WDXS31 PGTW 232100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 016// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 10.0S 57.9E INITIAL INTENSITY: 40 KTS GEOGRAPHIC REFERENCE: 607 NM NORTH OF PORT LOUIS, MAURITIUS MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 18 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY EXPOSED LOW LEVEL CIRCULATION CENTER AND A SINGLE, DEEP CONVECTIVE PLUME DISPLACED TO THE WEST-SOUTHWEST. THE CONVECTIVE BURST EXHIBITS WEAK OUTFLOW ON THE POLEWARD SIDE, CONSISTENT WITH THE HEAVILY SHEARED ENVIRONMENT. A PARTIAL SCATTEROMETER IMAGE FROM METOP-B AT 231748Z WAS HELPFUL IN LOCATING THE CENTER POSITION WITH MEDIUM CONFIDENCE. WIND RADII WERE ADJUSTED BASED OFF OF THIS PARTIAL DATA, WHICH SHOWED PRIMARILY THE WESTERN AND PART OF THE SOUTHERN PORTIONS OF THE SURFACE WIND FIELD. ADDITIONALLY, A 231828Z GMI 37GHZ MICROWAVE IMAGE MISSED THE CENTER BUT CAPTURED THE LOW TO MID-LEVEL RAIN BAND STRUCTURE IN THE SOUTHEASTERN QUADRANT, WHICH IS STILL WELL-ORGANIZED. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON PERSISTENCE, THE PARTIAL SCATTEROMETRY DATA, AND THE GOOD AGREEMENT OF THE AGENCY FIXES AND OBJECTIVE ESTIMATES. INITIAL WIND RADII BASIS: PARTIAL SCATTEROMETRY DATA AND OTHERWISE PERSISTENCE. CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A SUBTROPICAL RIDGE CENTERED TO THE SOUTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS DEMS: T3.0 - 45 KTS FIMP: T2.5 - 35 KTS CIMSS ADT: 37 KTS AT 231800Z CIMSS AIDT: 43 KTS AT 231700Z CIMSS D-MINT: 36 KTS AT 231403Z CIMSS D-PRINT: 34 KTS AT 231800Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 25-30 KTS SST: 26-27 CELSIUS OUTFLOW: WEAK POLEWARD OTHER FACTORS: THE SYSTEM HAS BECOME DECOUPLED DUE TO UNFAVORABLE VERTICAL WIND SHEAR. ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THE FORECAST HAS BEEN TRUNCATED TO 72 HOURS. FORECAST DISCUSSION: TC 04S IS FORECAST TO REMAIN IN SHALLOW LATITUDES ON THE EQUATORWARD PERIPHERY OF A SUBTROPICAL RIDGE TO THE SOUTHEAST, WHICH WILL DRIVE THE SYSTEM STEADILY WESTWARD. THE EXPECTED TRACK LIES NORTH OF THE MIDPOINT BETWEEN NORTHERN MADAGASCAR AND SEYCHELLES. THIS TRACK WILL BECOME PROBLEMATIC DUE TO BORDERLINE SEA SURFACE TEMPERATURES OF 26C, NOW THROUGH TAU 48, WHICH THEN DIP BELOW 26C ONCE 04S ARRIVES NW OF MADAGASCAR. THIS, PLUS THE UNFAVORABLE AMOUNTS OF VERTICAL WIND SHEAR AND DRY AIR THAT HAVE PLAGUED THE SYSTEM IN THE PAST 24 HOURS, WILL WEAKEN WHAT REMAINS OF THE VORTEX. THE EXPOSED LOW LEVEL CIRCULATION CENTER WILL BROADEN AND WEAKEN TO BELOW THRESHOLDS, BRINGING ABOUT DISSIPATION OVER WATER BY TAU 72. MODEL DISCUSSION: THERE IS LITTLE CHANGE FROM PREVIOUS FORECASTS DUE TO FAIRLY GOOD AND CONSISTENT MODEL GUIDANCE. TRACK GUIDANCE SPREADS TO 96 NM AT TAU 24 AND 217 NM BY TAU 48, TO ILLUSTRATE THE VARIANCE. DIRECTIONALLY, THERE IS GOOD AGREEMENT WITH THE WEST-NORTHWESTWARD TRACK, BUT TRACK PROXIMITY TO THE STEERING RIDGE IS ALSO AFFECTING THE ALONG-TRACK SPEEDS AS CAN BE EXPECTED. THE SOUTHERNMOST SOLUTION IS THE GFS, AND THE NORTHERN EDGE OF THE ENVELOPE IS BRACKETED BY ECMWF-BASED GUIDANCE. THE JTWC FORECAST IS PLACED WITH MEDIUM CONFIDENCE NEAR THE TRACK CONSENSUS AND FAVORING THE MORE NORTHERN SOLUTIONS. REGARDING INTENSITY, ONLY THE HAFS MODEL STILL BELIEVES IN THE ABILITY OF TC 04W TO INTENSIFY THROUGH TAU 24, BUT THE MAJORITY OF INTENSITY AIDS PREDICT A FLATLINED INTENSITY THROUGH ABOUT TAU 36 AND A GRADUAL DOWNWARD TREND THEREAFTER. THE AIDS THAT ACCOUNT FOR DECAY ARE SUGGESTING AN IMMEDIATE BUT GRADUAL DECREASE AT A STEADY RATE. WITH THE INEVITABLE DISSIPATION IN THE NEAR-TERM, THE JTWC INTENSITY FORECAST IS PREDICTED WITH HIGH CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM INTENSITY 00-72 HR: HIGH// NNNN