WDXS31 PGTW 092100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 24S (IVONE) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 17.4S 83.1E INITIAL INTENSITY: 50 KTS GEOGRAPHIC REFERENCE: 876 NM SOUTHEAST OF DIEGO GARCIA MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 12 KTS SIGNIFICANT WAVE HEIGHT: 27 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL CYCLONE 24S (IVONE) WITH A LARGE AREA OF CONVECTION OBSCURING THE CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC). NORTHERLY SHEAR CAN BE SEE ACTING ON THE SYSTEM ALONG WITH DRY AIR WITHIN THE NORTHERN SEMICIRCLE, CAUSING THE LOW-LEVEL CLOUD LINES TO BE EXPOSED IN THAT REGION. THESE TWO FACTORS ARE INHIBITING CONVECTIVE DEVELOPMENT WITHIN THE NORTHERN PORTION OF THE SYSTEM. ENVIRONMENTAL ANALYSIS INDICATES THAT 24S IS IN A MARGINALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY THE DRY AIR AND NORTHERLY VERTICAL WIND SHEAR OFFSET BY THE STRONG POLEWARD OUTFLOW AND WARM (27-28 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED EIR. ADDITIONALLY, A 091616Z METOP-B ASCAT IMAGE REVAELED AN ELONGATED LLCC IN THE NORTH-SOUTH AXIS, SUPPORTING THE INITIAL POSITION. THE INITIAL INTENSITY OF 50 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE MENTIONED ASCAT SHOWING A WSATH OF 40 KTS WITHIN THE SOUTHERN SEMICIRCLE AND ALL AGENCIES AGREEING ON A T3.0 ESTIMATE. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS KNES: T3.0 - 45 KTS DEMS: T3.0 - 45 KTS FMEE: T3.0 - 45 KTS FIMP: T3.0 - 45 KTS CIMSS SATCON: 44 KTS AT 091749Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 10-15 KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: DRY AIR ENTRAINMENT ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 24S IS FORECAST TO CONTINUE TRACKING SOUTH-SOUTHWESTWARD ALONG THE WESTERN PERIPHERY OF THE STR THROUGH TAU 24. AFTER TAU 24, A BUILDING RIDGE WILL FORM SOUTH OF THE SYSTEM AND BECOME THE PRIMARY STEERING INFLUENCE. THIS WILL DRIVE 24S WESTWARD THROUGH TAU 72. NEAR TAU 72, 24S WILL BEGIN TO ROUND THE RIDGE AND WILL TURN SOUTHWARD. REGARDING INTENSITY, 24S IS FORECAST TO MAINTAIN INTENSITY THROUGH TAU 24 AS SHEAR REMAINS RELATIVELY LOW. HOWEVER, AFTER TAU 24, SHEAR INCREASES TO AROUND 30 KTS. THE SHEAR ALONG WITH DRY AIR ENTRAINMENT WILL CAUSE 24S TO WEAKEN, THOUGH ONLY MARGINALLY DUE TO THE ROBUST POLEWARD OUTFLOW CHANNEL. NEAR TAU 96, SHEAR WILL INCREASE TO OVER 35 KTS AND DRY AIR WILL COMPLETELY ERODE THE VORTEX, CAUSING DISSIPATION BY TAU 120. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72 WITH A 180 NM CROSS-TRACK SPREAD AT TAU 72. AFTER TAU 72, MODELS DIVERGE AND CROSS-TRACK SPREAD INCREASES TO AROUND 500 NM AT TAU 120. NAVGEM TAKES THE SYSTEM NEARLY STRAIGHT WESTWARD WHILE THE ECMWF ENSEMBLE MEAN RACES THE VORTEX SOUTHWARD. AS A RESULT, THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS IN MODERATE AGREEMENT, WITH MODELS OVERALL PORTRAYING MARGINAL WEAKENING THROUGH TAU 72 AND QUICKER WEAKENING AFTERWARD. THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN