WDXS32 PGTW 081500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 18.3S 56.6E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 115 NM NORTH-NORTHWEST OF PORT LOUIS, MAURITIUS MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL CYCLONE (TC) 21S (GEZANI) REMAINS QUASI-STATIONARY AND PERSISTS IN A SUB-OPTIMAL STATE IN THE FACE OF PERSISTENT MODERATE VERTICAL WIND SHEAR (VWS) AND DRY AIR ENTRAINMENT. OVER THE PAST FEW HOURS, ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTED A FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC), WITH FRAGMENTED AND FLARING CONVECTION IN THE NORTHEASTERN QUADRANT. OVER THE PAST COUPLE OF HOURS HOWEVER, CONVECTION HAS INCREASED AND STARTED TO ORGANIZE AND MOVE CLOSER TO THE LLCC, WHICH IS NOW OBSCURED; THIS MORPHOLOGY SUGGESTS AN EXPECTED ABATEMENT IN SHEAR VALUES. A 081157Z GMI MICROWAVE IMAGERY SUITE REVEALED A SHALLOW AND ILL-DEFINED VORTEX, WITH SHALLOW RAIN BANDS IN THE WESTERN SEMI-CIRCLE UNDER THE INFLUENCE OF DRY MID-LEVEL AIR, CONTRASTED BY MORE VIGOROUS CONVECTIVE BANDS TO THE NORTHEAST. ANIMATED WATER VAPOR IMAGERY INDICATES A DEVELOPING EQUATORWARD OUTFLOW CHANNEL, EVIDENCED BY TRANSVERSE BANDS WITHIN THE CIRRUS CANOPY. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PREVIOUSLY EXPOSED LLCC. THE INITIAL INTENSITY IS ASSESSED WITH LOW CONFIDENCE, CONSERVATIVELY ASSESSED AT 35 KNOTS TO ALIGN WITH THE ASCENDING TREND IN OBJECTIVE INTENSITY ESTIMATES. ENVIRONMENTAL CONDITIONS ARE TRANSITIONING TOWARD A MORE CONDUCIVE REGIME, CHARACTERIZED BY HIGH OCEAN HEAT CONTENT (OHC), INCIPIENT OUTFLOW ALOFT, AND SUBSIDING VWS. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT WITH RIDGING TO THE WEST AND EAST, AND TROUGHING TO THE SOUTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS DEMS: T2.0 - 30 KTS FMEE: T2.0 - 30 KTS CIMSS ADT: 35 KTS AT 081200Z CIMSS AIDT: 38 KTS AT 081100Z CIMSS D-MINT: 31 KTS AT 081201Z CIMSS D-PRINT: 33 KTS AT 081200Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE VWS: 10-15 KTS SST: 28-29 CELSIUS OUTFLOW: WEAK EQUATORWARD OTHER FACTORS: DRY AIR AND SUBSIDENCE ACROSS THE SOUTHWESTERN FLANK. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: LOW INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: AS TC 21S REMAINS TRAPPED IN A WEAK STEERING ENVIRONMENT, IT IS FORECAST TO DRIFT ERRATICALLY WEST-NORTHWESTWARD OVER THE NEXT 12 HOURS. A MORE DEFINITIVE WESTWARD VECTOR IS FORECAST AFTER TAU 12 AS A DEEP-LAYER SUBTROPICAL RIDGE (STR) ESTABLISHES AN EASTWARD EXTENSION FROM SOUTHERN MADAGASCAR, SUPPLANTING THE TROUGH CURRENTLY RESIDING SOUTHEAST OF MAURITIUS. TC 20S WILL ACCELERATE WESTWARD THROUGH TAU 48, WITH LANDFALL EXPECTED ALONG THE EASTERN COAST OF MADAGASCAR NEAR TAU 48. POST-LANDFALL, THE MID-LEVEL VORTICITY MAXIMUM WILL RAPIDLY TRANSIT THE CENTRAL MADAGASCAR OROGRAPHIC BARRIER, RE-EMERGING ON THE WESTERN COAST BY TAU 72. THE SYSTEM WILL THEN UNDERGO RAPID RE-ORGANIZATION AND CONTINUE ON A WEST-SOUTHWESTWARD TRAJECTORY. GLOBAL DETERMINISTIC MODELS EXHIBIT SIGNIFICANT DIVERGENCE REGARDING THE RIDGE PATTERN; THE GFS DEPICTS A ROBUST, ZONALLY-ORIENTED RIDGE DRIVING A WESTWARD TRACK, WHILE THE ECMWF IDENTIFIES A COL OR WEAKNESS IN THE RIDGE, INDUCING A MORE POLEWARD COMPONENT. REGARDING INTENSITY, A REDUCTION IN VWS IS FACILITATING INCREASED CONVECTIVE ORGANIZATION. THE COMPACT RADIUS OF THE SYSTEM WILL ACCELERATE AXISYMMETRIZATION; ONCE A VERTICALLY ALIGNED VORTEX IS ESTABLISHED WITHIN A MOIST THERMODYNAMIC PROFILE AND ENHANCED POLEWARD EXHAUST, RAPID INTENSIFICATION (RI) IS PROBABLE. THE SYSTEM IS FORECAST TO RAPIDLY INTENSIFY (RI) BEGINNING AT TAU 12, AND CONTINUE RIGHT UP TO LANDFALL. THE FORECAST CALLS FOR A PEAK INTENSITY OF 75 KNOTS, BUT THE ACTUAL PEAK WILL LIKELY REACH 80-85 KNOTS AND OCCUR BETWEEN TAU 36 AND TAU 48. THE SYSTEM WILL RAPIDLY WEAKEN AS IT CROSSES MADAGASCAR, DOWN TO 30 KNOTS BY TAU 72. ONCE REESTABLISHED IN THE MOZAMBIQUE CHANNEL, THE SYSTEM WILL COMMENCE A SECONDARY RI PHASE AS IT PASSES OVER VERY WARM SSTS, WITH LOW VWS, AND STRONG POLEWARD OUTFLOW. MODEL DISCUSSION: MODEL GUIDANCE IS IN GENERAL AGREEMENT ON THE OVERALL TRACK SCENARIO, BUT SHOWS A MODEST AMOUNT OF UNCERTAINTY IN BOTH CROSS- AND ALONG-TRACK ASPECTS, PARTICULARLY AFTER TAU 72. THROUGH TAU 48, THE GUIDANCE ENVELOPE IS WELL-CONSTRAINED. AFTER TAU 72, CROSS-TRACK BIFURCATION INCREASES TO 290NM BY TAU 120. THE ECMWF TRACKS THE SYSTEM SOUTHWARD TOWARD EUROPA ISLAND, WHEREAS THE GFS/GEFS FAVOR A FLATTER TRAJECTORY TOWARD BEIRA, MOZAMBIQUE. THE JTWC FORECAST IS CONSISTENT WITH THE ECMWF SOLUTION THROUGH TAU 48, THEN TRANSITIONS TO THE GDM FNV3 TRACKER THROUGH TAU 120. CONFIDENCE IS MEDIUM THROUGH TAU 72 AND LOW THROUGH TAU 120. INTENSITY GUIDANCE IS MIXED, WITH THE SHIPS (GFS AND NAVGEM) MARKING THE LOWER END OF THE SPECTRUM WITH A PEAK OF JUST 50 KNOTS, WHILE THE HWRF AND CONSENSUS ARE DEPICTING A 65 KNOT PEAK AND THE HAFS-A AND GDM FNV3 ARE REACHING 75-80 KNOTS. RI GUIDANCE INCLUDING RIDE ARE TRIPPED, SUGGESTING A PEAK OF 80 KNOTS, WHILE THE COAMPS-TC ENSEMBLE RI PROBABILITIES ARE REACHING AS HIGH AS 65 PERCENT. THE JTWC THROUGH TAU 48 IS PLACED NEAR THE HAFS-A MODEL WITH A POTENTIAL. ALL MODEL GUIDANCE SUPPORTS A RAPID WEAKENING THROUGH TAU 72, FOLLOWED BY ADDITIONAL NEAR-RAPID INTENSIFICATION FROM TAU 96 TO TAU 120. THE ENSEMBLE RI PROBABILITIES REACH APPROXIMATELY 40 PERCENT IN THE LATER PORTION OF THE FORECAST. THE JTWC LONG-RANGE FORECAST IS PLACED NEAR THE CONSENSUS MEAN, WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN