As we head into October the peak of the hurricane season is well behind us and the time of year when the longest-lasting, and usually the most powerful hurricanes form, is also rapidly fading away. We call the period from mid August to late September the “Cape Verde” season, named for the Cape Verde Islands just west of the African coastline. Waves of low pressure moving west off of Africa have a large, warm Atlantic Ocean ahead of them for tropical development and if these areas of low pressure turn into tropical storms or hurricanes, we call them “Cape Verde” storms. With so much time over the water before they get close to the Caribbean Island or the United States, these storms can get quite large and strong and last more than a week if conditions are right. This year, Hurricane Katia and Irene were strong Cape Verde hurricanes, and both became major (Category 3 or larger) with Katia becoming a Category 4 storm for a short time. Then again, Maria was a weak hurricane also originating from a Cape Verde wave of low pressure.
As we head into October, the favorable wind flow for these storms shifts southward while the waves of energy coming off of Africa get weaker. Sometimes the change happens slowly and sometimes it’s like a light switch turning off, but either way the Cape Verde Season comes to an end around this time of year. I have the latest satellite image next to this article with my quick analysis.
I found three weak waves in the area, one which is well southeast of the Cape Verde Islands with two more over central Africa…much too far south for development over the Atlantic Ocean. You can also see two other weak storms from Cape Verde waves…Philippe and Ophelia…neither of which has been very impressive and unlikely to become hurricanes. We have had only three hurricanes for the entire 2011 season through the end of September.
Note the high pressure areas over northern Africa as the jet stream over Europe shifts further south with numerous storm systems there. As the jet stream shifts south, it pushes the high pressure areas further south and that shuts off the area of tropical wave development over north-central Africa. Rarely do we see any development if the African waves that stay south of 10 degrees north latitude (dashed line), and you can see that the remaining waves are all south of 10 degrees north latitude. This signals the end of the Cape Verde Season, which produced only three hurricanes this year (Irene, Katia and Maria). Any new storms developing in the next 2 months will need to be in the Gulf of Mexico, Caribbean Sea or the southwest Atlantic Ocean for any chance at hitting the United States…and right now I don’t expect much more excitement this year. There may be an October storm that threatens the southeastern states, but that would be it.
Assuming we accept that Irene was a hurricane at landfall (and that is doubtful), it was the only hurricane to brush the United States in the past 4 years and assuming we don’t see a Category 5 storm this year (which we won’t), it has also been more than 4 years since the last Category 5 storm, Felix in 2007. The last Category 5 to get close to the United States was Wilma in 2005 and the last one to hit the United States was Andrew in 1992. The below normal trend in global cyclone activity continues and as the Atlantic Ocean cools we’ll see even fewer hurricanes and fewer powerful hurricanes over the next two decades.
2011 has been a quiet season…a lot of named storms but most rather weak and short-lived…but that was not expected by any official hurricane forecasters this year…another weather surprise for 2011. :)