WDPN32 PGTW 121500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 19W (CHANTHU) WARNING NR 026// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 26.2N 122.6E INITIAL INTENSITY: 95 KTS GEOGRAPHIC REFERENCE: 89 NM EAST-NORTHEAST OF TAIPEI, TAIWAN MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 10 KTS SIGNIFICANT WAVE HEIGHT: 38 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS THAT TY 19W HAS UNDERGONE SIGNIFICANT WEAKENING OVER THE PAST SIX HOURS, WITH CONVECTIVE CLOUD TOPS WARMING SIGNIFICANTLY AND THE EYE COOLING AND FILLING. HIGH RESOLUTION MODELING INDICATES DRY, DESCENDING AIR IS BEING ENTRAINED INTO THE SOUTHWESTERN SIDE OF THE SYSTEM. ADDITIONALLY, CIMSS SHEAR ANALYSIS REVEALS MODERATE SOUTHWESTERLY SHEAR IS NOW IMPINGING ON THE SYSTEM. FINALLY, OVER THE PAST SIX HOURS THE SYSTEM HAS MOVED OVER AN EDDY OF LOW OHC. THE COMBINATION OF ALL THESE FACTORS HAS LED TO THE RAPID WEAKENING. ANIMATED RADAR IMAGERY DEPICTS THE DEVELOPMENT OF A MOAT OF WEAK RADAR RETURNS SURROUNDING A COMPACT CORE OF CONVECTION WHICH IS WRAPPING INTO THE ILL-DEFINED LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON RJTD RADAR FIXES AND THE 10-NM RAGGED EYE IN THE EIR. THE INITIAL INTENSITY IS ASSESSED AT 95 KNOTS WITH MEDIUM CONFIDENCE BASED ON A BLEND OF THE AGENCY CURRENT INTENSITY ESTIMATES BETWEEN 5.0-5.5. AGENCY DATA-T NUMBERS ARE MUCH LOWER, IN THE T4.5 (77 KTS) RANGE, WHILE THE ADT AND SATCON APPEAR UNREASONABLY HIGH BASED ON THE RAPIDLY DETERIORATING STRUCTURE. A 121106Z ASCAT-B PASS PROVIDED SOLID DATA TO DRAMATICALLY REDUCE THE SIZE OF THE WIND FIELD. ANALYSIS REVEALS AN INCREASINGLY MARGINAL ENVIRONMENT WITH INCREASING SOUTHWESTERLY SHEAR, DECREASING SSTS AND STEADILY DECREASING OUTFLOW ALOFT. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T5.0 - 90 KTS RJTD: T6.0 - 115 KTS KNES: T5.5 - 102 KTS CIMSS SATCON: 114 KTS AT 121038Z CIMSS ADT: 104 KTS AT 121140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD OTHER FACTORS: WEAKENING EQUATORWARD OUTFLOW CHANNEL DISPLACED WELL TO THE EAST. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TY 19W CONTINUES TO TRACK GENERALLY NORTHWARD ALONG THE WESTERN PERIPHERY OF A NORTH-SOUTH ORIENTED STR TO THE EAST. THE PAST SIX HOURS HAS SEEN THE TRACK TREND A BIT FURTHER EAST OF NORTH THAN EXPECTED, BUT THIS IS LIKELY JUST A WOBBLE AND THE OVERALL TRACK IS FORECAST TO BE NORTH THROUGH THE NEXT 12 TO 24 HOURS. BETWEEN TAU 24 AND 36, A DEEP STR IS EXPECTED TO BUILD TO THE NORTHWEST OF THE SYSTEM WITH RIDGING EXTENDING EASTWARD ACROSS THE KOREAN PENINSULA, COCOONING TY 19W IN A COMPETING STEERING ENVIRONMENT. IN RESPONSE, TY 19W WILL SLOW AND BECOME QUASI-STATIONARY AND STALL IN THE VICINITY OF SHANGHAI BETWEEN TAU 36 AND 72. BY TAU 72 A FAIRLY STRONG MID-LATITUDE MAJOR SHORTWAVE TROUGH WILL BEGIN TO ERODE THE STR TO THE WEST, WHILE THE STR TO THE EAST MOVES SOUTHWEST AND BUILDS. THE COMBINATION OF THE REORIENTATION OF STR TO THE EAST AND THE APPROACHING TROUGH FROM THE WEST WILL KICK TY 19W ONTO AN ACCELERATING EASTWARD TRACK. AS THE SYSTEM SKIRTS THE SOUTHERN KOREA COASTLINE, IT WILL BECOME EMBEDDED UNDER STRONG WESTERLY FLOW ALOFT AND BEGIN EXTRATROPICAL TRANSITION (ETT). THE UPPER-LEVEL OUTFLOW PATTERN HAS SHIFTED OVER THE PAST 12 HOURS, WITH A 200MB ANTI-CYCLONE NOW FIRMLY IN PLACE JUST EAST OF THE SYSTEM. WHILE THIS POSITIONING SUPPORTS A STRONG POLEWARD OUTFLOW CHANNEL, THE PREVIOUSLY STRONG EQUATORWARD CHANNEL HAS WEAKENED. THIS PATTERN IS EXPECTED TO CONTINUE AND THE REDUCTION TO ONE OUTFLOW CHANNEL WILL BE THE PRIMARY MECHANISM TO WEAKEN THE SYSTEM OVER THE NEXT 24 TO 36 HOURS. BY TAU 48 A 200MB RIDGE WILL MOVE DIRECTLY OVER THE SYSTEM, EFFECTIVELY CHOKING OFF ANY SIGNIFICANT OUTFLOW MECHANISM AND LEADING TO MORE RAPID WEAKENING THROUGH TAU 72. ONCE THE SYSTEM STARTS MOVING EAST IT WILL TAP INTO ROBUST DIVERGENT OUTFLOW AHEAD OF THE APPROACHING TROUGH, BUT RAPIDLY INCREASING VWS WILL OFFSET THIS OUTFLOW RESULTING IN A STEADY INTENSITY THROUGH THE REMAINDER OF THE FORECAST. MODEL DISCUSSION: TRACK GUIDANCE REMAINS IN GOOD AGREEMENT WITH MINIMAL CROSS- AND ALONG-TRACK SPREAD THROUGH TAU 24. BETWEEN TAU 36 AND 72 HOWEVER, THE MODELS DEPICT WHAT IS KNOWN AS A SQUASHED SPIDER PATTERN WITH TRACKERS RANDOMLY SCATTERED WITHIN A 150NM CIRCLE CENTERED OVER SHANGHAI. THIS IS NOT UNEXPECTED DUE TO THE WEAK AND COMPETING STEERING INFLUENCES, THOUGH IT LENDS VERY HIGH UNCERTAINTY TO THE TRACK FORECAST THROUGH TAU 72. THE JTWC FORECAST LIES ON THE EASTERN SIDE OF THE GUIDANCE CLUSTER THROUGH TAU 72. TRACK GUIDANCE FIRMS UP AFTER TAU 72, WITH THE BULK OF THE TRACKERS TIGHTLY CLUSTERED ON A TRACK OVER THE SOUTHERN COAST OF KOREA THROUGH TAU 120. THE EGRR AND GALWEM ARE THE NOTABLE OUTLIERS, TRACKING THE SYSTEM OVER CENTRAL KYUSHU. THE EXTENDED FORECAST LIES TO THE NORTH OF THE CONSENSUS MEAN WITH LOW CONFIDENCE DUE TO THE UNCERTAINTIES AROUND THE TIMING OF THE TURN TO THE EAST. INTENSITY GUIDANCE IS IN GOOD AGREEMENT ON THE STEADY WEAKENING TREND, THOUGH THE JTWC FORECAST LIES ABOUT FIVE KNOTS HIGHER THAN THE CONSENSUS MEAN, WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: LOW TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN