WDPN31 PGTW 120300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 15W (FIFTEEN) WARNING NR 06// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL DEPRESSION 15W (FIFTEEN), LOCATED APPROXIMATELY 417 NM NORTH-NORTHWEST OF KOROR, HAS TRACKED WESTWARD AT 17 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS THE DEEP CONVECTIVE BANDING ALONG THE PERIPHERY OF THE LOW LEVEL CIRCULATION CENTER (LLCC) HAS BEEN STEADILY IMPROVING OVER THE PAST FEW HOURS. A RECENT 112057Z SSMIS MICROWAVE PASS SHOWS THE POSSIBLE EARLY STAGES OF AN EYE WALL DEVELOPING. THE CURRENT INTENSITY REMAINS AT 30 KNOTS AS DVORAK ESTIMATES FROM PGTW AND KNES HAVE BEEN PERSISTENT OVER THE PAST SIX HOURS. UPPER LEVEL ANALYSIS INDICATES TD 15W REMAINS SOUTH OF A SUBTROPICAL RIDGE (STR) AND IS IN A WEAKLY DIVERGENT REGION, PROVIDING MARGINAL SOUTHWESTERLY OUTFLOW. VERTICAL WIND SHEAR (VWS) HAS BEEN SLOWLY WEAKENING, CURRENTLY AT LOW TO MODERATE LEVELS (10 TO 15 KNOTS), AS THE TRACK SPEED FOR TD 15W REMAINS BETWEEN 18 TO 23 KNOTS AND IS IN PHASE WITH THE UPPER LEVEL FLOW. TD 15W CONTINUES TRACKING WESTWARD UNDER THE STEERING INFLUENCE OF THE DEEP LAYERED REFLECTION OF THE STR LOCATED TO THE NORTH OF THE SYSTEM. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TD 15W IS FORECAST TO TRACK WESTWARD TO WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF THE STR, BUT WILL UNDERGO A PERIOD OF DECREASED SPEEDS AS A MIGRATORY UPPER-LEVEL TROUGH PASSES TO THE NORTH OF THE RIDGE BETWEEN TAU 36 AND 48. FAVORABLE ENVIRONMENTAL CONDITIONS ARE EXPECTED TO PERSIST WITH HIGH SEA SURFACE TEMPERATURES, IMPROVING UPPER LEVEL OUTFLOW, AND DECREASING VWS ALLOWING TD 15W TO GRADUALLY INTENSIFY, REACHING TO TYPHOON STRENGTH BY TAU 48. TD 15W IS FORECAST TO MAKE LANDFALL IN EASTERN LUZON AROUND TAU 60, LEADING TO A TEMPORARY WEAKENING TREND IN THE LATER TAUS. C. TD 15W IS EXPECTED TO MAINTAIN A WEST-NORTHWESTWARD TRACK THROUGH THE REMAINDER OF THE FORECAST PERIOD AS THE STR REBUILDS TO THE NORTH. BY TAU 96 THE FAVORABLE ENVIRONMENT WILL CAUSE TD 15W TO INTENSIFY AS IT APPROACHES THE COAST OF CHINA, MAKING LANDFALL TO THE NORTH OF HAINAN AROUND TAU 120. AVAILABLE MODEL GUIDANCE REMAINS IN GOOD AGREEMENT. DUE TO THE UNCERTAINTY OF INTENSITY AND TRACK SPEEDS IN THE EXTENDED TAUS, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST.// NNNN