subj/prognostic reasoning for tropical depression 21w warning nr 20//
rmks/
1. For meteorologists.
2. 12 hour summary and analysis.
   A. Tropical depression (td) 21w (peipah) has continued to weaken
over the past 12 hours and has fallen below tropical storm intensity.
Animated multispectral and water vapor imagery depicts a lack of deep
convection associated with the system. The weakening was due to the 
continued entrainment of drier Continental air and the loss of good
poleward outflow. The storm has increased track speed to the south-
west over the past 12 hours. 
   B. The current storm intensity is based on Dvorak satellite in-
tensity estimates that range from 25 to 30 knots. Current upper level
analysis depicts weak divergence aloft and weak vertical wind shear.
Poleward outflow has decreased significantly over the past 12 hours
and equatorward outflow remains hindered. TD 21w has tracked south-
westward at 07 knots over the past 06 hours.
   A. The forecast philosophy has only slightly changed since the
last prognostic reasoning. Forecast intensities have been lowered
due to the weakening trend over the past 12 hours. The system is 
dissipating earlier than prior forecasts as well.
   B. The low to mid-level subtropical ridge to the northwest will
remain the primary steering influence. This will cause the system to
track generally southwestward throughout the forecast period. Outflow
will continue to be hindered, which will result in a continued weak-
ening of the system. Marginal ocean heat content along the forecast
track will also contribute to the weakening of the system. The system
will completely dissipate by tau 36 with the remnants tracking over
southeast Vietnam prior to tau 48.
Forecast team: Bravo//