WDPN31 PGTW 091500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 24W (ETAU) WARNING NR 005// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 24W (ETAU), LOCATED APPROXIMATELY 273 NM SOUTHEAST OF DA NANG, VIETNAM, HAS TRACKED WESTWARD-SOUTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY (EIR) DEPICTS A WEAKENING SYSTEM, WITH DEEP CONVECTION SHEARED TO THE WEST OF A RECENTLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC). A 091121Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS THE DEEP CONVECTIVE CORE OF THE SYSTEM IS NOW BECOMING DECOUPLED FROM THE LLCC, AND ALONG WITH THE EXPOSED LLCC, PROVIDES HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY REMAINS AT 40 KNOTS, HEDGED SLIGHTLY HIGHER THAN THE MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 (35 KNOTS) FROM PGTW/RJTD IN LIGHT OF AN ADVANCED DVORAK TECHNIQUE (ADT) ESTIMATE OF T3.0 (40 KNOTS). UPPER-LEVEL ANALYSIS REVEALS A RAPIDLY DETERIORATING ENVIRONMENT, WITH COOLING SSTS (26-27 DEG CELSIUS), AND MODERATE TO HIGH (20-25 KNOTS) EASTERLY VWS, OFFSETTING THE CONTINUED ROBUST WESTWARD OUTFLOW. DUE TO THE EFFECTS OF THE DECOUPLING OVER THE PAST SIX HOURS, THE PRIMARY STEERING INFLUENCE IS LIKELY SHIFTING TO THE LOW LEVEL EAST-NORTHEASTERLY FLOW, RESULTING IN THE WEST- SOUTHWESTWARD TRACK MOTION SEEN IN THE PAST SIX HOURS. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 24W IS FORECAST TO CONTINUE TRACKING WEST-SOUTHWESTWARD THROUGH TAU 12, AND IS EXPECTED TO MAKE LANDFALL ALONG THE SOUTH- CENTRAL VIETNAMESE COAST BETWEEN TAU 12 AND TAU 18. AS THE SYSTEM APPROACHES THE COASTLINE, IT WILL ENCOUNTER COOLER (26 DEG CELSIUS), AND INCREASING VWS, CONTINUING THE DECOUPLING OF THE CORE, RESULTING IN A SLOW WEAKENING TREND PRIOR TO LANDFALL. AFTER LANDFALL, THE SYSTEM WILL TRACK INLAND AND DISSIPATE OVER CENTRAL CAMBODIA BY TAU 36. NUMERICAL MODEL GUIDANCE IS IN TIGHT AGREEMENT WITH A SPREAD OF 25NM AT LANDFALL WITH SOMEWHAT INCREASING SPREAD THEREAFTER AS THE VARIOUS MODELS LOSE THE VORTEX SIGNATURE AS THE SYSTEM RAPIDLY DECAYS OVER LAND. THE JTWC FORECAST IS SET SLIGHTLY SOUTH OF THE MULTI-MODEL CONSENSUS IN LIGHT OF THE RECENT TRACK MOTION AND SHIFT IN THE STEERING MECHANISM WITH HIGH CONFIDENCE.// NNNN