WDPN31 PGTW 010900
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING 
NR 010//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 394 NM 
EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTH-
NORTHWESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED 
MULTISPECTRAL SATELLITE IMAGERY DEPICTS A TIGHT, WELL-DEFINED AND 
FULLY EXPOSED LOW-LEVEL CIRCULATION (LLC) WITH FLARING DEEP 
CONVECTION DEVELOPING ALONG A CONVERGENT BOUNDARY IN THE 
SOUTHWESTERN QUADRANT OF THE CIRCULATION. THIS CONVECTIVE ACTIVITY 
IS BEING FURTHER SHEARED AWAY FROM THE LLC BY MODERATE TO STRONG (20-
25 KTS) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) ASSOCIATED WITH A 
MESOSCALE TUTT CELL LOCATED JUST TO THE EAST OF THE LLCC. THE 
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE FULLY 
EXPOSED LLCC JUST OFFSHORE THE SOUTHEASTERN TIP OF SAMAR. THE 
INITIAL INTENSITY HAS BEEN LOWERED TO 35 KNOTS WITH MODERATE 
CONFIDENCE, IN LINE WITH MULTI-AGENCY DVORAK CURRENT INTENSITY 
ESTIMATES RANGING BETWEEN T2.0 (30KTS) AND T2.5 (35 KTS) FROM PGTW, 
RJTD AND KNES, AND A T2.7 ADT ESTIMATE. OVERALL THE ENVIRONMENT HAS 
BECOME INCREASINGLY HOSTILE, WITH THE STRONG (20-25 KTS) VWS AND 
CONVERGENT FLOW ALOFT ASSOCIATED WITH THE TUTT CELL OFFSETTING THE 
VERY WARM (30C) SSTS AND GOOD EQUATORWARD OUTFLOW. TS 04W HAS FULLY 
DECOUPLED AND THE LLCC IS MOVING SOMEWHAT ERRATICALLY, WITH A 
GENERAL TREND TOWARDS THE NORTH-NORTHWEST ALONG THE WESTERN 
PERIPHERY OF THE DEEP STR TO THE NORTH AND NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 04W IS FORECAST TO CONTINUE TRACKING NORTH-NORTHWESTWARD, 
SKIRTING THE EASTERN PORTIONS OF SAMAR AND SOUTHEASTERN LUZON BEFORE 
MOVING OVER NORTH-CENTRAL LUZON BY TAU 48. THROUGH TAU 24, THE 
AFOREMENTIONED TUTT CELL WILL CLOSELY FOLLOW THE TRACK OF THE LLCC, 
PRODUCING CONVERGENT FLOW OVER TOP OF THE SYSTEM AND PROHIBITING ANY 
REESTABLISHMENT OF CORE CONVECTION OR INTENSIFICATION. BY TAU 36 THE 
TUTT CELL WILL DISSIPATE, LEADING TO A MARGINALLY MORE SUPPORTIVE 
OUTFLOW ENVIRONMENT, BUT LACKING SUFFICIENT TIME OVER WATER, THERE 
WILL BE LITTLE TO NO INTENSIFICATION PRIOR TO LANDFALL ALONG THE 
SOUTHEAST COAST OF NORTHERN LUZON JUST AFTER TAU 36. ONCE ASHORE, 
THE RUGGED TERRAIN OF NORTHERN LUZON WILL SERVE TO DISRUPT THE LOW 
LEVEL CIRCULATION (LLC), LEADING TO A SLOW WEAKENING TREND. THE 
REMAINING LLC WILL EMERGE INTO THE LUZON STRAIT AROUND TAU 60 AND 
BEGIN ACCELERATING NORTHEASTWARD INTO A BREAK IN THE STR. THE 
MAJORITY OF AVAILABLE NUMERICAL MODEL GUIDANCE HAS SHIFTED SLIGHTLY 
EASTWARD, AND THE BULK OF THE 12Z ECMWF ENSEMBLE MEMBERS INDICATE A 
TRACK ALONG THE EASTERN SIDE OF LUZON. THE JTWC FORECAST TRACK LIES 
ALONG THE EASTERN SIDE OF THE GUIDANCE ENVELOPE THROUGH TAU 72 WITH 
MODERATE CONFIDENCE.
   C. IN THE EXTENDED PERIOD, THE WESTERN PORTIONS OF THE STR WILL 
CONTINUE TO ERODE ALLOWING THE SYSTEM TO RECURVE NORTHEASTWARD. 
WHILE SSTS ARE SUFFICIENTLY WARM IN THE LUZON STRAIT TO SUPPORT A 
TROPICAL CYCLONE, THE IMPACT OF RAPIDLY INCREASING VWS ASSOCIATED 
WITH A MID-LATITUDE SHORTWAVE TROUGH EXPECTED TO DIG OVER THE EAST 
CHINA SEA, WILL SERVE TO LIMIT ANY POTENTIAL INTENSIFICATION. THE 
SYSTEM IS EXPECTED TO MAINTAIN INTENSITY AS IT RACES NORTHEASTWARD 
WITH THE EFFECTS INCREASED POLEWARD OUTFLOW OFFSETTING THE HIGH VWS. 
TS 04W IS EXPECTED TO BEGIN EXTRA-TROPICAL TRANSITION (ETT) NEAR TAU 
96 BUT MAY DISSIPATE PRIOR TO COMPLETING ETT NEAR TAU 120. MODEL 
GUIDANCE REMAINS IN RELATIVELY GOOD AGREEMENT IN THE ACROSS-TRACK 
SOLUTIONS THROUGH TAU 120, BUT SHOWS SIGNIFICANT ALONG-TRACK DELTAS 
AFTER TAU 72 WITH NEARLY 1000NM ALONG-TRACK SPREAD BY TAU 120 
BETWEEN THE GFS AND UKMET ENSEMBLE MEAN. THE JTWC FORECAST IS 
POSITIONED ON THE EASTERN EDGE OF THE GUIDANCE ENVELOPE AND SLIGHTLY 
SLOWER THAN THE CONSENSUS MEAN. IN LIGHT OF THE UNCERTAINTY 
ASSOCIATED WITH THE IMPACT OF THE FORECASTED LAND INTERACTION AND 
THE DOWNSTREAM EFFECTS OF THAT INTERACTION, THERE IS LOW CONFIDENCE 
IN THE EXTENDED PORTION OF THE JTWC FORECAST.//
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