MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 009// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 108 NM WEST OF KADENA AB, HAS TRACKED NORTH-NORTHWESTWARD AT 13 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (AMSI) REVEALS CONVECTIVE BANDING, PRIMARILY ON THE EAST, WINDING INTO A BROAD, PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER. THERE IS HIGH CONFIDENCE IN THE INITIAL POSITION BASED ON THE AMSI, RADAR IMAGERY, AND SURROUNDING OBSERVATIONS. THE INITIAL INTENSITY OF 55 KTS IS ON THE HIGH SIDE OF DVORAK INTENSITY ESTIMATES RANGING FROM T3.0-T3.5 (45-55 KTS) DUE TO OKINAWA OBSERVATIONS UP TO 55 KTS. THE ENVIRONMENT, WITH MODERATE (10-15 KT) VERTICAL WIND SHEAR (VWS), GOOD EQUATORWARD AND POLEWARD OUTFLOW, AND WARM (27-28 CELSIUS) SEA SURFACE TEMPERATURE (SST), IS SUPPORTIVE OVERALL, HOWEVER, SOME DRY AIR ENTRAINING INTO AND SUPPRESSING CONVECTION ON THE WESTERN HALF OF THE SYSTEM IS EXPECTED TO LIMIT FURTHER INTENSIFICATION. TS 18W IS TRACKING POLEWARD ALONG THE WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR). 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. AROUND TAU 24, TS 18W WILL HAVE ROUNDED THE STR AXIS AND BEGIN TURNING NORTHEASTWARD. THERE IS A BRIEF PERIOD WHERE THE ENVIRONMENT IS MARGINALLY SUPPORTIVE BEFORE STRONG VWS, LAND INTERACTION, AND COOL SST BEGIN TO CAUSE GRADUAL WEAKENING. BY TAU 48, TS 18W WILL BECOME ENTRENCHED IN THE MID-LATITUDE BAROCLINIC ZONE AND BEGIN EXTRATROPICAL TRANSITION (ETT). AFTER COMPLETING ETT (NO LATER THAN TAU 72), A GALE-FORCE COLD CORE LOW WILL REMAIN. NUMERICAL GUIDANCE IS IN STRONG AGREEMENT THROUGH TAU 36. AFTER TAU 36, CROSS-TRACK AGREEMENT REMAINS GOOD BUT ALONG-TRACK DIFFERENCES CAUSE THE SPREAD AMONG THE MODELS TO INCREASE. BASED ON STRONG AGREEMENT AND SHORT DURATION OF FORECAST, THERE IS GOOD CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN