WDXS31 PGTW 200900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04S (CHENGE) WARNING NR 009// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 9.0S 67.0E INITIAL INTENSITY: 50 KTS GEOGRAPHIC REFERENCE: 346 NM WEST-SOUTHWEST OF DIEGO GARCIA MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 09 KTS SIGNIFICANT WAVE HEIGHT: 23 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL CYCLONE (TC) 04S STEADILY INTENSIFIED OVER THE PAST DAY, AND THE PRESENTATION IN THE ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) IS MUCH IMPROVED. THE PREVIOUSLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) APPEARS TO HAVE TUCKED UP UNDER A DEVELOPING CDO FEATURE. THE MOST RECENT MICROWAVE IMAGERY FROM A 200227Z SSMIS PASS SHOWS THAT THE LLCC WAS STILL EXPOSED TO THE EAST OF THE DEVELOPING CONVECTION AT THAT TIME, WITH LOW-LEVEL BANDING FEATURES EVIDENT TO THE EAST IN THE DRIER AIR THAT IS PRESENT IN THAT HEMISPHERE. ANIMATED WATER VAPOR IMAGERY AND CIMSS AMV ANALYSIS INDICATE THAT THE CONVECTION IS SUCCESSFULLY PUSHING BACK AGAINST THE PERSISTENT EASTERLY SHEAR, AT LEAST AT THE OUTFLOW LAYER AROUND 200MB. THE AMVS SHOW THE FLOW SPLITTING AS IT MEETS THE OUTFLOW BOUNDARY, FLOWING BOTH EQUATORWARD AND POLEWARD. THIS SETUP IS CAUSING A LOCALIZED REDUCTION IN THE DEEP-LAYER SHEAR AND ALLOWING TC 04S TO BECOME MORE SYMMETRIZED AND TO INTENSIFY. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE ANIMATED MSI AND A LONG EXTRAPOLATION OF THE EARLIER SSMIS IMAGE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AN AVERAGE OF THE OBJECTIVE AND SUBJECTIVE INTENSITY ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS HAVE BECOME MORE FAVORABLE, WITH REDUCED SHEAR, WARM SSTS AND STRONG POLEWARD AND WEAKER EQUATORWARD OUTFLOW. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS FMEE: T3.5 - 55 KTS FIMP: T3.0 - 45 KTS CIMSS SATCON: 52 KTS AT 200430Z CIMSS ADT: 57 KTS AT 200530Z CIMSS AIDT: 49 KTS AT 200530Z CIMSS D-MINT: 47 KTS AT 200227Z CIMSS D-PRINT: 49 KTS AT 200530Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 04S HAS SLOWED DOWN TO ABOUT SIX KNOTS OF FORWARD SPEED, AND HAS SHIFTED SLIGHTLY MORE SOUTHWESTWARD, AS IT MOVES THROUGH AN AREA OF WEAKENED STEERING GRADIENT INDUCED BY A WEAKNESS IN THE RIDGE TO THE SOUTH. THE WEAKNESS IN THE RIDGE WILL PERSIST FOR ANOTHER 12 TO 24 HOURS BEFORE THE RIDGE BUILDS ONCE MORE, ALLOWING TC 04S TO SPEED UP A BIT AFTER TAU 24. THE GLOBAL MODELS, SPECIFICALLY GFS AND ECMWF, DEPICT TWO VERY DIFFERENT SCENARIOS IN THE EVOLUTION OF THE RIDGE TO THE SOUTH. THE GFS DEPICTS A RAPIDLY BUILDING RIDGE THAT MOVES QUICKLY WESTWARD, WHILE EXPANDING NORTHWARD. THIS RESULTS IN A MUCH SLOWER TRACK THAT PUSHES TC 04S NORTHWARD TOWARDS THE SEYCHELLES BY THE END OF THE FORECAST. THE ECMWF, AND THE OTHER MEMBERS OF THE CONSENSUS, ALSO DEPICT THE RIDGE BUILDING, BUT IT MOVES WEST AT A MUCH SLOWER RATE AND REMAINS ON A MORE EAST-WEST ORIENTATION, WHICH ALLOWS TC 04S TO TRACK GENERALLY WESTWARD AFTER TAU 48, PASSING NORTH OF THE NORTHERN TIP OF MADAGASCAR. ANALYSIS OF THE UPPER-LEVEL FIELDS INDICATES THAT THE ECMWF IS INITIALIZING BETTER, AND THIS FORECAST SCENARIO IS CURRENTLY FAVORED, WITH TC 04S FORECAST TO TRACK WEST-SOUTHWEST THROUGH TAU 48, THEN WESTWARD THROUGH THE END OF THE FORECAST PERIOD. THE SYSTEM HAS CLEARLY BECOME MORE ORGANIZED AND THE VORTEX HAS APPARENTLY BEEN ABLE TO SYMMETRIZE AND ALIGN VERTICALLY, SETTING UP A PERIOD OF INTENSIFICATION. THIS TREND IS EXPECTED TO CONTINUE FOR AT LEAST THE NEXT 12 HOURS OR SO, AND THE FORECAST CALLS FOR THE TC 04S TO INTENSIFY UP TO 55 KNOTS, WITH THE POSSIBILITY THAT IT COULD MAKE A RUN AT 60 KNOTS WITH THE NEXT 24 HOURS. AFTER TAU 24, SHEAR INCREASES SHARPLY, AND DRY AIR IS FORECAST TO PUSH IN AND ENGULF THE SYSTEM, WHICH WILL LEAD TO A STEADY WEAKENING TREND THROUGH THE REMAINDER OF THE FORECAST. DISSIPATION BELOW WARNING CRITERIA IS FORECAST BY TAU 120. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS SPLIT INTO TWO GROUPINGS. THE FIRST IS COMPRISED OF THE GFS AND GEFS MEAN, WHICH TRACKS THE SYSTEM WEST THROUGH TAU 48, THEN NORTHWEST TOWARDS THE SEYCHELLES BY TAU 120. THE OTHER GROUPING IS COMPRISED OF THE REMAINDER OF THE CONSENSUS MEMBERS INCLUDING ECMWF, ECENS MEAN, EC-AIFS, GALWEM, NAVGEM, EGRR AND THE CONSENSUS MEAN, WHICH CONFINED IN A RELATIVELY TIGHT (115NM SPREAD AT TAU 120) ENVELOPE THAT TRACKS SOUTHWESTWARD TO TAU 48, THEN WESTWARD THROUGH TAU 120. THE GEFS AND ECENS ENSEMBLES ARE SHOWING INCREASING DISPERSION THROUGH THE FORECAST PERIOD, WITH ECENS MEMBERS SPREAD ACROSS ALMOST 600NM ON THE NORTH-SOUTH AXIS, WHILE GEFS IS ONLY SLIGHTLY BETTER. THE JTWC FORECAST IS POSITIONED ON THE NORTHERN EDGE OF THIS ENVELOPE, CLOSEST TO THE ECMWF AND EC-AIFS TRACKERS, WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS MIXED, WITH A FAIRLY WIDE SPREAD OF POTENTIAL OUTCOMES. THE FRIA RAPID INTENSIFICATION (RI) AID HAS TRIPPED, AND SOMEWHAT SURPRISINGLY, THE SHIPS-NAVGEM IS DEPICTING A HIGH RATE OF INTENSIFICATION AND IS THE HIGHEST OF THE CONSENSUS MEMBERS, REACHING 70 KNOTS BY TAU 48. THE MESOSCALE MODELS ON THE OTHER HAND ARE SIGNIFICANTLY LOWER AND DO NOT BELIEVE THE SYSTEM WILL INTENSIFY FURTHER, INDICATING WEAKENING THROUGH THE ENTIRE FORECAST. THE JTWC FORECAST IS PLACED NEAR THE CONSENSUS MEAN, WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN