MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 01W (VONGFONG) WARNING NR 017// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 01W (VONGFONG), LOCATED APPROXIMATELY 96 NM NORTH OF MANILA, PHILIPPINES, HAS TRACKED NORTH-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. TROPICAL STORM (TS) 01W (VONGFONG), LOCATED APPROXIMATELY 38 NM NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED NORTH-NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY (EIR) SHOWS DEEP CONVECTION CONTINUING TO ERODE AS THE LOW-LEVEL CIRCULATION CENTER (LLCC) INTERACTS WITH THE MOUNTAINOUS ISLAND OF LUZON. THE INITIAL INTENSITY IS ASSESSED AT 45 KNOTS BASED ON MULTI-PLATFORM CIRA WINDFIELD ANALYSES WITH DVORAK ANALYSES RANGING FROM T3.5 TO T2.5 FROM RJTD AND RCTP OVER THE LAST 6 HOURS. THE INITIAL POSITION IS BASED WITH FAIR CONFIDENCE BASED ON SURFACE OBSERVATIONS AND ANIMATED EIR. POLEWARD OUTFLOW IS GOOD AHEAD OF AN APPROACHING MID-LATITUDE TROUGH, NEARBY SEA SURFACE TEMPERATURES (SST) ARE WARM (30-31 DEGREES CELSIUS), AND VERTICAL WIND SHEAR IS MODERATE (15-20 KNOTS). TS 01W IS TRACKING ALONG THE WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) TO THE EAST. 3. FORECAST REASONING. A. THERE HAS BEEN NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY, EXCEPT OVER-WATER DISSIPATION IS ANTICIPATED BEYOND TAU 48 RATHER THAN EXTRATROPICAL TRANSITION. B. TS 01W IS FORECAST TO TRACK NORTHWARD ALONG THE WESTERN PERIPHERY OF THE STEERING STR THROUGH TAU 12 TO 24. AN UPPER-LEVEL MID-LATITUDE TROUGH IS APPROACHING FROM THE NORTHWEST, WHICH WILL ERODE THE STR BEYOND TAU 24. THIS WILL ALLOW 01W TO TURN TOWARD THE NORTHEAST AND ACCELERATE, WITH GOOD AGREEMENT AMONG THE DYNAMICAL MODEL GUIDANCE. TS 01W IS FORECAST TO CONTINUE TO WEAKEN, EVEN AFTER IT EMERGES BACK OVER WARM WATER, DUE TO INCREASED VERTICAL WIND SHEAR AHEAD OF THE APPROACHING TROUGH. BEYOND TAU 36, TS 01W WILL ACCELERATE AND BEGIN TO DISSIPATE OVER WATER AS IT IS ABSORBED BY THE BAROCLINIC ZONE. EXTRATROPICAL TRANSITION IS NOT EXPECTED, GIVEN THE DECOUPLED STRUCTURE OF TS 01W AND WEAK UPPER-LEVEL DYNAMICS. DEPENDING ON HOW MUCH OF THE WARM-CORE STRUCTURE OF TS 01W REMAINS AFTER IT EMERGES FROM LUZON, TS 01W COULD DISSIPATE SOONER THAN FORECAST. GIVEN THE TIGHT CLUSTERING IN DYNAMICAL MODEL GUIDANCE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. // NNNN NNNN