MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 007// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 18W (TAPAH), LOCATED APPROXIMATELY 147 NM SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTH-NORTHEASTWARD AT 11 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS CONVECTIVE BANDING WRAPPING INTO A PARTIALLY EXPOSED LOW LEVEL CIRCULATION (LLC) WHICH APPEARS TO HAVE BECOME MORE SYMMETRICAL OVER THE PAST SIX HOURS. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON A 200945Z F-17 SSMIS 91 GHZ MICROWAVE IMAGE DEPICTING A LLC WITH DEEP CONVECTION EXTENDING TO THE SOUTH AND SOUTHEAST. EIR REVEALS THAT THIS CONVECTION IS WRAPPING INTO THE NORTHERN AND EASTERN QUADRANTS OF THE SYSTEM. THE INITIAL INTENSITY OF 50 KNOTS IS BASED ON DVORAK ESTIMATES OF T3.0 (50 KTS) BY RJTD AND T3.5 (55 KTS) BY PGTW. DUE TO THE BROAD NATURE OF THE SYSTEM, DVORAK ESTIMATES HAVE BEEN CONSISTENTLY LOW THROUGHOUT THE PREVIOUS FORECAST PERIODS, LENDING GOOD CONFIDENCE TO THE INITIAL INTENSITY ESTIMATE. UPPER LEVEL ANALYSIS INDICATES THAT TS 18W HAS MOVED INTO A REGION OF LOW TO MODERATE (10-15 KTS) VERTICAL WIND SHEAR (VWS) WITH MODERATE EQUATORWARD OUTFLOW. THE WARM (28-29 CELSIUS) SEA SURFACE TEMPERATURE (SST) REMAINS CONDUCIVE FOR INTENSIFICATION AS TS 18W TRACKS NORTH- NORTHWESTWARD ALONG THE PERIPHERY OF A SUBTROPICAL RIDGE (STR) LOCATED TO THE NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. FOLLOWING ITS PRIMARY STEERING MECHANISM, TS 18W WILL TRACK NORTH-NORTHWESTWARD ALONG THE STR FOR THE NEXT 24 HOURS. ALONG THIS ROUTE, TS 18W WILL INTENSIFY TO ITS PEAK INTENSITY (65 KTS) BY TAU 36. BETWEEN TAU 30 AND 36, A MID-LATITUDE TROUGH, CURRENTLY LOCATED TO THE NORTHWEST, WILL MOVE NORTH OF THE SYSTEM AND BEGIN TO ENHANCE POLEWARD OUTFLOW PRIOR TO THE SYSTEM BECOMING EMBEDDED IN THE TROUGH. CONCURRENTLY, TS 18W WILL ROUND THE STR AXIS AND BEGIN A NORTHEAST TRACK TOWARDS THE SEA OF JAPAN (SOJ). AT TAU 48, VWS WILL INCREASE AND SST WILL BEGIN TO COOL, SLOWLY ERODING THE SYSTEM AS IT INTERACTS WITH LAND UPON TRANSITING INTO THE SOJ. BEFORE TAU 72, TS 18W WILL BEGIN EXTRATROPICAL TRANSITION (ETT) AS IT ACCELERATES ALONG A NORTHEAST TRACK. NUMERICAL GUIDANCE IS IN GOOD AGREEMENT THROUGH THIS PORTION OF THE FORECAST, SUPPORTING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. C. AFTER TAU 72, TS 18W WILL CONTINUE TO WEAKEN DUE TO HIGH VWS VALUES AS IT BECOMES EMBEDDED IN THE MID-LATITUDE TROUGH. PRIOR TO MAKING LANDFALL AT HOKKAIDO, TS 18W WILL TRANSITION INTO AN EXTRATROPICAL CYCLONE. THE SYSTEM WILL RETURN TO THE PACIFIC OCEAN BY TAU 84 AND CONTINUE ON A NORTHEASTERLY COURSE FOR THE REMAINDER OF THE FORECAST. MODEL GUIDANCE IS IN GOOD AGREEMENT FOR THE EXTENDED PORTION OF THE FORECAST, LENDING TO HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN