MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (TEMBIN) WARNING NR 40// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 15W (TEMBIN), LOCATED APPROXIMATELY 155 NM NORTHEAST OF TAIPEI, TAIWAN, HAS TRACKED NORTH-NORTHEASTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY, IN ADDITION TO RADAR IMAGERY FROM ISHIGAKIJIMA, JAPAN, SHOWS THE LOW LEVEL CIRCULATION CENTER (LLCC) REMAINS ON THE NORTHEASTERN QUADRANT OF A LARGE, BROAD REGION OF DEEP CONVECTION. BASED ON A 281328Z ASCAT PASS, THE WINDS NEAR THE CENTER ARE STRONGEST WITHIN THE SOUTHEASTERN QUADRANT OF THE LLCC, AND INDICATE SPEEDS AROUND 50 KNOTS. THIS CURRENT INTENSITY OF 50 KNOTS IS BASED ON THE ASCAT PASS, AND IS SLIGHTLY HIGHER THAN DVORAK ESTIMATES FROM PGTW BUT IS IN GOOD AGREEMENT WITH THE 3.0/3.5 DVORAK FROM RJTD. UPPER LEVEL ANALYSIS INDICATES TS 15W IS SLIGHTLY POLEWARD OF A SUBTROPICAL RIDGE (STR) AXIS THAT EXTENDS FROM CENTRAL JAPAN TO DIRECTLY OVER TAIWAN. THE UPPER LEVEL REFLECTION OF THE DEEP STR INDICATES WINDS FROM THE EAST ARE PRODUCING LOW TO MODERATE LEVELS OF VERTICAL WIND SHEAR (VWS) ALONG THE EASTERN AND NORTHEASTERN PORTIONS OF THE LLCC. TS 15W IS CURRENTLY TRACKING AROUND THE MID- LEVEL REFLECTION OF THE STR WHICH HAS BECOME THE DOMINANT STEERING MECHANISM. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 15W IS FORECAST TO TRACK NORTH-NORTHEASTWARD THROUGH TAU 36 UNDER THE INFLUENCE OF THE STR. THIS IS NOT EXPECTED TO SHIFT UNTIL THE SYSTEM MAKES LANDFALL AROUND TAU 36 AND SHIFTS MORE EASTWARND AS THE STR SHIFTS IN RESPONSE TO THE REMNANTS OF EXTRATROPICAL STORM 16W (BOLAVEN) TRACKING TO THE NORTH OF THE STR. TS 15W WILL STRUGGLE TO MAINTAIN INTENSITY OVER THE NEXT 12 HOURS IN AN INCREASINGLY HARSH UPPER LEVEL ENVIRONMENT AS VWS INCREASES AS TS 15W MOVES FURTHER POLEWARD OF THE UPPER LEVEL STR AXIS. SEA SURFACE TEMPERATURES (SSTS) REMAIN FAVORABLE AT 26 TO 28 CELSIUS AND WILL BE THE PRIMARY FACTOR ALLOWING TS 15W TO MAINTAIN INTENSITY FOR THE NEXT 12 HOURS. THE STEADY INCREASE IN VWS WILL BEGIN TO WEAKEN THE SYSTEM BEYOND TAU 12. SSTS ARE EXPECTED TO SLOWLY DECREASE THROUGH LANDFALL, APPROXIMATELY TAU 36, WHICH WILL ALSO FURTHER WEAKEN TS 15W. BY TAU 36 INCREASING INTERACTION WITH AN APPROACHING MID- LATITUDE TROUGH WILL BEGIN THE EXTRATROPICAL TRANSITION PROCESS. FRICTIONAL EFFECTS AND CONTINUED INTERACTION WITH THE TROUGH WILL LEAD TO COMPLETE EXTRATROPICAL SYSTEM BY TAU 48. MODEL GUIDANCE IS IN GOOD AGREEMENT WITH THE TRACK, BUT APPEARS TO SLOW THE SYSTEM AS IT APPOACHES THE KOREAN PENINSULA. MODEL FIELDS APPEAR TO INDICATE THE LLCC IS LOST WITHIN THE APPROACHING TROUGH, THUS LOSE THE ABILITY TO TRACK THE REMNANTS OF THE LLCC. BASED ON THIS ASSESSMENT, THE CURRENT FORECAST KEEPS THE TRACK FASTER THAN THE CONSENSUS OF MODEL GUIDANCE, BUT STILL HAS HIGH CONFIDENCE.// NNNN NNNN