WDPN31 PGTW 200900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 25W (TWENTYFIVE) WARNING NR 003// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 21.2N 128.6E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 321 NM SOUTH OF KADENA AB MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 08 KTS SIGNIFICANT WAVE HEIGHT: 11 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAGGED AND DISORGANIZED LOW LEVEL CIRCULATION (LLC), THOUGH CURRENT IMAGERY DOES NOT SHOW A DISTINCT CIRCULATION CENTER. THE CLOUD FIELD IS DEFINED BY A BROAD SWATH OF LOW-LEVEL, STABLE, STRATOCUMULUS CLOUDS TO THE NORTH AND WEST OF THE CENTER, ASSOCIATED WITH A NORTHEAST SURGE EVENT. THERE IS SOME SMALL-SCALE FLARING CONVECTION JUST SOUTH OF THE ASSESSED CENTER, BUT IT REMAINS DISORGANIZED AND HAS NOT PERSISTED FOR MORE THAN A COUPLE OF HOURS. A 200439Z AMSR2 37GHZ MICROWAVE IMAGE DEPICTED A WEAK LOW EMISSIVITY REGION (LER) AND LOW-LEVEL FRAGMENTED SPIRAL BANDING FEATURES, SANDWICHED BETWEEN AREAS OF DISORGANIZED CONVECTIVE ACTIVITY, LENDING MEDIUM CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY REMAINS SET AT 25 KNOTS WITH MEDIUM CONFIDENCE, HEDGED A BIT LOWER THAN THE BULK OF AGENCY AND OBJECTIVE FIX ESTIMATES IN LIGHT OF A 200439Z AMSR2 WINDSPEED IMAGE WHICH SHOWED 25-27 KNOT WINDS TO THE NORTHWEST IN THE GRADIENT FLOW ASSOCIATED WITH THE NORTHEAST SURGE AND 20-23 KNOTS IN THE OTHER QUADRANTS. ANALYSIS REVEALS A GENERALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY WARM SSTS AND LOW VWS, BUT OFFSET SOMEWHAT BY AN INFLUX OF RELATIVELY STABLE AIR FROM THE NORTH AND WEAK OUTFLOW ALOFT. THE SYSTEM HAS MOVED ON A WEST-NORTHWEST TRACK OVER THE PAST 12 HOURS TOWARDS A WEAKNESS IN THE SUBTROPICAL RIDGE (STR) TO THE NORTH. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SOUTHERN SIDE OF A SUBTROPICAL RIDGE (STR) TO THE NORTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS KNES: T2.5 - 35 KTS CIMSS ADT: 33 KTS AT 200540Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 5-10 KTS SST: 28-29 CELSIUS OUTFLOW: WEAK EQUATORWARD OTHER FACTORS: COOL, STABLE AIR ENTRAINING INTO THE SYSTEM FROM THE NORTH. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH 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: AS MENTIONED ABOVE, OVER THE PAST 12 HOURS, TD 25W HAS TRACKED ON A SLIGHT WEST-NORTHWEST TRAJECTORY, TOWARDS A WEAKNESS IN THE RIDGE WHICH HAS BEEN INDUCED BY A PASSING TROUGH TO THE NORTH. THE STR IS EXPECTED TO BUILD IN AND REPOSITION TO POINT DUE NORTH OF TD 25W WITHIN THE NEXT 12 TO 18 HOURS, AND BE THE PREDOMINANT STEERING INFLUENCE THROUGH AT LEAST TAU 48. IN RESPONSE, TD 25W SHOULD IMMINENTLY SHIFT OVER TO A DUE WESTWARD TRACK AND CONTINUE THIS GENERAL MOTION THROUGH THE LUZON STRAIT. SHORTLY AFTER THE SYSTEM MOVES WEST OF THE STRAIT THE STEERING PATTERN WILL SHIFT ONCE MORE, WITH A BUILDING STR OVER SOUTHERN CHINA TAKING OVER AS THE PRIMARY STEERING MECHANISM. THIS, COMBINED WITH THE INFLUENCE OF ANOTHER NORTHEAST SURGE SPILLING SOUTHWARD OUT OF THE TAIWAN STRAIT, WILL TURN TD 25W ONTO A SOUTHWESTWARD TRACK THROUGH THE REMAINDER OF THE FORECAST PERIOD. DUE TO THE CONTINUED PRESENCE OF A RELATIVELY STABLE AIRMASS TO THE WEST AND NORTH OF THE SYSTEM, AND THE CONTINUED DISORGANIZED NATURE OF THE CIRCULATION, LITTLE TO NO INTENSIFICATION IS EXPECTED THROUGH THE FIRST 12 HOURS OR SO. AS THE SYSTEM SLIDES FURTHER WEST, THE AIR MASS TO THE WEST WILL MODIFY AND BECOME LESS STABLE, WHICH WHEN COMBINED WITH PERSISTENT LOW VWS AND IMPROVED OUTFLOW, WILL ALLOW FOR STEADY INTENSIFICATION TO A PEAK OF 50 KNOTS OR SLIGHTLY HIGHER BETWEEN TAU 48 AND TAU 72. BY TAU 72, THE INFLUENCE OF THE NORTHEASTERLY FLOW COMING SOUTH FROM THE TAIWAN STRAIT WILL BEGIN TO INFLUENCE THE SYSTEM, INJECTING COOLER, DRIER AND MORE STABLE AIR INTO THE WESTERN PORTION OF THE CIRCULATION. THIS COMBINED WITH MARGINALLY INCREASED SHEAR AND REDUCED SSTS LEFT IN THE WAKE OF TY 23W WILL LEAD TO STEADY WEAKENING THROUGH THE REMAINDER OF THE FORECAST. MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE WITH NOTABLE EXCEPTION OF NAVGEM, IS IN OVERALL GOOD AGREEMENT ON THE GENERAL TRACK. NAVGEM IS THE SOLE OUTLIER, TAKING THE SYSTEM TO LANDFALL OVER TAIWAN, THEN TURNING SHARPLY SOUTHWARD INTO THE SCS BY TAU 120. THE REMAINDER OF THE TRACK GUIDANCE IS CONSTRAINED IN A TIGHT 70NM ENVELOPE AT TAU 48, WHICH DECREASES TO LESS THAN 50NM AT TAU 72 BEFORE WIDENING STEADILY TO A 270NM WIDE ENVELOPE AT TAU 120. THE GFS IS CONSISTENTLY THE FASTEST OF THE MODELS, PARTICULARLY THROUGH TAU 48, WHILE THE UKMET LAGS BEHIND THE REST OF THE PACK. THE JTWC FORECAST LIES ON THE NORTHERN EDGE OF THE GUIDANCE THROUGH TAU 24 TO REFLECT THE RECENT TRACK MOTION, THEN REJOINS THE MAIN PACK, BUT REMAINING JUST NORTH OF THE CONSENSUS MEAN, THROUGH THE REMAINDER OF THE FORECAST. INTENSITY GUIDANCE IS IN FAIRLY GOOD AGREEMENT, WITH THE COAMPS-TC AND HWRF ON THE LOWER END OF THE SPECTRUM, PEAKING AROUND 35 KNOTS, WHILE THE GFS AND DECAY-SHIPS (GFS) MARK THE HIGHER END, PEAKING AROUND 50 KNOTS. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST, IN CONCURRENCE WITH THE MODELS NOTED ABOVE, ABOUT 10 KNOTS HIGHER THAN THE CONSENSUS MEAN. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN