WDPN31 PGTW 130300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 28W (YAMANEKO) WARNING NR 003// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 22.1N 166.0E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 171 NM NORTH OF WAKE ISLAND MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 03 KTS SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A DETERIORATED STRUCTURE, PARTICULARLY WHEN COMPARED TO JUST THREE HOURS AGO. THE SYSTEM LIKELY PEAKED OUT IN INTENSITY AROUND THE 122100Z HOUR, WHEN CONVECTION WAS AT ITS MAXIMUM, CLOUD TOPS WERE THE COLDEST AND THE CONVECTIVE BLOWOFF WAS ABLE TO PUSH BACK AGAINST THE STRONG WESTERLY SHEAR. BUT THE HALCYON DAYS ARE NOW OVER, WITH CLOUD TOP TEMPERATURES WARMING AND CONVECTION RAPIDLY WEAKENING AND BECOMING MORE DISORGANIZED, ALLOWING FOR THE SHEAR TO TAKE MORE OF A DETRIMENTAL EFFECT. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON SPIRAL BAND ANALYSIS OF THE LOW-LEVEL BANDS SEEN IN THE CLEAR AIR TO THE WEST OF THE CONVECTIVE MASS, CLARIFIED BY TWO PARTIAL ASCAT PASSES FROM THE 2200Z HOUR, WHICH HELPED CONSTRAIN THE POSITION. THE INITIAL INTENSITY REMAINS SET AT 35 KNOTS, SLIGHTLY BELOW THE PGTW, KNES AND PHFO DVORAK CURRENT INTENSITY ESTIMATES WHICH SEEM A BIT TOO HIGH BASED ON THE SATELLITE REPRESENTATION, AND MORE IN LINE WITH THE DEEP MULTINET AND OPEN-AIIR ESTIMATES. THE AFOREMENTIONED ASCAT PASSES INDICATED THE 35 KNOT WIND FIELD IS LIMITED TO THE NORTHEAST SECTOR OF THE SYSTEM, UNDER THE CONVECTION. THE ENVIRONMENT IS BECOMING INCREASINGLY HOSTILE, WITH WIND SHEAR ESTIMATED BY CIMSS AT OVER 30 KNOTS OFFSETTING THE ROBUST POLEWARD OUTFLOW AND WARM SSTS. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: COMPETING STEERING PATTERN, WITH A STRONG TRANSITORY RIDGE TO THE NORTH AND WEAKER SUBTROPICAL RIDGE (STR) TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T3.0 - 45 KTS RJTD: T2.0 - 30 KTS KNES: T3.0 - 45 KTS PHFO: T3.0 - 45 KTS CIMSS ADT: 34 KTS AT 122340Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 30+ KTS SST: 27-28 CELSIUS OUTFLOW: STRONG POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH INITIAL INTENSITY: HIGH INITIAL WIND RADII: HIGH 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL STORM 28W HAS SLOWED SIGNIFICANTLY, DOWN TO ABOUT THREE KNOTS, AS IT HAS RUN HEADLONG INTO THE SOUTHERN SIDE OF A SHALLOW TRANSITORY RIDGE CENTERED NEAR 35N 160E. THIS SLOW DRIFT NORTHWARD IS EXPECTED TO CONTINUE FOR THE NEXT 12 TO 18 HOURS, UNTIL THE RAPIDLY MOVING RIDGE TO THE NORTH MOVES EAST OF THE STORMS LONGITUDE. ONCE THIS OCCURS, THE SYSTEM WILL BE EJECTED OUT TO THE NORTHEAST, ACCELERATING STEADILY ALONG THE ENHANCED GRADIENT BETWEEN A HIGH-AMPLITUDE RIDGE TO THE EAST AND A STRONG, NEGATIVELY TILTED TROUGH RAPIDLY MOVING IN FROM THE WEST. TRACK SPEEDS ARE EXPECTED TO INCREASE TO 25 KNOTS OR MORE BY THE END OF THE FORECAST. IN TERMS OF INTENSITY, AS MENTIONED IN THE PREVIOUS SECTION, THE SYSTEM LIKELY PEAKED AROUND 2100Z, WHEN RAW ADT VALUES REACHED T2.7, THE SYSTEM HAS A SYMMETRICAL CENTRAL DENSE OVERCAST AND TEMPERATURES REACHED ALMOST -90C IN THE OVERSHOOTING TOPS. NOW THAT THE CONVECTION HAS WARMED BY ABOUT 15C, AND BECOME MORE DISORGANIZED, THE STRONG SHEAR IS BEGINNING TO EXERT DOMINANCE OVER THE ROBUST POLEWARD OUTFLOW AND THE SYSTEM IS BEGINNING TO DECOUPLE. THE STRONG AND PERSISTENT SHEAR MEANS THAT TS 28W IS NOT LIKELY TO BE ABLE TO MAINTAIN STRONG CORE CONVECTION GOING FORWARD, BUT THE LOW-LEVEL WIND FIELD WILL TAKE TIME TO SPIN DOWN, THUS INTENSITY IS EXPECTED TO REMAIN STEADY THROUGH TAU 36. WHILE MANUAL AND AUTOMATED PHASE CLASSIFICATIONS REMAIN FIRMLY TROPICAL, THE SYSTEM IS TRENDING MORE TOWARDS BEING SUBTROPICAL AND A BRIEF PERIOD OF SUBTROPICAL TRANSITION IS ANTICIPATED AROUND TAU 24. BUT AS IT MOVES NORTH OF THE 26C ISOTHERM AROUND TAU 24, IT WILL QUICKLY TRANSITION TO AN EXTRATROPICAL LOW AS IT MOVES UNDER A STRONG 200MB JET STREAM AND COMES INTO PHASE WITH A VERY STRONG, NEGATIVELY TILTED TROUGH MOVING IN FROM THE WEST. EXTRATROPICAL TRANSITION (ETT) IS FORECAST NO LATER THAN TAU 48, POSSIBLY AS EARLY AS TAU 36. AS THE SYSTEM UNDERGOES ETT IT IS ANTICIPATED TO INTENSIFY DUE TO STRONG BAROCLINIC FORCING. MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN OVERALL GOOD AGREEMENT, WITH A MODEST DEGREE OF CROSS-TRACK SPREAD THROUGH THE FORECAST PERIOD. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS ONE, AND LIES NEAR THE MULTI-MODEL CONSENSUS MEAN, THOUGH DUE TO THE UNCERTAINTY IN THE SHORT-TERM TRACK MOTION, CONFIDENCE IS MEDIUM. THE JTWC INTENSITY FORECAST LIES BELOW THE BULK OF THE GUIDANCE, WHICH SHOWS INTENSIFICATION UP TO 50 KNOTS BY TAU 12, FOLLOWED BY WEAKENING AND THEN REINTENSIFICATION AFTER TAU 36 AS THE SYSTEM UNDERGOES ETT. THE JTWC FORECAST DEVIATES FROM THE GUIDANCE IN THE SHORT-TERM DUE TO THE CURRENT DECOUPLING SEEN IN THE SATELLITE IMAGERY AND THE FORECASTED PERSISTENCE OF THE STRONG SHEAR, THOUGH LATER INTENSIFICATION REMAINS POSSIBLE IN THE LATER TAUS DUE TO THE STRONG BAROCLINIC FORCING INDUCED BY THE NEGATIVELY TILTED TROUGH INTERACTION. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM// NNNN