WDPN31 PGTW 030300
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (CHOI-WAN) 
WARNING NR 018//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 541 NM
SOUTH-SOUTHWEST OF TAIPEI, TAIWAN, HAS TRACKED NORTHWESTWARD AT 15 
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY (MSI) SHOWS A SYSTEM THAT HAS DEEPENED, ALBEIT WITH A 
PARTIALLY EXPOSED LOW LEVEL CIRCULATION (LLC) AS THE BROAD CENTRAL 
CONVECTION HAS BECOME OFFSET TO THE SOUTHWEST. THE INITIAL POSITION 
IS PLACED WITH HIGH CONFIDENCE USING LOW CLOUD TRACING IN THE MSI 
LOOP AND EXTRAPOLATED FROM NEARBY SURFACE OBSERVATIONS. THE INITIAL 
INTENSITY OF 35 KTS IS SET WITH FAIR CONFIDENCE BASED ON AN OVERALL 
ASSESSMENT OF DVORAK ESTIMATES OF T2.0/30KTS (PGTW), T2.5/35KTS 
(ADT), AND SATELLITE CONSENSUS (SATCON) OF 43KTS AND REFLECTS THE 
DEEPENED STATE. THE SYSTEM IS IN A MARGINAL ENVIRONMENT WITH GOOD 
WESTWARD AND EQUATORWARD OUTFLOW AND VERY WARM (30-31C) SEA SURFACE 
TEMPERATURES, OFFSET BY MODERATE TO STRONG (20-25KT) VERTICAL WIND 
SHEAR FROM THE NORTHEAST. THE CYCLONE IS TRACKING ALONG THE WESTERN 
PERIPHERY OF THE SUBTROPICAL RIDGE (STR) TO THE EAST-NORTHEAST. 
3. FORECAST REASONING.
   A. THERE IS A CHANGE TO THE FORECAST PHILOSOPHY. THE FORECAST 
TRACK HAS BEEN EXTENDED TO TAU 120 TO INCLUDE EXTRA-TROPICAL 
TRANSITION.
   B. TS 04W WILL CONTINUE TO TRACK NORTH-NORTHWESTWARD OVER THE 
NEXT 12 HOURS, MAKING THE TURN NORTH-NORTHEASTWARD THEN 
NORTHEASTWARD AFTER TAU 12 UNDER THE STEERING INFLUENCE OF THE STR. 
THE SYSTEM IS FORECAST TO REMAIN AT 35KTS UNDER THE AFOREMENTIONED 
MARGINAL CONDITIONS AS IT ROUNDS THE RIDGE AXIS. BY TAU 36, TS 04W 
WILL WEAKEN TO 30 KTS DUE TO INCREASING VWS ASSOCIATED WITH STRONG 
NORTHEASTERLY FLOW FROM A SECONDARY STR ANCHORED OVER SOUTHEASTERN 
CHINA. THE SYSTEM WILL REMAIN AT 30 KTS AS IT PASSES NORTH OF KADENA 
AIR BASE BEFORE TAU 48 AND UP TO TAU 72 AS IT CONTINUES 
NORTHEASTWARD. THE FORECAST TRACK IS LAID IN GOOD AGREEMENT WITH MODEL 
CONSENSUS FOR THE FIRST 24 HOURS. AFTER TAU 24, THE FORECAST TRACK IS 
MORE TO THE RIGHT OF FORECAST CONSENSUS TO OFFSET NAVGEM BEING THE 
EXTREME LEFT OUTLIER WITH A NORTHWARD TRACK TOWARD THE TAIWAN STRAIT.
   C. AFTER TAU 72, TS CHOI-WAN WILL ACCELERATE NORTHEASTWARD ON THE 
POLEWARD SIDE OF THE STEERING STR. CONCURRENTLY BY TAU 96, IT WILL 
BEGIN EXTRA-TROPICAL TRANSITION (ETT) AS IT ENTERS THE BAROCLINIC 
ZONE AND WILL COMPLETE ETT SOUTH OF YOKOSUKA, JAPAN, BY TAU 120. 
THERE IS A DISTINCT POSSIBILITY THAT TS 04W WILL DISSIPATE BEFORE 
ETT. NUMERICAL MODELS REMAIN IN GOOD AGREEMENT, LENDING FAIR 
CONFIDENCE TO THE EXTENDED PORTION OF THE JTWC TRACK FORECAST.//
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