WDXS31 PGTW 080300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (MITCHELL) WARNING NR 009// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 20.7S 115.6E INITIAL INTENSITY: 75 KTS GEOGRAPHIC REFERENCE: 127 NM NORTHEAST OF LEARMONTH, AUSTRALIA MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 24 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL CYCLONE 20S (MITCHELL) WITH SLIGHTLY WARMING CLOUD TOPS OVER THE PAST 6 HOURS. ADDITIONALLY, THE RADAR EYE IN ANIMATED RADAR IMAGERY FROM LEARMONTH SHOWS A DISTINCT WEAKNESS IN THE EASTERN SEMICIRCLE, HINTING AT A SLIGHT WEAKENING TREND. THE PERSISTENT DRY AIR ENTRAINMENT AND MODERATE NORTHEASTERLY VERTICAL WIND SHEAR APPEAR TO BE TAKING A TOLL ON THE SYSTEM. SURFACE OBSERVATIONS FROM THE BARROW ISLAND AIRPORT REPORTED MAXIMUM SUSTAINED WINDS OF 59 KTS AND A PRESSURE MINIMUM OF 977 MB AS THE CENTER PASSED OVER THE ISLAND. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE ANIMATED RADAR IMAGERY. THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. ENVIRONMENTAL ANALYSIS NOW INDICATES THAT 20S IS IN A MARGINALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG POLEWARD AND EQUATORWARD OUTFLOW ALOFT, MODERATE (15-20 KTS) NORTHEASTERLY VERTICAL WIND SHEAR, DRY AIR ENTRAINMENT, AND WARM (29-30 C) SEA SURFACE TEMPERATURES. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED OVER EASTERN AUSTRALIA. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T4.5 - 77 KTS APRF: T4.5 - 77 KTS CIMSS ADT: 82 KTS AT 080000Z CIMSS AIDT: 80 KTS AT 080000Z CIMSS D-MINT: 70 KTS AT 072122Z CIMSS D-PRINT: 66 KTS AT 080020Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 28-29 CELSIUS OUTFLOW: STRONG POLEWARD AND EQUATORWARD OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE WEST AND NORTH 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 20S IS FORECAST TO TRACK SOUTHWESTWARD, ALONG THE WESTERN PERIPHERY OF THE STR THROUGH TAU 12. 20S IS FORECAST TO MAKE LANDFALL WITHIN THE EXMOUTH GULF, SOUTH OF LEARMONTH, RIGHT AROUND TAU 18. AFTER TAU 18, 20S WILL TURN SOUTHWARD AS IT ROUNDS THE WESTERN EXTENT OF THE STR WITH INCREASING TRACK SPEEDS. BY TAU 36, 20S WILL HAVE ROUNDED THE RIDGE AXIS AND WILL COMMENCE A SOUTHEASTWARD TURN AS IT TRACKS FURTHER INLAND INTO WESTERN AUSTRALIA. IN TERMS OF INTENSITY, 20S IS FORECAST TO CONTINUE MARGINALLY WEAKENING OVER THE NEXT 12 HOURS AS IT ENTERS THE EXMOUTH GULF DUE TO THE DRY AIR ENTRAINMENT AND NORTHEASTERLY SHEAR. LANDFALL INTENSITY IS FORECAST TO BE AROUND 65-70 KTS. AFTER LANDFALL, 20S WILL BEGIN TO STEADILY WEAKEN IN RESPONSE TO INCREASING NORTHERLY SHEAR, DRY AIR ENTRAINMENT, AND FRICTIONAL EFFECTS. DISSIPATION IS FORECAST TO OCCUR NEAR TAU 48, AS THE SYSTEM TRACKS SOUTHEAST OF SHARK BAY. MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN VERY GOOD AGREEMENT WITH THE EXCEPTION OF NAVGEM WHICH CONTINUES TO DISPLAY A FURTHER WESTWARD TRACK. DISCOUNTING NAVGEM, THERE IS A MERE 25 NM CROSS-TRACK SPREAD AT TAU 48, WITH AN ALONG-TRACK SPREAD OF AROUND 100 NM. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE ALTERED CONSENSUS (REMOVING NAVGEM) WITH HIGH CONFIDENCE. INTENSITY GUIDANCE AGREES ON STEADY WEAKENING THROUGHOUT THE FORECAST PERIOD WITH NO SIGNIFICANT OUTLIERS. THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE CONSENSUS WITH MEDIUM CONFIDENCE DUE TO INHERENT UNCERTAINTIES REGARDING TERRAIN EFFECTS AND THE CORRESPONDING WEAKENING TREND OVER LAND. FORECAST CONFIDENCE: TRACK 00-72 HR: HIGH INTENSITY 00-72 HR: MEDIUM// NNNN