gulf of Mexico, so development in this part of the tropics isn't simply limited to August and September.

Wind shear ripped apart Tropical Storm Harvey in the eastern Caribbean Sea only for it to roar back as a Cat. 4 Hurricane Harvey in the Gulf. It then slammed and flooded out parts of coastal Texas.

Every hurricane should be taken seriously, particularly those in the Caribbean Sea, Gulf of Mexico, or off the Southeast coast.

But much more often than not, Cape Verde hurricanes - even stronger ones - have to clear some atmospheric hurdles before they become a U.S. threat.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.