WDPN31 PGTW 021500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (CHOI-WAN) 
WARNING NR 016//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 04W (CHOI-WAN), LOCATED APPROXIMATELY 51 
NM SOUTH-SOUTHWEST OF MANILA, PHILIPPINES, HAS TRACKED NORTHWESTWARD 
AT 
09 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) 
SATELLITE IMAGERY AND ANIMATED RADAR DATA FROM PAGASA, SHOW THAT TD 
04W HAS BECOME A BIT MORE ORGANIZED OVER THE PAST SIX HOURS, WITH THE 
RADAR DATA SHOWING THE NASCENT CONVECTIVE BANDING TO GOOD EFFECT. 
WHILE THE RADAR DATA IS FAIRLY CONCLUSIVE, THE SATELLITE DEPICTION 
STILL SHOWS A RATHER DISORGANIZED STRUCTURE, WITH MODERATE NORTHERLY 
VWS AND SOME MID-LEVEL DRY AIR INHIBITING CONVECTIVE DEVELOPMENT ON 
THE NORTH SIDE OF THE SYSTEM, WHILE A MASS OF FLARING CONVECTION HAS 
DEVELOPED TO THE WEST OVER THE SOUTH CHINA SEA. THE INITIAL POSITION, 
OVER THE SOUTHERN PORTION OF LUZON NEAR CALATAGAN, IS ASSESSED WITH 
HIGH CONFIDENCE BASED PRIMARILY ON THE WEAK SPIRAL BANDS EVIDENT IN 
THE ANIMATED RADAR DATA AND SURFACE OBSERVATIONS. THE INITIAL 
INTENSITY REMAINS SET AT 30 KTS, HEDGED SLIGHTLY LOWER THAN A DVORAK 
CURRENT INTENSITY ESTIMATE OF T2.5 (35 KTS) FROM RJTD DUE TO THE 
OVERALL LACK OF A DISTINCT CONVECTIVE CORE AND THE OVERALL 
DISORGANIZED NATURE OF THE CONVECTION. ENVIRONMENTAL CONDITIONS HAVE 
REMAINED WEAKLY FAVORABLE FOR ADDITIONAL DEVELOPMENT, CHARACTERIZED BY 
VERY WARM (30-31C) SSTS, GOOD EQUATORWARD AND WESTWARD OUTFLOW AND 
MODERATE (15-20 KT) VWS. THE UPPER-LEVEL TROUGH THAT HAS BEEN CLOSELY 
TRACKING THE LLCC HAS BEGUN TO DISSIPATE, BUT ITS EFFECTS ARE STILL 
SEEN IN THE DOWNWARD VERTICAL MOTION ON THE NORTHERN PORTION OF THE 
SYSTEM. TD 04W IS TRACKING NORTHWEST ALONG THE WESTERN PERIPHERY OF 
THE DEEP STR TO THE NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. OVER THE NEXT 24 HOURS, TD 04W IS FORECAST TO CONTINUE TRACKING 
NORTHWESTWARD ALONG THE SOUTHWEST PERIPHERY OF THE STEERING RIDGE, 
SKIRTING THE WESTERN COAST OF LUZON. DURING THE SAME TIMEFRAME, THE 
AFOREMENTIONED UPPER-LEVEL TROUGH WILL DISSIPATE, ALLOWING FOR 
DEVELOPMENT OF DIFFLUENT EASTERLY FLOW ALOFT OVER TOP OF THE SYSTEM, 
LEADING TO A SHORT PERIOD OF INTENSIFICATION THOUGH TAU 24. THE SYSTEM 
IS EXPECTED TO REACH A PEAK INTENSITY OF 35 KTS BY TAU 12 UNDER THESE 
SLIGHTLY IMPROVED CONDITIONS. BY TAU 36 THE SYSTEM WILL HAVE ROUNDED 
THE RIDGE AXIS AND BEGIN ACCELERATING NORTHEASTWARD TO THE EAST OF 
TAIWAN. AS IT MOVES POLEWARD IT WILL ENCOUNTER STEADILY INCREASING VWS 
AND CONVERGENT UPPER-LEVEL FLOW ALONG THE SOUTH SIDE OF A 200MB 
ANTICYCLONE, LEADING TO STEADY WEAKENING AND ULTIMATE DISSIPATION NO 
LATER THAN TAU 72 AS THE SYSTEM NEARS THE SOUTHERN RYUKU ISLANDS. 
DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH THE DURATION OF 
THE FORECAST, WITH A MAXIMUM CROSS-TRACK SPREAD OF 100 NM AT TAU 36 AS 
THE SYSTEM ROUNDS THE RIDGE AXIS. CONSISTENT WITH EXPECTATIONS FOR 
RECURVING SYSTEM, ALONG-TRACK SPREAD INCREASES STEADILY AFTER TAU 48 
AS THE SYSTEM ACCELERATES NORTHEASTWARD. THE JTWC TRACK LIES ON THE 
WESTERN SIDE OF THE GUIDANCE ENVELOPE THROUGH THE DURATION OF THE 
FORECAST. SOME NEAR-TERM DEVIATION RIGHT OR LEFT OF THE FORECAST TRACK 
IS EXPECTED AS THE LLCC INTERACTS WITH THE SHORELINE OF WESTERN LUZON 
BUT CONFIDENCE IN THE OVERALL TRACK IS HIGH.//
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