WDXS32 PGTW 151500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (GEZANI) WARNING NR 016// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 26.4S 40.1E INITIAL INTENSITY: 80 KTS GEOGRAPHIC REFERENCE: 252 NM SOUTH OF EUROPA ISLAND MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 28 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) IMAGERY REVEALS THE CONVECTIVE CANOPY ASSOCIATED WITH TROPICAL CYCLONE (TC) 21S (GEZANI) AS IT IS DETERIORATING DUE TO MARGINALLY UNFAVORABLE ENVIRONMENTAL CONDITIONS. DRY AIR ENTERING THE CORE OF THE VORTEX, COMBINED WITH INCREASING VERTICAL WIND SHEAR (VSW) ARE OFFSET ONLY BY WARM (27-28 C) SEA SURFACE TEMPERATURES (SST) AND MODERATE RADIAL OUTFLOW. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON EXTRAPOLATION FROM A 151017Z GPM GMI MICROWAVE SUITE INDICATING INCREASED VERTICAL TILT OF THE VORTEX. THE INITIAL INTENSITY OF 80 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE 150622Z METOP-C UHR ASCAT PASS INDICATING 75- 80 KTS THROUGHOUT THE WESTERN PART OF THE CIRCULATION. THE OBJECTIVE INTENSITY AIDS LISTED BELOW ARE LIKELY OVERESTIMATING THE WEAKENING, DUE TO REDUCED DEEP-LAYER CONVECTION. INITIAL WIND RADII BASIS: 150622Z METOP-C UHR ASCAT AND 150626Z METOP-C ASCAT IMAGES CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST WITH TEMPORARY IMPACTS FROM A PASSING LONG-WAVE BAROCLINIC TROUGH TO THE SOUTH AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T4.5 - 77 KTS DEMS: T4.5 - 77 KTS CIMSS SATCON: 62 KTS AT 151128Z CIMSS ADT: 55 KTS AT 151200Z CIMSS AIDT: 59 KTS AT 151100Z CIMSS D-MINT: 69 KTS AT 151038Z CIMSS D-PRINT: 64 KTS AT 151230Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 15-20 KTS SST: 27-28 CELSIUS OUTFLOW: MODERATE RADIAL ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: A DEEP-LAYER, BAROCLINIC TROUGH PROPAGATING EASTWARD AND LOCATED TO THE SOUTH OF TC 21S IS CURRENTLY DISRUPTING THE OTHERWISE DOMINANT STEERING INFLUENCE OF THE SUBTROPICAL RIDGE CENTERED TO THE SOUTHEAST. AS A RESULT, THE SYSTEM IS FORECAST TO CONTINUE TRACKING NORTHEASTWARD FOR THE NEXT 24 HOURS. AFTER THE PASSING OF THE TROUGH, THE NOW STRENGTHENING STR WILL BE POSITIONED DIRECTLY TO THE EAST OF TC GEZANI AND WILL EXERT ITS INFLUENCE THROUGH NAVIGATING THE SYSTEM POLEWARD. AS THE VORTEX UNDERGOES VERTICAL DECOUPLING AND BECOMES INCREASINGLY SHALLOW BETWEEN TAUS 36 AND 48, IT IS FORECAST TO INITIATE ITS EXTRATROPICAL TRANSITION (ETT), WITH THE BAROCLINIC TRANSFORMATION EXPECTED TO BE COMPLETE BY TAU 72. IN REGARD TO INTENSITY, THE VWS IS EXPECTED TO INCREASE STEADILY, ESPECIALLY DURING THE ETT. WHILE THE CYCLONE IS THEN ADVECTED INTO A DECIDEDLY HOSTILE ENVIRONMENT CHARACTERIZED BY DIMINISHED OCEANIC HEAT CONTENT AND CONTINUOUS DRY AIR INTRUSION, CONTINUOUS WEAKENING IS PREDICTED TOWARDS THE END OF THE CURRENT FORECAST PERIOD. MODEL DISCUSSION: CONFIDENCE IN THE OVERALL TRACK FORECAST HAS SHOWN IMPROVEMENT AND IS ASSESSED AS MEDIUM. TIMELINE OF THE POLEWARD TURN, AS WELL AS A CROSS-TRACK SPREAD OF 130 NM AROUND TAU 48 ARE THE MAIN CONTRIBUTORS TO UNCERTAINTY. OTHERWISE, ALL MODELS AGREE ON THE GENERAL STEERING PATTERN AND RESULTING TRACK PATH THROUGHOUT THE FORECAST PERIOD. NAVGEM REMAINS A NOTABLE OUTLIER PROJECTING THE SLOWEST AND WESTERNMOST TRACK. OFFICIAL JTWC FORECAST IS LAID CLOSE TO THE MULTI-MODEL CONSENSUS, OFFSETTING SLIGHTLY FOR OUTLYING GUIDANCE FROM NAVGEM. IN TERMS OF INTENSITY, GUIDANCE IS IN GOOD AGREEMENT PROJECTING STEADY WEAKENING AS THE SYSTEM TRANSITIONS INTO A COLD-CORE LOW AND THEREFORE THE JTWC FORECAST IS LAID IN CLOSE PROXIMITY TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM INTENSITY 00-72 HR: HIGH// NNNN