WDPN33 PGTW 072100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 14W (CHAN-HOM) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 32.0N 158.1E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 959 NM EAST-SOUTHEAST OF MISAWA, JAPAN MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 16 KTS SIGNIFICANT WAVE HEIGHT: 22 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A MOSTLY EXPOSED LOW-LEVEL CIRCULATION CENTER WITH FLARING CONVECTION LIMITED TO THE NORTHERN EDGE NEAR THE LLCC. THE LLCC AND WEAK LOW-LEVEL CLOUD BANDS SURROUNDING IT HAVE SEPARATED FROM THE LARGER EXTENSION OF CONVECTIVE ACTIVITY WHICH IS NOW DISPLACED WELL TO THE SOUTHEAST. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC IN IR IMAGERY. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE CIMSS OBJECTIVE INTENSITY ESTIMATES AND THE PERSISTENT STRUCTURE OBSERVED IN EIR. THE ENVIRONMENT IS CHARACTERIZED AS MARGINALLY UNFAVORABLE WITH MODERATE VERTICAL WIND SHEAR (VWS), 26-27 C SEA SURFACE TEMPERATURES, AND POOR OUTFLOW. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SOUTHERN FLANK OF A WESTWARD EXTENSION OF THE SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.0 - 30 KTS RJTD: T1.5 - 25 KTS RCTP: T2.0 - 30 KTS CIMSS SATCON: 31 KTS AT 071810Z CIMSS ADT: 32 KTS AT 071810Z CIMSS AIDT: 32 KTS AT 071810Z CIMSS D-MINT: 29 KTS AT 071554Z CIMSS D-PRINT: 29 KTS AT 071830Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 15-20 KTS SST: 26-27 CELSIUS OUTFLOW: WEAK POLEWARD OTHER FACTORS: SIGNIFICANT DRY AIR ENTRAINMENT ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 14W (CHAN-HOM) HAS TRACKED WEST-SOUTHWESTWARD OVER THE PAST 6 HOURS AT A HEALTHY 16 KT TRANSLATION SPEED UNDER THE STEERING INFLUENCE OF THE RIDGE EXTENSION TO THE NORTHEAST. THE STEERING RIDGE WILL BEGIN WEAKENING AND RETREAT TO THE EAST AROUND TAU 12 AS A SEPARATE STR BUILDS IN FROM THE WEST. THE STEERING ENVIRONMENT WILL BECOME WEAK AS A RESULT AND INDUCE A SLOWER NORTHWESTWARD TRACK SPEED BETWEEN TAU 24-48. AFTER TAU 48, THE RIDGE TO THE NORTHEAST IS EXPECTED TO BUILD BACK WESTWARD AND REGAIN CONTROL OF THE STEERING ENVIRONMENT, RESULTING IN A WEST-NORTHWESTWARD TRACK AS THE SYSTEM ACCELERATES TOWARD HONSHU. THE SYSTEM WILL HAVE AN OPPORTUNITY TO CONSOLIDATE AND INTENSIFY WHILE IT REMAINS IN THE WEAK STEERING ENVIRONMENT AND REACH A PEAK OF BETWEEN TAU 60-72. AS 14W APPROACHES HONSHU IT WILL ENCOUNTER COOLER SEA SURFACE TEMPERATURES AND A COMPLEX MIDLATITUDE SYNOPTIC ENVIRONMENT THAT FAVORS SUBTROPIC TRANSITION BEGINNING AROUND TAU 96 AND COMPLETING BY TAU 120. MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT ABOUT THE WEST-NORTHWESTWARD TRAJECTORY AND MODULATIONS INDUCED BY THE EVOLVING STEERING ENVIRONMENT. THE CROSS-TRACK SPREAD OF 125 NM AT TAU 72 IS ATTRIBUTABLE TO THE COMPLEX STEERING ENVIRONMENT AND MIDLATITUDE ENVIRONMENTAL INFLUENCE ON THE SYSTEM STRUCTURE. CROSS-TRACK SPREAD INCREASES TO 320 NM BY TAU 120 FOR THE SAME REASONS. THE JTWC TRACK FORECAST IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER DUE TO UNCERTAINTY ABOUT THE SUBTROPICAL TRANSITION SCENARIO. MODEL INTENSITY GUIDANCE IS IN GOOD AGREEMENT ABOUT GRADUAL INTENSIFICATION FOLLOWED BY A WEAKENING TREND AFTER TAU 72. THE JTWC INTENSITY FORECAST IS PLACED NEAR THE CONSENSUS WITH MEDIUM CONFIDENCE THROUGHOUT THE FORECAST PERIOD. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 00-72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN