WDPN31 PGTW 210300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 25W (TWENTYFIVE) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 20.4N 124.6E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 404 NM NORTH-NORTHEAST OF MANILA, PHILIPPINES MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: TROPICAL DEPRESSION 25W CONTINUES TO SLOWLY INTENSIFY IN A MARGINALLY FAVORABLE ENVIRONMENTAL OVER THE PAST 12 HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAGGED DISORGANIZED SYSTEM, BUT HAS RETAINED A SMALL CORE WITH DEEP CONVECTION FIRING ON THE WESTERN SIDE OF THE OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC). AN EARLIER 202118Z SSMIS 91GHZ MICROWAVE IMAGE REVEALS SHALLOW BANDING WRAPPING INTO AN ELONGATED LOW-LEVEL CIRCULATION CENTER POSITIONED ALONG THE NORTHEASTERN EDGE OF A CURVED CONVECTIVE BAND. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SSMIS MICROWAVE IMAGE ALONG WITH AGENCY DVORAK POSITION FIXES. THE INITIAL INTENSITY OF 30 KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A COMBINATION OF OBJECTIVE AND SUBJECTIVE DVORAK INTENSITY ESTIMATES WHICH REMAIN ON THE HIGH END OF INTENSITY ESTIMATES. ENVIRONMENTAL CONDITIONS ARE MARGINALLY FAVORABLE WITH DRY STABLE AIR TO THE NORTH, MODERATE EQUATORWARD OUTFLOW, WARM (29-30C) SEA SURFACE TEMPERATURES (SSTS), AND LOW (5-10KTS) VERTICAL WIND SHEAR (VWS). THESE ENVIRONMENTAL FACTORS FOR FORECAST TO PREVENT RAPID CONSOLIDATION AND STRENGTHENING. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED OVER TAIWAN. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS KNES: T2.0 - 30 KTS CIMSS ADT: 39 KTS AT 202340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 5-10 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE EQUATORWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM 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 25W'S FORECAST TRACK REMAINS VERY SIMILAR TO PREVIOUS FORECASTS, AS THE SYSTEM GENERALLY TRACKS WEST-NORTHWESTWARD ALONG THE SOUTHEASTERN PERIPHERY OF A DEEP- LAYER SUBTROPICAL RIDGE (STR) POSITIONED OVER TAIWAN. THIS GENERAL WESTWARD TRACK WILL CONTINUE THROUGH TAU 24. BY TAU 36, THE SYSTEM WILL THEN PASS THROUGH THE LUZON STRAIGHT AND MOVE INTO THE SOUTH CHINA SEA WITH A SLIGHT TRACK POLEWARD AS A WEAK TROUGH PASSES TO THE NORTH THAT WILL INDUCE A WEAKNESS WITHIN THE STEERING RIDGE. BY TAU 48, 25W WILL THEN REACH AN INFLECTION POINT WHEREBY THE PRIMARY STEERING MECHANISM BECOMES A DEEP LAYER RIDGE OVER EAST CHINA. THIS STEERING MECHANISM WILL THEN STEER THE SYSTEM GENERALLY SOUTHWESTWARD TOWARD SOUTHERN VIETNAM THROUGH THE REMAINED OF THE FORECAST PERIOD. TD 25W WILL SLOWLY INTENSIFY THROUGH THE NEXT 24 HOURS DUE TO THE INITIAL HIGHLY ASYMMETRICAL LLCC ALONG WITH THE DRY STABLE AIR OVER THE NORTHERN PERIPHERY. HOWEVER, AFTER TAU 48 THE SYSTEM MOVES INTO A MORE CONDUCIVE ENVIRONMENTAL WITH IMPROVED UPPER-LEVEL OUTFLOW WHICH WILL ALLOW FOR A FAST INTENSIFICATION PERIOD. AFTER TAU 72, DECREASING SEA SURFACE TEMPERATURES (SSTS) LEFT IN THE WAKE OF TY 23W ALONG WITH UPPER-LEVEL CONVERGENT EASTERLY FLOW WILL RESULT IN STEADY WEAKENING THROUGH TAU 120. MODEL DISCUSSION: NUMERICAL TRACK MODELS ARE IN TIGHT AGREEMENT THROUGH 72 WITH A LARGER SPREAD UP TO 370 NM BY TAU 120. PAST TAU 48 AFUI TAKES AN UNREALISTIC ABRUPT TURN WESTWARD WHILE THE BULK OF THE GUIDANCE CONTINUES THE SYSTEM SOUTHWESTWARD AND CROSS-TRACK SPREAD INCREASE. ADDITIONALLY, THE GFS ENSEMBLE (GEFS) SOLUTIONS SUPPORT THE JTWC FORECAST TRACK THROUGH TAU 72, WITH INCREASING CROSS-TRACK SPREAD BY TAU 120. FOR THESE REASON THE JTWC FORECAST TRACK IS PLACED WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS UNCERTAIN, WITH AVAILABLE GUIDANCE INDICATING A SPREAD BETWEEN 30 KNOTS (COTI) AND 50 KNOTS (DECAY SHIPS), ALONG WITH HWRF NOW SHOWING A MUCH LOW PEAK OF 40 KNOTS FROM THE PREVIOUS MODEL RUN. THE JTWC FORECAST IS HEDGED SLIGHTLY ABOVE THE SHIPS GUIDANCE, AND THE CONSENSUS MEAN TO MAINTAIN CONTINUITY WITH PREVIOUS INTENSITY GUIDANCE. THEREFORE, CONFIDENCE IS LOW IN THE FORECAST INTENSITY. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: LOW INTENSITY 72-120 HR: LOW// NNNN