Tuesday, September 13, 2011

Tropical Depression 18W Update #2

Contrary to our forecast yesterday, Tropical Depression 18W (Former Bagyong "Onyok") veered away from its westerly course and has now exited the Philippine Area of Responsibility. It is now moving east northeastward at around 10kph. Maximum sustained winds remain at 55kph gusting to 75kph.

So what is causing this abrupt and erratic movement? TD 18W right now is still weak and in terms of structure, is vertically "short" meaning it has not extended yet into the upper levels of the atmosphere. Normally tropical cyclones are steered by upper-air features--in the Pacific we have the Subtropical Ridge. Currently, TD 18W is weak and shallow enough that features in the mid and lower-levels of the atmosphere have higher influence on the movement instead of upper-levels. With 18W's case, we have a near-equatorial ridge to the south that is steering the storm to the east. However, we are still expecting TD 18W to resume its westward movement once it strengthens enough or at least becomes more influenced by the ridge to the north.

A look the VIS Image shows the partially exposed low-level circulation center moving under the intense convective activity (brighter white colors). I also outlined the blue arrows and the cyclonic circulation that shows the clouds trying to wrap around the storm center. I also included the boundary line of the Philippine Area of Responsibility.

VIS Image from NRLMRY

The general forecast for TD 18W remains largely unchanged. We are still expecting this TD to intensify into a tropical storm tomorrow and eventually affect Okinawa by Thursday or Friday. Computer models are actually in very good agreement with the short to medium-range timeline. They do, however, diverge quite a bit once we get into the weekend (or 5-day period). Some models take 18W towards Kyushu in Japan, while some take it south towards Taiwan. The difference will depend mainly on the strength of an approaching trough that would weaken the ridge. After that trough passes, we are expecting a ridge to rebuild in Eastern China and that would push 18W down towards Taiwan.

My forecast is slightly displaced north of JTWC's track and is in line with the model consensus.

We'll have another update tomorrow, so please stay tuned!
(Issued 0830 UTC) 430pm PhT

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