29 July 2016

Africa is awakening

The official Atlantic hurricane season is six months long, from June 1 through November 30.  But activity is concentrated in a much shorter timeframe.  If we think in terms of Accumulated Cyclone Energy, or ACE, roughly 52% of the entire season's ACE occurs in just one month between August 25 and September 25.
[ACE is a metric that accounts for the intensity and the longevity of a storm, and can be added up across all storms]

Tropical cyclones in the Atlantic can form in the Caribbean Sea, Gulf of Mexico, off the U.S. east coast, and in the far eastern Atlantic off the coast of Africa.  The preferred formation locations vary throughout the season and follow the warmer water, lower wind shear, and pre-existing "triggers" or disturbances.  African easterly waves are generated over continental tropical Africa and move westward toward the Atlantic Ocean.  During August and September, these AEWs can be potent pre-existing disturbances with persistent thunderstorm activity and large-scale rotation.

According to a paper by Dr. Chris Landsea in 1993, only ~60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, but ~85% of major hurricanes have their origins as AEWs.  From the time an AEW leaves the African coast, it typically takes about 11-14 days for that system to reach the United States (if it survives that long and is steered that way).

I'm bringing this up because we are now at the point in the season where we look to Africa for the majority of tropical cyclone embryos.  In fact, there are some AEWs out there today, one of which may become a tropical depression rather soon.

There are three AEWs evident on this satellite image: a sloppy one on the far left of the image, one near the middle just south of the Cape Verde islands, and a small one crossing into southern Mali. The one near the Cape Verde islands is the one of greatest interest. A close-up of it is shown below -- you can already see spiral banding features wrapping around a center of circulation.

Although its prospects for development are fairly high in the next day or two, the longer-range outlook is grim for this... it will face increasing wind shear and drier air in the days ahead and most likely dissipate long before it reaches the Lesser Antilles.

If it should continue to develop this weekend and become strong enough to be upgraded to a tropical storm, the next name on the list is Earl.  As I mentioned in Wednesday's post, this would be the 7th time the name Earl is used.  Just for fun, the tracks of the previous six Earls are shown below: