WDXS32 PGTW 091500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 18.0S 53.8E INITIAL INTENSITY: 60 KTS GEOGRAPHIC REFERENCE: 172 NM NORTH-NORTHWEST OF ST DENIS MOVEMENT PAST 6 HOURS: WESTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 27 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL CYCLONE (TC) 21S (GEZANI) EXHIBITS CONTINUED RAPID CYCLOGENESIS AND INTENSIFICATION AS IT TRACKS ON A WESTERLY VECTOR TOWARDS MADAGASCAR. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) AND ENHANCED INFRARED (EIR) IMAGERY DEPICT A HIGHLY COMPACT CORE STRUCTURE, WITH VERTICAL HOT TOWERS (VHTS) WRAPPING AROUND THE LOW LEVEL CIRCULATION CENTER (LLCC), YET TO ACHIEVE THE TEMPORAL PERSISTENCE REQUISITE FOR ESTABLISHING A DEFINITIVE EYE FEATURE. EIR ANALYSIS HAS INTERMITTENTLY SUGGESTED NASCENT OCULAR FORMATION OVER THE PREVIOUS 12 HOURS; HOWEVER, PERSISTENT MID-LEVEL EAST- NORTHEASTERLY VERTICAL WIND SHEAR (VWS) HAS HITHERTO INHIBITED STRUCTURAL CONSOLIDATION. A 091117Z GMI AND 091016Z AMSR2 MICROWAVE PASS REVEALED A MODERATE VORTEX TILT, THOUGH VERTICAL ALIGNMENT HAS IMPROVED SINCE THE PREVIOUS SYNOPTIC PERIOD. THE AMSR2 COLOR ENHANCED 36GHZ IMAGE ALSO REVEALED A CYAN RING AROUND A NASCENT MICROWAVE EYE FEATURE, SUGGESTING A HIGH POTENTIAL FOR RAPID INTENSIFICATION (RI) IN THE NEAR FUTURE. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON AN EXTRAPOLATION OF THE MICROWAVE IMAGES NOTED ABOVE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, BIASED HIGHER THEN THE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED BELOW TO ACCOUNT FOR POTENTIAL INTENSITY UNDERESTIMATION INHERENT TO SMALL-CORE, COMPACT SYSTEMS. ENVIRONMENTAL CONDITIONS ARE HIGHLY FAVORABLE, WITH LOW (AND DECREASING) VERTICAL WIND SHEAR (VWS), WARM SSTS, AND STRONG POLEWARD OUTFLOW. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) COMPLEX EXTENDING ZONALLY FROM EAST OF SOUTH AFRICA TO SOUTHEAST OF MAURITIUS. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.5 - 55 KTS DEMS: T3.5 - 55 KTS CIMSS SATCON: 55 KTS AT 091230Z CIMSS ADT: 53 KTS AT 091200Z CIMSS AIDT: 49 KTS AT 091200Z CIMSS D-MINT: 46 KTS AT 091117Z CIMSS D-PRINT: 51 KTS AT 091130Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE VWS: 5-10 KTS SST: 28-29 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: SLIGHTLY ELEVATED MID-LEVEL NORTHERLY SHEAR; WEDGE OF DRY MID-LEVEL AIR ALONG THE EASTERN FLANK. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 21S IS FORECAST TO MAINTAIN A WESTWARD TRACK ALONG THE NORTHERN PERIPHERY OF A DEEP-LAYER STR POSITIONED TOO THE SOUTH OVER THE NEXT 24 TO 36 HOURS. LANDFALL ALONG THE EASTERN COAST OF MADAGASCAR NEAR THOAMASINA IS EXPECTED SHORTLY AFTER TAU 24. POST- LANDFALL, TC 21S WILL EXECUTE A GRADUAL SOUTHWESTWARD RECURVATURE AS THE STEERING RIDGE REORIENTS TO A SOUTHWEST-NORTHEAST AXIS. THE LLCC IS PROJECTED TO FULLY DECOUPLE FROM THE MID-LEVEL VORTEX UPON INTERACTION WITH THE OROGRAPHIC BARRIER OF THE CENTRAL MADAGASCAR MASSIF; THE SURFACE CENTER WILL LIKELY BE ATTENUATED WHILE MID-LEVEL VORTICITY TRANSITS RAPIDLY SOUTHWESTWARD, REACHING THE WESTERN LITTORAL BY TAU 48. UPON EMERGENCE OVER THE MOZAMBIQUE CHANNEL, THE VORTEX WILL RE-ESTABLISH SURFACE COUPLING AND CONTINUE A SOUTHWESTWARD TRACK UNTIL TAU 96. SUBSEQUENTLY, THE SYSTEM WILL TURN SHARPLY POLEWARD INTO A WEAKNESS IN THE RIDGE INDUCED BY THE PASSAGE OF A DEEP-LAYER UPPER-LEVEL TROUGH. REGARDING INTENSITY, THE ENVIRONMENTAL PROFILE IS PRIMED FOR RI, CONTINGENT UPON VORTEX AXISYMMETRIZATION. WITH DIMINISHING SHEAR AND ENHANCED POLEWARD OUTFLOW, RI COMMENCEMENT IS ANTICIPATED WITHIN THE NEXT FEW HOURS. PEAK INTENSITY IS FORECAST AT 95 KTS AT TAU 24, THOUGH NEAR-SHORE INTENSIFICATION MAY YIELD A 100-KT MAXIMUM AT LANDFALL. TC 21S WILL RAPIDLY WEAKEN TO 30 KNOTS BY THE TIME IT REACHES THE WEST COAST OF MADAGASCAR BUT STRENGTHEN EXPEDITIOUSLY OVER OPEN WATER AS THE VORTEX BUILDS BACK DOWN TO THE SURFACE AND REORGANIZES. A SECONDARY INTENSIFICATION PHASE IS FORECAST THROUGH THE END OF THE FORECAST PERIOD, LEVERAGING FAVORABLE SHEAR AND ENHANCED POLEWARD OUTFLOW. WHILE NOT EXPLICITY FORECAST AT THE CURRENT TIME, THE POSSIBILITY OF A SECOND ROUND OF RI AFTER TAU 72 IS A DISTINCT PROBABILITY. MODEL DISCUSSION: THE DETERMINISTIC, ENSEMBLE AND ARTIFICIAL INTELLIGENCE (AI) MODEL PACKAGE EXHIBITS ROBUST SPATIAL CONCURRENCE, VALIDATING THE WESTWARD TRACK THROUGH TAU 36 FOLLOWED BY RECURVATURE AND POLEWARD ACCELERATION AFTER 96. THE GFS-GEFS REPRESENT THE NORTHERN AND WESTERN CROSS-TRACK OUTLIERS, WHILE THE ECMWF-ECENS DEFINE THE SOUTHERN AND EASTERN BOUNDARY OF THE GUIDANCE ENVELOPE. CROSS-TRACK SPREAD EXPANDS TO 250NM BY TAU 120. THE GFS AND GEFS DEPICT THE MOST GRADUAL TURN AND MARK THE NORTHERN AND WESTERN BOUNDARY OF THE GUIDANCE ENVELOPE, WHILE THE ECMWF AND ECENS DEPICT THE SHARPEST AND FASTEST TURN AND OUTLINE THE SOUTHERN AND EASTERN EDGE OF THE ENVELOPE. CROSS-TRACK SPREAD GRADUALLY INCREASES TO 250NM BY THE END OF THE FORECAST PERIOD. THE REMAINDER OF THE GUIDANCE IS EQUALLY SPREAD BETWEEN THE TWO OUTLIERS. THE JTWC FORECAST IS PLACED JUST NORTH AND WEST OF THE CONSENSUS MEAN, CONSISTENT WITH THE CONSENSUS OF THE AI MEMBERS, WITH HIGH CONFIDENCE THROUGH TAU 72, AND MEDIUM CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT ON THE OVERALL SHAPE OF THE TREND, WITH A MODEST AMOUNT OF SPREAD. SHIPS AND COAMPS-TC (NAVGEM) CONSTITUTE THE CONSERVATIVE LOWER BOUND (70 KTS), WHEREAS HAFS-A, HWRF, AND COAMPS- TC (GFS) PROJECT PRE-LANDFALL PEAKS OF 85-90 KTS. STATISTICAL RI AIDS (RICN, RIDE, RIPA) HAVE BEEN TRIGGERED, SUPPORTING PEAKS OF 100-105 KTS. BOTH THE COAMPS-TC AND ECMWF ENSEMBLE RI PROBABILITIES EXCEED 40 PERCENT (COAMPS-TC IS 90 PERCENT). THE JTWC FORECAST FOLLOWS THE RI GUIDANCE THROUGH TAU 24, THEN HAFS-A THROUGH TAU 48. DURING THE SECOND INTENSIFICATION PERIOD, ALL OF THE GUIDANCE SAVE FOR THE SHIPS, DEPICTS A STEEP INTENSIFICATION TREND (THE HWRF IN FACT REACHES 115 KTS BY TAU 120). THE JTWC FORECAST IS ON THE HIGHER EDGE OF THE ENVELOPE AND ABOVE ALL OF THE GUIDANCE EXCEPT FOR THE HWRF DURING THE EXTENDED FORECAST. CONFIDENCE IS MEDIUM THROUGHOUT THE FORECAST PERIOD. FORECAST CONFIDENCE: TRACK 00-72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN