MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 012// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 283 NM SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHEASTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS AN EXPANSIVE CLOUD FIELD AND A BROAD EYE-LIKE FEATURE. THE INITIAL POSITION IS PLACE WITH FAIR CONFIDENCE BASED ON A 211601Z GMI 37GHZ MICROWAVE IMAGE, WHICH SHOWS THE BROAD NATURE OF THE SYSTEM. THE INTENSITY OF 60 KNOTS IS HEDGED BETWEEN DVORAK CURRENT INTENSITY (CI) ESTIMATES OF T4.0 (65 KNOTS) FROM PGTW AND T3.5 (55 KNOTS) FROM KNES. ENVIRONMENTAL ANALYSIS REVEALS THAT TS 18W IS EXPERIENCING NEUTRAL (15 TO 20 KNOT) VERTICAL WIND SHEAR. TS 18W HAS A ROBUST 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 STR THE TRACK WILL SHIFT FROM NORTH-NORTHEASTWARD TO NORTHEASTWARD. FAVORABLE SEA SURFACE TEMPERATURES AND STRONG POLEWARD OUTFLOW OFFSET BY INCREASING VERTICAL WIND SHEAR WILL ALLOW FOR A GRADUAL DECREASE IN INTENSITY. AS TS 18W TRACKS AROUND THE STR IT WILL BEGIN TO INTERACT WITH THE MIDLATITUDE WESTERLIES. INTERACTION WITH THE BAROCLINIC ZONE WILL CAUSE TS 18W TO BEGIN EXTRATROPICAL TRANSITION (ETT) BY TAU 24 AND 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