subj/prognostic reasoning for typhoon 09w warning nr08//
rmks/
1. For meteorologists.
2. 12 hour summary and analysis.
   A. Typhoon (TY) 09w (Sepat) has continued to organize and 
intensify over the past 12 hours. Recent microwave satellite imagery,
including a 132104z SSMI 85ghz image, shows a well-defined eye
feature. Forward track speed has increased in response to a
developing subtropical steering ridge to the north.
   B. TY 09w continues to intensify under the influence of decreasing
vertical wind shear and good poleward and equatorward diffluence. An
upper level low to the north that has enhanced poleward outflow
continues to fill, and upper level ridging building in its place is
beginning to disrupt the favorable poleward outflow channel. Current
Dvorak satellite intensity estimates of 4.5 from pgtw, 5.0 from rjtd,
and analysis of storm signature in recent microwave satellite imagery
support an initial intensity of 80 knots.
3. Forecast reasoning.  
   A. The forecast philosophy for ts 09w has not changed
significantly since the last issue of the prognostic reasoning.
   B. TY 09w is expected to continue generally westward during
the next 24 hours under the influence of a developing subtropical
ridge to the north. Ridging is also forecast to build to the south
and east of the storm during this period and, after tau 24, will
begin to introduce a northward component to storm motion. With TY 09w 
vertical wind shear and equatorward diffluence are expected to remain
favorable through tau 72. However, poleward outflow may become more
restricted as subtropical ridging builds to the north. Therefore the
current forecast calls for a steady intensification at a less than
climatological rate.
   C. The trough currently extending southward along the east coast
of China is expected to move out over the next 48 hours. Therefore,
subtropical ridging is expected to remain strong to the north of the
system during the extended period. The orientation of this ridge will
induce a northwestward storm track between tau 72 and tau 120. During
the extended period, TY 09w is expected to weaken slightly in
response to lowering ocean heat content and slightly higher vertical
wind shear from the remnants of the baroclinic zone over eastern
China. Land fall over central Taiwan will further weaken and slow the
system.
Forecast team: Charlie//