1. For meteorologists.
2. 12 hour summary and analysis.
   A. Typhoon (TY) 21w (peipah) has continued to intensify under the
combined influences of low vertical wind shear, favorable upper level
outflow and high ocean heat content. A banding eye has appeared in
recent microwave satellite imagery, including a 040938z windsat pass.
Interaction with the island of Luzon is disrupting the western portion
of the storm circulation, hampering development of associated convec-
tion in that region of the storm.
   B. The current storm intensity is based on Dvorak satellite inten-
sity T-numbers of 4.0/4.0 from both pgtw and rjtd. Storm position is
based on satellite position estimates from pgtw and rjtd and a host
of recent microwave satellite images. Recent animated water vapor
satellite imagery reveals that a well-developed mesoscale anticyclone
aloft is providing good radial outflow and that a strong poleward
outflow channel discussed in previous prognostic reasoning bulletins
persists.
3. Forecast reasoning.
   A. The forecast reasoning has not changed significantly since the
previous issue of the prognostic reasoning bulletin.
   B. TY 21w is expected to continue tracking westward for the next
24 hours under the steering influence of a western Pacific subtropi-
cal ridge extension to the north. A mid-latitude trough moving east-
Ward through southeastern China is expected to transit eastward over
the next day, allowing ridging to build in its wake over the northern
South China Sea. This second, developing ridge is expected to influ-
ence storm motion after tau 24, inducing both a reduction in forward
track speed and a gradual turn toward the southwest. As the steering
influence of the second steering ridge becomes more pronounced after
tau 48, track speeds will increase. TY 21w will weaken over central
Luzon during the next several hours before reemerging into the south
China sea around tau 12. Thereafter, the storm is expected to rein-
tensify slowly as increasing vertical wind shear and marginal ocean
heat content counter the effect of favorable equatorward outflow. 
   C. The storm will continue tracking generally southwestward in
the extended Taus with a slight increase in the westward component
of motion occurring as the steering ridge axis assumes a more zonal
orientation to the north. The current forecast calls for landfall
in southeastern Vietnam between tau 96 and tau 120. As the system
tracks over land after tau 96, dissipation will begin.
Forecast team: Alpha.//