What is annular?
An annular hurricane is a very symmetric hurricane what very strong convection throughout its structure. This allows it to not have to undergo eyewall replacement cycles (where the older dry eyewalls get cycled out every once in a while). This means that an annular hurricane doesn't undergo the power fluctuations that most hurricanes have. It also looks like a donut. Here is a picture of katrina when she was annular:
http://commons.wikimedia.org/wiki/Image:NOAA-Hurricane-Katrina-Aug28-05-2145UTC.jpg
I don't think Rita is annular yet. You can see that she isn't as symmetric as Katrina was and she has a stronger spiral pattern:
From Wikipedia:
Annular hurricanes have a large, symmetric eye surrounded by a ring of intense convection, with hardly any convection (i.e. bands) elsewhere. These types of storms are not prone to fluctuations in intensity related to the eyewall replacement cycles typically seen in intense tropical cyclones.
http://en.wikipedia.org/wiki/Annular_hurricane
