Here's an excerpt:

The hardest thing to explain is when hurricanes become Annular.Very few hurricanes can reach this as Atmospheric condtions need to be nearly perfect,a good idea to see wether this is the case is wether the outflow is impressive right the way around the hurricane,which was infact the case in Isabel.Annular hurricanes are rare,however they tend to be very powerful in nature and the strongest hurricanes tend to be Annular in nature.Because of how hard it is to attain its very rare that a hurricane can become annular for more then 12hrs simply because the need for perfect enviormental condtions.

http://www.net-weather.co.uk/forum/index.php?showtopic=19418

Steve