MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (RAMMASUN) WARNING NR 12// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 09W (RAMMASUN), LOCATED APPROXIMATELY 268 NM NORTH-NORTHWEST OF YAP, HAS TRACKED WESTWARD AT 20 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS AN UNCHANGED STRUCTURE WITH SUSTAINED CONVECTIVE BANDS ALONG THE LEADING WESTERN EDGE OF THE SYSTEM AND HINTS AT A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). A 130451Z AMSU-B MICROWAVE IMAGE SHOWS DEEP CONVECTION HAS PERSISTED ALONG THE WESTERN PORTION OF A POORLY DEFINED LLCC. UPPER LEVEL ANALYSIS REVEALS A CONTINUED MARGINAL ENVIRONMENT CHARACTERIZED BY WEAKLY DIFFLUENT EASTERLY OUTFLOW AND MODERATE (15 TO 20 KNOTS) VERTICAL WIND SHEAR (VWS). THERE IS LOW CONFIDENCE IN THE CURRENT POSITION WHICH IS BASED UPON THE AFOREMENTIONED SATELLITE IMAGERY ALONG WITH POSITIONAL FIXES FROM PGTW AND RJTD THAT ARE 45NM SPREAD APART. THE INITIAL INTENSITY IS BASED UPON THE UNCHANGED STRUCTURE AND CONGRUENT DVORAK INTENSITY ESTIMATES FROM ALL AGENCIES. TS 09W IS QUICKLY TRACKING WEST ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE (STR) THAT IS ANCHORED NORTH OF THE SYSTEM. 3. FORECAST REASONING. A. THE FORECAST TRACK SPEEDS HAVE BEEN INCREASED TO ACCOUNT FOR THE PAST 12 HOUR TRACK SPEED INCREASES AND FORECAST INTENSITY HAS BEEN DECREASED IN THE EARLY TERM DUE TO EARLIER THAN EXPECTED LAND INTERACTION WITH LUZON. B. TS 09W IS FORECAST TO CONTINUE WESTWARD FOR THE NEXT 48 HOURS ALONG THE SOUTHERN PERIPHERY OF THE ENTRENCHED STR. AFTER TAU 48, A MID-LATITUDE TROUGH IS EXPECTED TO DIG ACROSS EASTERN CHINA, WHICH WILL MODIFY THE STR LEADING TO A SLIGHT WEST-NORTHWESTWARD DEFLECTION IN TRACK AS THE SYSTEM MAKES LANDFALL INTO CENTRAL LUZON. THE CURRENT MARGINAL UPPER LEVEL ENVIRONMENT IS FORECAST TO PERSIST FOR THE NEXT 48 HOURS WHICH WILL LEAD TO A LESS THAN CLIMATOLOGICAL RATE OF INTENSIFICATION, THAT IS NOW EXPECTED TO JUST REACH TYPHOON STRENGTH BEFORE MAKING LANDFALL. AFTER LANDFALL, THE ROUGH TERRAIN WILL WEAKEN THE SYSTEM BEFORE IT RE-EMERGES IN THE SOUTH CHINA SEA AS A MODERATE TROPICAL STORM AFTER TAU 72. C. IN THE EXTENDED TAUS, TS RAMMASUN IS EXPECTED TO CONTINUE TO TRACK WEST-NORTHWESTWARD ACROSS THE SOUTH CHINA SEA AND TOWARDS HAINAN ISLAND. WARM SEA SURFACE TEMPERATURES, DECREASED VWS, AND INCREASED OUTFLOW WILL ALLOW FOR RE-INTENSIFICATION. MODEL GUIDANCE IS IN GOOD AGREEMENT; HOWEVER, DUE TO THE LOW CONFIDENCE IN THE INITIAL POSITION AND THE RECENT TRACK SPEED CHANGES, THE CONFIDENCE IN THE FORECAST REMAINS LOW.// NNNN NNNN