msgid/genadmin/navpacmetoccen Pearl Harbor hi/jtwc//
subj/prognostic reasoning for tropical storm 09w warning nr04//
rmks/wdpn31 pgtw 130300
1. For meteorologists.
2. 12 hour summary and analysis.
   A. Tropical Storm (TS) 09w (Sepat) has developed and
intensified over the past 12 hours, as depicted by a 122127z
SSMI microwave satellite image which revealed a banding eye
feature. Outflow has remained favorable and the vertical wind
shear has reduced slightly. The storm has remained in a weak
steering environment and has track slowly west-southwestward.
   B. Ts 09w remains located at the southeastern end of low to
mid-layer troughing extending toward the tropics from the east
China sea. The current position is based on satellite fixes
from pgtw and rjtd, as well as a recent 122335z ssmis microwave
satellite image. Dvorak satellite intensity estimates of 3.5
from pgtw and 3.0 from rjtd support an intensity of 50 knots.
The aforementioned microwave banding eye feature also supports
this intensity estimate. An upper level anticyclone that was to
the southwest of the storm is beginning to build over the storm.
Vertical wind shear is low to moderate and radial outflow continues
to improve.
   A. The forecast philosophy for ts 09w has changed slightly
from the previous prog reasoning. Due to recent intensification,
the intensity forecast has been increased at all Taus.
   B. Ts 09w is expected to move slowly westward to west-
southwestward over the next 12 to 24 hours. After that time,
strong 700 mb to 500 mb ridging is expected to develop to the
north of the circulation, which will initiate a more rapid
westward track through tau 72. The storm will continue to in-
tensify as the anticyclone builds over the system and vertical
wind shear decreases. Poleward outflow into the upper low to
the north will remain strong in the near term and radial
outflow will increase through the forecast period. Overall,
the upper level environment is forecast to improve and
therefore intensification will occur at a climatological rate
through tau 72.
   C. In the extended period, the storm is expected to track
slightly poleward in response to an extensive trough over
eastern China. The subtropical ridge is expected to extend
eastward enough to prevent a recurvature of the storm before
it reaches the coast of China, but a general northwestward
track is expected. Intensification will be slowed by a slight
increase in vertical wind shear and the extension of the sub-
tropical ridge will begin to decrease the poleward outflow.
A large spread in the numerical models remains, although, the
numerical aids are in better agreement than they previously were.
The EGRR, GFDN, ECMWF and JTYM depict a weaker ridge and a sharper
poleward turn. The NOGAPS, necp GFS, WBAR and JGSM display a
scenario in which the subtropical ridge is stronger and extends
farther eastward, resulting in a more westward track initially
with a later poleward turn. This forecast continues to be slightly
west of the track consensus in the extended period, favoring the
solutions depicting stronger and more continuous subtropical
ridging to the north.
Forecast team: Bravo//