WDXS31 PGTW 012100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (HERMAN) WARNING NR 015// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 19.8S 106.8E INITIAL INTENSITY: 70 KTS GEOGRAPHIC REFERENCE: 435 NM WEST-NORTHWEST OF LEARMONTH, AUSTRALIA MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 04 KTS SIGNIFICANT WAVE HEIGHT: 20 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION SHEARED OVER THE WESTERN QUADRANT OF THE LLCC. A 011807Z ATMS 165GHZ IMAGE REVEALS AN AREA OF DEEP CONVECTION OVER THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH OVERALL LOW CONFIDENCE DUE TO THE LACK OF HIGH RESOLUTION MICROWAVE IMAGERY AND VISIBLE IMAGERY. THE INITIAL INTENSITY OF 70 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK ESTIMATES AND OBJECTIVE DVORAK ESTIMATES. ENVIRONMENTAL CONDITIONS ARE MARGINALLY UNFAVORABLE WITH MODERATE (15-20 KNOTS) VERTICAL WIND SHEAR (VWS), MODERATE EASTWARD OUTFLOW, (26-27C) SEA SURFACE TEMPERATURES (SSTS) AND DRY AIR ENTRAINMENT. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP LAYER SUBTROPICAL RIDGE (STR). AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T5.0 - 90 KTS KNES: T5.0 - 90 KTS APRF: T4.5 - 77 KTS CIMSS SATCON: 69 KTS AT 011431Z CIMSS ADT: 84 KTS AT 011730Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 15-20 KTS SST: 26-27 CELSIUS OUTFLOW: MODERATE EASTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: OVER THE PAST SIX HOURS, TC 17S HAS BEEN TRACKING SOUTH-SOUTHWESTWARD ON THE WESTERN PERIPHERY OF A STR POSITIONED TO THE EAST WITH A DEEP MIDLATITUDE TROUGH POSITIONED TO THE SOUTH-SOUTHWEST. THE SYSTEM IS FORECAST TO GRADUALLY TURN TOWARD THE WEST-SOUTHWEST BRIEFLY BEFORE TURNING SHARPLY WEST-NORTHWESTWARD AFTER TAU 24. AFTER TAU 24, THE MIDLATITUDE TROUGH WILL TRACK EASTWARD AND A STRONG LOW TO MID LEVEL STR IS EXPECTED TO BECOME THE DOMINATE STEERING FEATURE TO THE SOUTH, WHICH WILL STEER THE SYSTEM WEST-NORTHWESTWARD THROUGH THE FORECAST PERIOD. TC 17S SHOULD RAPIDLY WEAKEN OVER THE NEXT 24 HOURS, AS ENVIRONMENTAL CONDITIONS DEGRADE STEADILY WITH SIGNIFICANT DRY AIR ENTRAINMENT, INCREASING VWS AND MARGINALLY-FAVORABLE SSTS VALUES AROUND 25-26C. TC 17S WILL, THEREFORE, DISSIPATE BY TAU 72. MODEL DISCUSSION: NUMERICAL MODELS ARE IN GOOD AGREEMENT WITH A 130 NM SPREAD IN SOLUTIONS AT TAU 72 LEADING HIGH CONFIDENCE TO THE TRACK FORECAST. THE JTWC TRACK IS CLOSE TO THE MULTI-MODEL CONSENSUS THROUGH 48 HOURS AND SLIGHTLY RIGHT OF CONSENSUS THROUGH OUT THE REST OF THE FORECAST. RELIABLE INTENSITY GUIDANCE SUPPORTS A RAPID WEAKENING TREND DUE TO IMPEDING ENVIRONMENTAL FACTORS TO 35 KNOTS BETWEEN TAU 24-36 LEADING HIGH CONFIDENCE TO THE INTENSITY FORECAST. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM// NNNN