MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 010// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 397 NM SOUTH-SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTHWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY REVEALS A CONSOLIDATING SYSTEM WITH A LARGE, 45NM DIAMETER RAGGED EYE AND EXTENSIVE SPIRAL BANDING OVER THE EASTERN SEMICIRCLE. A 21/0147Z ASCAT-B IMAGE SHOWS A LARGE, OBLONG CENTER WITH A SWATH OF 50-60 KNOT WINDS DISPLACED OVER THE EASTERN SEMICIRCLE. SURFACE OBSERVATIONS FROM OKINAWA INDICATE SUSTAINED 40-50 KNOT WINDS WITH GUSTS OF 70-80 KNOTS WITHIN THE SPIRAL BANDING. THERE IS GOOD CONFIDENCE IN THE CURRENT POSITION BASED ON THE EYE FEATURE AND HOURLY RADAR FIXES. THE 21/0423Z AMSR2 IMAGE SHOWS IMPROVED DEEP CONVECTIVE BANDING WRAPPING INTO A DEFINED LOW-LEVEL CIRCULATION CENTER. THE CURRENT INTENSITY IS INCREASED SLIGHTLY TO 60 KNOTS, WHICH IS CONSISTENT WITH THE PGTW DVORAK ESTIMATE OF T4.0 (65 KNOTS) AND RECENT SURFACE OBSERVATIONS. 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. THE 34-KNOT AND 50-KNOT WIND RADII HAVE BEEN ADJUSTED BASED ON A DETAILED ANALYSIS OF RECENT ASCAT IMAGERY. 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 24, TS 18W WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) AS IT INTERACTS WITH THE BAROCLINIC ZONE AND MIDLATITUDE WESTERLIES. AFTER TAU 36, TS 18W WILL ACCELERATE NORTHEASTWARD AND WILL BECOME EMBEDDED IN THE STRONG MIDLATITUDE WESTERLIES OVER THE SEA OF JAPAN, COMPLETING ETT BY TAU 48 WITH CLEAR FRONTAL CHARACTERISTICS. AFTER COMPLETING ETT, A GALE- FORCE COLD CORE LOW WILL REMAIN AND TRACK NORTH OF MISAWA AB. NUMERICAL GUIDANCE IS IN TIGHT AGREEMENT THROUGH TAU 48. AFTER TAU 48, NUMERICAL GUIDANCE DIVERGES WITH SIGNIFICANT DIFFERENCES IN TRACK SPEED. OVERALL, THERE IS GOOD CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN