WDPN32 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 21W (TWENTYONE), LOCATED APPROXIMATELY 163 NM
WEST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 18 KNOTS OVER
THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY
SHOWS THE CENTRAL CONVECTION HAS GREATLY DEEPENED BUT THE WRAP INTO
AN OBSCURED LOW LEVEL CIRCULATION REMAINS LOOSE. THE CURRENT POSITION
IS PLACED WITH GOOD CONFIDENCE BASED A COMPOSITE RADAR LOOP FROM
PAGASA AND LINES UP WITH A SEMI-CLOSED CURVED BAND FEATURE IN THE
121025Z SSMIS. THE INITIAL INTENSITY OF 30 KNOTS IS BASED ON AN
OVERALL ASSESSMENT OF AGENCY AND OBJECTIVE DVORAK ESTIMATES.
ENVIRONMENTAL ANALYSIS INDICATES THE SYSTEM IS IN AN AREA OF LIGHT
(05-10 KNOT) VERTICAL WIND SHEAR AND GOOD EQUATORWARD OUTFLOW. IT IS
ALSO TRACKING OVER WARM (29-30 CELSIUS) SEA SURFACE TEMPERATURES. 
THE CYCLONE IS TRACKING UNDER THE INFLUENCE OF A SUB-TROPICAL RIDGE 
(STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM
THE PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TD 21W WILL TRACK WEST-NORTHWESTWARD TOWARD THE GULF OF TONKIN
UNDER THE INFLUENCE OF THE STR WHICH WILL REMAIN THE PRIMARY STEERING
MECHANISM FOR THE DURATION OF THE FORECAST. THE AFOREMENTIONED
MARGINALLY FAVORABLE ENVIRONMENT WILL CAUSE STEADY INTENSIFICATION TO
A PEAK OF 60 KNOTS BY TAU 48. 
   C. AFTER TAU 72, TD 21W WILL CONTINUE TO TRACK TO THE WEST-
NORTHWEST UNDER THE INFLUENCE OF THE STR, MAKE A FINAL LANDFALL OVER 
NORTHERN VIETNAM THEN DRAG INTO NORTHERN CAMBODIA. INTERACTION WITH 
TERRAIN WILL CAUSE ITS GRADUAL DECAY AND EVENTUAL DISSIPATION OVER 
LAND BY TAU 120. DYNAMIC MODEL GUIDANCE IS IN OVERALL GENERAL 
AGREEMENT, LENDING FAIR CONFIDENCE IN THE JTWC TRACK FORECAST.//
NNNN