Australian Severe Weather Forum

I have been observing system 15W for a several days now and it has developed into a well organized super typhoon called Choi-wan.
 
sat 1 image was a day or so earlier also showing typhoon Koppu. You will noticed no eye has formed in Choi-wan at this stage ( Sat image courtesy of day image)

The second sat imagery posted will highlight the intense system with  deep convective banding wrapping into a center with a well developed eye. The warm sea surface temperature has the allowed the system to continue to intensify.

Yesterday 15/09/09, Choi-wan had sustained winds estimated at 230 kilometers per hour (145 miles per hour or 125 knots), according to the Joint Typhoon Warning Center. Choi-wan was strengthening. 24 hours later, the storm has reached Category 5 status with sustained winds of 260 km/hr (160 mph or 140 knots). The Joint Typhoon Warning Center expected the storm to intensify a little more.
 
Previous warning stated the wave height to be 38 feet but I would expect that to increase over the next few hours or day. JTWC expects the storm to undergo extra-tropical transitioning to the south of Japan.

(This transition usually occurs when the system nears its end of being tropical, which is usually between latitude 30deg and 40 deg. The storm will go from a warm core to a cold core storm. Extra-tropical, also known as mid latitude or baroclinic storms,get their energy from the horizontal temperature contrast that exists in the atmosphere. They are a low system with associated cold fronts, warm fronts and ocluded fronts.)  

At this stage according to computer models the super storm will track northwest and should miss the mainland. There are two small islands in its path.

Col

Attached is a great VIS image and cross section from NRL yesterday.

Over the last 6 or so hours (18/09/09) Super typhoon Choi-wan has weakened slightly into a cat 4 storm but is still rated as a super typhoon. It  is undergoing an interesting metamorphosis at present by developing a 2nd eye wall which is not uncommon for the waters it is in and the size of the storm.

Strong typhoons sometimes develop a double eye-walled structure, which means they undertake a process of an "eye wall replacement cycle" where a new eye wall develops and replaces an existing one. The cycle begins with a concentrated ring of convection that develops outside the eye wall. The ring of convection then propagates inward leading to a double-eye. The inner eye wall eventually dissipates while the outer intensifies and moves inward. The double-eye-walled structure usually marks the end of an episode of intensification and may last for a day or two. as the system's inner eyewall diminishes, it will temporarily weaken in intensity before restrengthening as the outer eyewall contracts.

Choi- wan will weaken as it leaves the tropics over the next few days and become extra-tropical as discussed in the first post. The wave height at the peak yesterday reached 41 feet.

You will notice in the sat images from NASA day image and JTWC the double eye wall forming.

Col

This page shows an animation of the eywall replacement cycle:

http://cimss.ssec.wisc.edu/tropic/real-time/marti/2009_15W/webManager/basicGifDisplay48.html