MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 013// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 210 NM SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHEASTWARD AT 14 KNOTS OVER THE PAST SIX HOURS. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON A 220000Z HIMAWARI VISIBLE SATELLITE IMAGE, WHICH SHOWS A BROAD AND PARTIALLY EXPOSED LOW LEVEL CIRCULATION. THE INITIAL INTENSITY OF 55 KNOTS IS BASED ON A DVORAK CURRENT INTENSITY (CI) ESTIMATE OF T3.5 (55 KNOTS) FROM PGTW. ENVIRONMENTAL ANALYSIS REVEALS THAT TS 18W IS EXPERIENCING NEUTRAL TO UNFAVORABLE (20 TO 25 KNOT) VERTICAL WIND SHEAR. THE STRONG VERTICAL WIND SHEAR HAS CAUSED THE SYSTEM TO BEGIN TO DECOUPLE WITH THE REMAINING DEEP CONVECTION OFFSET TO THE NORTHEAST. TS 18W CONTINUES TO HAVE A STRONG POLEWARD OUTFLOW CHANNEL BUT LITTLE TO NO EQUATORWARD OUTFLOW. SEA SURFACE TEMPERATURES REMAIN FAVORABLE, BETWEEN 27 AND 28 DEGREES CELSIUS. TS 18W IS TRACKING NORTH-NORTHEASTWARD WHILE UNDER THE INFLUENCE OF A SUBTROPICAL RIDGE (STR) LOCATED TO THE EAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. THE AFOREMENTIONED STR WILL REMAIN THE PRIMARY STEERING FEATURE IN THE SHORT TERM. AS TS 18W TRACKS AROUND THE PERIPHERY OF THE STR THE TRACK WILL SHIFT FROM NORTH-NORTHEASTWARD TO NORTHEASTWARD. INCREASING VERTICAL WIND SHEAR WILL CAUSE TS 18W TO STEADILY DECREASE IN INTENSITY. BY TAU 24, INTERACTION WITH THE MIDLATITUDE WESTERLIES AND THE BAROCLINIC ZONE WILL CAUSE TS 18W TO BEGIN EXTRATROPICAL TRANSITION (ETT). TS 18W WILL COMPLETE ETT BY TAU 36. AFTER ETT, A GALE FORCE WIND FIELD WILL REMAIN AND RAPIDLY TRACK NORTHEASTWARD. DYNAMIC MODEL GUIDANCE REMAINS IN GOOD AGREEMENT THROUGH TAU 36 WITH AN INCREASING ALONG-TRACK SPREAD BY TAU 48. THE JTWC FORECAST TRACK IS PLACED NEAR THE MULTI-MODEL CONSENSUS. OVERALL, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN