WDPN31 PGTW 052100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (MUN) WARNING NR 017// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 31.7N 148.7E INITIAL INTENSITY: 50 KTS GEOGRAPHIC REFERENCE: 499 NM EAST-SOUTHEAST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 21 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL STORM 04W (MUN) WITH CONVECTION RUNNING ALONG THE NORTHERN SEMICIRCLE, EXTENDING TO THE WESTERN SIDE OF THE PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC). A SLOT OF DRY AIR IS BEING ENTRAINED INTO THE CENTER OF THE VORTEX FROM THE SOUTH, WHICH HAS BEEN HINDERING ANY FURTHER DEVELOPMENT OF THE SYSTEM. THE ENVIRONMENT IS NOW ANALYZED TO BE MARGINAL DUE TO DRY AIR CONTINUING TO BE PULLED INTO THE CENTER. OTHER ENVIRONMENTAL FACTORS ARE FAVORABLE THOUGH, CHARACTERIZED BY MODERATE POLEWARD OUTFLOW, LOW (5-10 KTS) VERTICAL WIND SHEAR, AND WARM (27-28 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN THE ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 50 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF AN EXTENSION OF RIDGING TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T2.5 - 35 KTS RCTP: T2.5 - 35 KTS KNES: T3.0 - 45 KTS CIMSS ADT: 32 KTS AT 051750Z CIMSS AIDT: 58 KTS AT 051750Z CIMSS DPRINT: 48 KTS AT 051750Z CIMSS DMINT: 53 KTS AT 051813Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL VWS: 5-10 KTS SST: 27-28 CELSIUS OUTFLOW: MODERATE POLEWARD OTHER FACTORS: SIGNIFICANT DRY AIR ENTRAINMENT ANALYSIS CONFIDENCE: INITIAL POSITION: LOW INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: MEDIUM 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: NO FURTHER INTENSIFICATION IS FORECAST TO OCCUR FORECAST DISCUSSION: TS 04W IS FORECAST TO BEGIN TURNING NORTHWARD AS RIDGING TO THE EAST BUILDS IN NEXT TO THE VORTEX. A NORTHWARD TRACK IS FORECAST TO CONTINUE THOUGH TAU 24 BEFORE A TURN BACK TO THE NORTH-NORTHWEST BEGINS, IN RESPONSE TO THE FURTHER DEVELOPMENT OF THE RIDGE. 04W IS THEN FORECAST TO ROUND THE RIDGE AXIS NEAR TAU 48 AND ACCELERATE TO THE NORTHEAST THROUGH TAU 72. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN AT TAU 48 AND BE COMPLETE BY TAU 72 AS THE VORTEX ENTERS COOLER WATERS AND BECOMES COMPLETELY ENGULFED BY DRY AIR. 04W IS FORECAST TO MAINTAIN INTENSITY THROUGH TAU 24 AS SEA SURFACE TEMPERATURES REMAIN RELATIVELY WARM AND SHEAR REMAINS LOW. AFTER TAU 24, THESE FACTORS BEGIN TO SIGNIFICANTLY WORSEN, CAUSING WEAKENING THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD. MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN MODERATE AGREEMENT WITH A 142 NM CROSS-TRACK SPREAD AT TAU 48. AFTER TAU 48, MODELS DIVERGE DUE TO DIFFERENCES IN HOW THE SHALLOW VORTEX INTERACTS WITH THE UPPER-LEVEL ENVIRONMENT. NAVGEM AND GALWEM BOTH RACE THE VORTEX OFF TO THE NORTHEAST AFTER TAU 48 WHILE THE REMAINDER OF GUIDANCE SUGGEST A SLOWER TURN TO THE NORTHEAST. THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE, CLOSE TO THE CONSENSUS THROUGH TAU 48 AND SLOWER THROUGH TAU 72. INTENSITY GUIDANCE IS NOW SPLIT, WITH SOME MODELS (GFS AND COAMPS TC) SUGGESTING SLIGHT INTENSIFICATION THROUGH TAU 24 WHILE HAFS-A SUGGESTS WEAKENING STARTING FROM TAU 0. ALL MODELS AGREE ON WEAKENING AFTER TAU 24. THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS THROUGHOUT THE FORECAST PERIOD WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN