|Google Maps shows you the Cape Verde Islands in|
relation to the rest of the Eastern Atlantic Basin
Typically we end up with approximately two Cape Verde type storms per season. July, August, and September are the prime months for development, but we can see them in June and October as well. Katia and Ophelia were classified as Cape Verde hurricanes last season. Other notable Cape Verde storms in recent years include Ike (2008), Ivan (2004), Frances (2003), Isabel (2003), and Floyd in 1999. Andrew, Hugo, Gilbert, Gloria, Donna, the 1938 Long Island Express, the hurricane of 1933, and the deadliest of all US storms, the 1900 Galveston hurricane were all also Cape Verde storms. So Cape Verde hurricanes are also associated with monster storms because when hitting the US (or elsewhere), many have been monster, deadly, and damaging storms.
Very rough estimates, using NOAA's historical hurricane archive, suggests roughly once every four years or so, one of these will hit the US Mainland. There may be more official or proven statistics out there on that, but I was unable to locate any. Of more interest: Of the 128 storms or remnants of storms to pass within 200 nautical miles of northern Delaware (which includes the entire Delaware Valley, west to Altoona and east to about 150 miles offshore of NJ) since the 1800s, roughly 16 would be classified as "true" Cape Verde storms (forming within about 600 miles of the Cape Verde Islands). About 1/3 of storms or remnants of storms impacting the Delaware Valley or Jersey Shore form east of the Lesser Antilles.
So the bottom line: The longest lasting, occasionally powerful hurricanes we see in the Atlantic are very often the Cape Verde storms. And we'll watch to see if our latest Cape Verde type storm, Isaac, amounts to another notch in the belt.