Annularity of Katia:

Over the past several hours, Katia has gone from what appeared to be a disorganized cyclone to having a very large, well defined eye. NHC had Katia as disorganized this morning at the 5am update, however, it appears they misunderstood the structure of Katia.

Katia is following a very classic evolution to an Annular state, as described in Annular Hurricanes by Knaff, Kossin, and DeMaria. The specific example I am refering to is Hurricane Howard in 1998 which has had nearly identical evolution to Katia over the past day. A day before the annular transition Howard had a small, nearly pinhole eye. This is very similar to what was seen on Katia yesterday with a very tiny, often ill-defined eye.

The transition continued with both storms with what appeared to be dry air entrainment. However, in both storms this large dry air bulge actually became a new, giant eye. Over the course of 10 hours Howard completed the transition from giant eye to Annular. Katia is now about three hours into this transition.

In reference, I am attaching the following pictures:

Katia shown over a several hour period on September 5th, 2011

Extracted page from Annular Hurricanes by Knaff, Kossin, and DeMaria showing Hurricane Howard, 1998