Tropical Storm CHOI-WAN Advisory Qui, 03-06

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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