subj/prognostic reasoning for tropical storm 20w warning nr 4//
rmks/
1. For meteorologists.
2. 12 hour summary and analysis.
   A. The system was upgraded to Tropical Storm (TS) status
at 29/06z based on animated metsat imagery indicating a much 
tighter, well-consolidated low-level circulation center (LLCC),
and significantly improved convective banding wrapping into the
southern semi-Circle of the storm. This improved banding is best
seen in the 290610z amsre image which depicts strong deep con-
vection wrapping into the center. Recent storm motion indicates that
the steering ridge has strengthened over the past 12 hours as ts 19w
moves further north. Thus, ts 20w has deviated significantly from 
the 29/00z forecast track which guided the storm over Hainan Island, 
and has moved west-northwestward towards north-central Vietnam over 
the past 12 hours. The dynamic models are in good agreement and in-
dicate a northwestward track into Vietnam.   
   B. Ts 20w is currently tracking northwestward under the steering
influence of the low- to mid-level subtropical ridge (str) positioned
north of the system. The current position is based on a well-defined 
LLCC evident in animated infrared satellite imagery. Outflow remains
primarily limited to the equatorward periphery of the system due to
the continued effects of the northeasterly flow originating from ts
19w which continues to impinge on the northeastern quadrant. The 
current intensity is based on Dvorak estimates of 45 knots from pgtw
and rjtd along with a 291109z qscat pass depicting unflagged 40 knots
winds near the center.
3. Forecast reasoning.
   A. Due to the previously mentioned deviation in track resulting
from the strengthening of the str to the north, this forecast rep-
resents a more westward track than was depicted in the previous
prognostic reasoning.
   B. Ts 20w is forecast to continue tracking west-northwestward
towards north-central Vietnam under the steering influence of the
str north of the system. Ts 20w will maintain intensity under gen-
erally favorable conditions, and ultimately make landfall approxi-
mately 160 nm south of hanoi, Vietnam near tau 24. As the system 
tracks inland it is expected to weaken significantly, dissipating
over land by tau 48. The available dynamic models are in good agree-
ment with this forecast. This forecast favors the consensus of avail-
able model aids which are in fair agreement throughout the forecast
period.
Forecast team: Charlie//